Comprehensive Study Notes on Canine Splanchnology, Anatomy, and Physiology

Introduction to Splanchnology & Body Cavities

Overview of Splanchnology

• Definition: Splanchnology is the branch of anatomy dealing with the study of the internal organs (viscera) of the digestive, respiratory, and urogenital systems.
• Etymology:
  • Splanchnon: Greek term meaning "internal organ" or "entrails."
  • Viscus (singular) / Viscera (plural): Derived from Latin, meaning "an internal organ, entrails."
• General Characteristics of Viscera:
  • Organs located within body cavities (thoracic, abdominal, pelvic) that are essential for maintaining individual life and ensuring species continuity.
  • Each system generally consists of a tubular tract lined with a mucous membrane that communicates with the exterior at one or both ends.
  • Organs are surrounded by and, if hollow, lined by specialized membrane layers.
• Classification of Viscera:
  • Hollow Viscera: Organs containing internal cavities (e.g., stomach, intestines, urinary bladder, uterus).
  • Solid (Parenchymatous) Viscera: Organs lacking internal cavities (e.g., liver, pancreas, kidneys, ovaries, testes).

Overview of Anatomical Systems and Fields of Study

• Skeletal System: Study = Osteology; Chief Structures = Bones.
• Articular System: Study = Arthrology; Chief Structures = Joints.
• Muscular System: Study = Myology; Chief Structures = Muscles.
• Digestive System: Study = Splanchnology; Chief Structures = Stomach, intestines, liver, pancreas.
• Respiratory System: Study = Splanchnology; Chief Structures = Lungs, airways.
• Urinary System: Study = Splanchnology; Chief Structures = Kidneys, urinary bladder.
• Reproductive System: Study = Splanchnology; Chief Structures = Ovaries, testes, reproductive tracts.
• Endocrine System: Study = Endocrinology; Chief Structures = Ductless glands.
• Nervous System: Study = Neurology; Chief Structures = Brain, spinal cord, nerves.
• Circulatory System: Study = Cardiology; Chief Structures = Heart, blood vessels.
• Sensory System: Study = Esthesiology; Chief Structures = Eye, ear.

Body Cavities

• Body cavities are housed within the body trunk and defined by vertebrae, ribs, and muscular structures. They are categorized into three main zones by relative size:
  1. Abdominal Cavity (Abdomen): Largest in size.
  2. Thoracic Cavity (Thorax): Second largest in size.
  3. Pelvic Cavity (Pelvis): Smallest in size.

Thoracic Cavity Boundaries and Structure

• Cranial Boundary: Cranial thoracic opening (thoracic inlet), formed dorsally by the first thoracic vertebra ($T1$), laterally by the first pair of ribs, and ventrally by the manubrium (the first segment of the sternum).
• Dorsal Boundary: Bodies of the thoracic vertebrae.
• Ventral Boundary: Sternum.
• Caudal Boundary: Diaphragm.
• Internal Contents: Lungs, heart, pleura (membranes covering the lungs), pleural cavities, pericardium (sac surrounding the heart), and pericardial cavity.

The Diaphragm

• Structure: A dome-shaped muscle located at the caudal limit of the thoracic cavity, separating it from the abdominal cavity.
• Innervation: Supplied by the phrenic nerve.
• Function: Primary muscle of respiration (breathing).
• Anatomical Openings (Hiatuses):
  1. Aortic Hiatus: Most dorsal opening. Transmits the aorta, azygous vein, and thoracic duct.
  2. Oesophageal Hiatus: Located centrally between muscle fibers. Transmits the esophagus, as well as the dorsal and ventral vagal nerve trunks.
  3. Caval Foramen: Situated within the central tendinous region (centrum tendineum). Transmits the caudal vena cava. The vessel wall is fused directly with the diaphragm, preventing movement of the vessel during diaphragmatic contractions.
• Structural Components: Sternal part, costal part, lumbar part (comprising the right crus and left crus), and tendinous center.

Pelvic Cavity Boundaries and Structure

• Boundaries: Enclosed by the two hip bones (os coxae: ilium, ischium, pubis), the sacrum, and the first two caudal vertebrae. Ventrally bounded by the pelvic symphysis.
• Internal Contents: Rectum, anal canal, and the pelvic parts of the urinary and reproductive viscera.

Canine Pelvic Obstetrics & Presentations

• Functional Importance: Serves as the birth canal for the fetus during parturition. Adequate diameter is required to prevent dystocia (difficult birth).
• Pelvic Inlet (Cranial Pelvic Aperture):
  • Dorsal Boundary: Sacral promontory.
  • Lateral Boundary: Arcuate line.
  • Ventral Boundary: Pecten of the pubis.
• Pelvic Diameters:
  • Conjugate Diameter: Distance from the sacrum to the pubis.
  • Transverse Diameter: Widest transverse width of the inlet.
• Pelvic Outlet (Caudal Pelvic Aperture):
  • Dorsal Boundary: 3rd to 4th caudal vertebrae.
  • Lateral Boundary: Sacrotuberous ligament and associated pelvic muscles.
  • Ventral Boundary: Ischiatic arch.
• Fetal Presentations in Canine Obstetrics:
  1. Anterior Presentation: Head first with head and forelegs extended. Ideal and most common presentation for natural whelping.
  2. Posterior Presentation: Tail and hind legs emerging first with legs extended caudally. Normal variation in the bitch.
  3. Breech Presentation: Rump/hips present first with hind limbs flexed forward along the fetal body. Classified as abnormal; requires close observation. If the bitch strains continuously without progress, immediate veterinary intervention is required.
  4. Two Front Legs Presentation: Head retained caudally or laterally while forelimbs present; requires manual or surgical veterinary assistance.

Digestive System: Oral Cavity & Associated Structures

Overview and Functions of Digestion

• Animal Classifications by Diet:
  • Herbivores: Consume plant matter exclusively.
  • Omnivores: Consume both plant and animal matter.
  • Carnivores: Consume animal tissue exclusively.
• Primary Function: Breakdown of ingested nutrients into molecular units for cellular absorption, energy production, tissue growth, and cellular repair.
• Types of Digestion:
  • Mechanical Digestion: Physical disruption and mixing of food items (e.g., mastication by teeth, churning by gastric smooth muscle layers).
  • Chemical Digestion: Enzymatic and acid cleavage of chemical bonds (e.g., gastric hydrochloric acid $HCl$ and pepsin breaking down proteins).

Segments of the Digestive System

• Digestive Tube (Alimentary Canal): Mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (cecum, colon, rectum), and anal canal.
• Digestive Glands: Major and minor salivary glands, liver, gallbladder, and pancreas.
• Digestive Accessory Organs: Lips, cheeks, tongue, teeth, and palate.

Oral Cavity (Cavum Oris)

• Boundaries:
  • Dorsally: Hard palate and soft palate.
  • Laterally: Cheeks (buccae) and dental arches.
  • Ventrally: Body of the mandible and mylohyoid muscles.
• Subdivisions:
  1. Vestibule: Space external to the teeth and alveolar arches, bounded by lips and cheeks.
  2. Oral Cavity Proper: Space internal to the dental arches, housing the tongue.
• Diastema: Toothless space separating the canine teeth from the premolars. Narrow in dogs compared to large gap seen in herbivores (ruminants, horses).
• Bony Anatomy of the Mouth:
  • Incisive bones and maxillae form the upper jaw.
  • Palatine bones form the roof of the mouth (hard palate).
  • Paired mandibles join at the midline mandibular symphysis to form the lower jaw and articulate with the temporal bones of the skull at the temporomandibular joint (TMJ).
• Functions of the Oral Cavity:
  1. Prehension: Grasping food using lips and tongue.
  2. Mastication: Chewing food into smaller boluses using tongue, cheeks, and teeth.
  3. Lubrication: Coating food with mucus and saliva to ease deglutition.
  4. Enzymatic Digestion: Initiates carbohydrate breakdown via salivary amylase in omnivores and herbivores. Absent in carnivores (dogs and cats) due to minimal oral residence time and absence of significant salivary amylase activity.

Lips (Labia Oris) and Cheeks (Buccae)

• Lips: Form the rostral and lateral boundaries of the vestibule. Upper lip (labium superius) and lower lip (labium inferius) meet at the angles of the mouth (angulus oris) to bound the oral fissure (rima oris).
  • Upper lips in dogs are termed flews; pendulous in heavy-muzzled breeds (Bulldog, Bloodhound, Hounds).
  • Philtrum: Central vertical groove along the hairless rostral muzzle of the upper lip.
  • Loose lip architecture creates a large vestibule, facilitating administration of liquid medications.
• Cheeks: Form the caudal portion of the lateral vestibular wall. Relatively small in dogs due to the wide oral fissure.

Gums (Gingiva)

• Firm, pink mucous membrane covering the alveolar processes of the maxilla and mandible, tightly investing the necks of teeth.
• Functions: Provides mechanical protection and structural support for teeth; creates a bacterial seal guarding underlying periodontal ligaments and alveolar bone.
• Anatomical Subdivisions:
  • Free (Marginal) Gingiva: Unattached collar surrounding the tooth neck, defining the gingival sulcus.
  • Attached Gingiva: Immobile tissue firmly bound to underlying alveolar bone to withstand masticatory forces.
  • Interdental Gingiva (Papilla): Tissue filling the interproximal space between adjacent teeth.
• Clinical Diagnostic Value of Gum Color:
  • Pink / Bubblegum Pink: Healthy tissue perfusion.
  • Pale / White: Indicates anemia, blood loss, severe shock, or systemic clotting disorders.
  • Yellow (Icteric): Indicates hemolysis, hyperbilirubinemia, or hepatic failure.
  • Blue / Purple / Grey (Cyanotic): Indicates hypoxia, severe hypothermia, respiratory distress, or cardiac failure.
  • Cherry Red / Bright Red: Indicates toxin exposure (e.g., carbon monoxide), systemic heatstroke, high blood pressure, or localized stomatitis/infection.
  • Brown / Black Spots: Normal melanin pigmentation in certain breeds (e.g., Chow Chow); abnormal new spots can indicate viral lesions or oral neoplasia (melanoma).
• Common Pathology: Plaque accumulation causes gingivitis (gastrointestinal/oral inflammation), which can progress to periodontitis (destruction of periodontal ligament and alveolar bone loss).

Palate

• Partition separating the respiratory tract (nasal cavities and nasopharynx) from the digestive tract (oral cavity and oropharynx).
• Hard Palate (Palatum Durum):
  • Rostral, bony portion formed by the palatine processes of the incisive, maxillary, and palatine bones.
  • Covered by specialized, heavily pigmented, and glandular mucosa.
  • Features 6 to 10 transverse ridges called palatine rugae (rugae = folds/wrinkles), separated by grooves. Widest at the level of the 4th upper cheek tooth (upper carnassial).
  • Divided symmetrically along the midline by the median palatine raphe (forms a ridge in dogs, but a groove in horses and ruminants).
  • Incisive Papilla: Small median eminence located immediately caudal to the upper central incisors. Flanked by orifices of the incisive ducts that communicate directly with the vomeronasal organ (Jacobson's organ), an olfactory receptor dedicated to detect pheromones and sexual chemical stimuli.
• Soft Palate (Palatum Molle / Velum Palatinum):
  • Caudal, flexible, muscular extension of the hard palate.
  • Elevates during deglutition to seal the nasopharynx, preventing entry of food/water into the nasal cavity.
• Brachycephalic Airway Syndrome (Elongated Soft Palate):
  • Congenital deformity prevalent in short-muzzled breeds (Pug, English Bulldog, Pekingese, Shar-Pei).
  • The soft palate extends excessively caudally, overlapping the epiglottis and obstructing the laryngeal opening.
  • Clinical Signs: Snorting, stertorous breathing, snoring, exercise intolerance, impaired thermoregulation (panting inefficiency), extreme vulnerability to heatstroke. Surgical excision (vulvoplasty/resection, often via laser) is indicated.

Tongue (Lingua)

• Muscular organ composed of interwoven intrinsic and extrinsic muscle bundles, situated on the floor of the oral cavity. Highly mobile, anchored to the hyoid apparatus and mandible.
• Functions: Prehension, taste sensation (gustation), bolus formation, fur grooming, thermoregulatory cooling via evaporative panting, and vocalization.
• Anatomical Subdivisions:
  • Root (Radix): Caudal one-third of the tongue, attached to the hyoid bone.
  • Body (Corpus): Elongated, middle portion featuring a dorsal median sulcus (groove).
  • Apex: Free, spatula-shaped rostral extremity.
• Lingual Frenulum: Ventral median fold of mucous membrane anchoring the tongue to the floor of the oral cavity.
• Lyssa: Unique anatomical structure located along the ventral median surface of the apex of carnivore tongues.
  • A rod-shaped, fusiform fibrous spicule extending from the apex to the level of the genioglossus muscle's vertical fibers.
  • Composed of a dense connective tissue capsule surrounding adipose tissue (prominent ventrally), striated muscle (middle/dorsal), cartilage, and receives motor innervation from the hypoglossal nerve (CN XII).

Lingual Papillae

1. Mechanical Papillae (Cornified epithelium, lack taste buds):
   • Filiform Papillae: Smallest and most abundant; cover the rostral two-thirds of the dorsal surface. Function to grasp food and aid in grooming.
     • Feline Adaptation: Cats possess heavily keratinized, backward-pointing filiform spines that act as micro-hooks to scrape meat from bones and groom coat hair effectively.
   • Conical Papillae: Larger than filiform, located on the root (caudal one-third). Direct food caudally toward the pharynx.
   • Marginal Papillae: Present along the lateral margins of the apex/body in newborn puppies to facilitate suckling; regress upon conversion to solid food.
2. Gustatory Papillae (Contain functional taste buds):
   • Fungiform Papillae: Mushroom-shaped projections scattered among filiform papillae on the apex and body.
   • Vallate (Circumvallate) Papillae: Largest and least numerous (typically 4 to 6 arranged in a V-shaped line rostral to the root). Each is encircled by a deep circular trench flanked by serous gustatory glands.
   • Foliate Papillae: Leaf-like ridges located on the lateral caudal margins of the tongue, containing serous glands.

Tongue Musculature & Innervation

• Intrinsic Muscle (Propria Linguae): Muscular fibers arranged in four ill-defined groups (longitudinalis dorsalis, longitudinalis ventralis, transversus linguae, verticalis linguae). Functions to alter tongue shape (bending, curling, grooving) and prevent accidental biting of the tongue.
• Extrinsic Muscles (Paired muscles moving the tongue relative to bone):
  1. Genioglossus: Arises from the mandibular symphysis; depresses the tongue (caudal fibers protract/protrude, rostral fibers retract the apex). Located medially.
  2. Hyoglossus: Arises from thyrohyoid and basihyoid bones; retracts and depresses the tongue. Located lateral to genioglossus.
  3. Styloglossus: Arises from the stylohyoid bone via 3 heads (short, rostral, long); retracts and elevates the tongue. Located laterally.
• Innervation:
  • Motor: Hypoglossal Nerve (CN XII).
  • Sensory & Gustatory: Trigeminal (CN V), Facial (CN VII), Glossopharyngeal (CN IX), and Vagus (CN X) nerves.

Salivary Glands

• Exocrine glands secreting saliva (composed of water, mucus, inorganic salts, and enzymes in non-carnivores) regulated by the autonomic nervous system (sympathetic and parasympathetic).
• Functions of Saliva: Moistens and lubricates food boluses to facilitate deglutition, moistens oral mucosa, aids thermoregulation via panting, cleanses oral tissues. Note: Feline and canine saliva lacks functional amylase.
• Major Salivary Glands:
  1. Parotid Gland: Located ventral to the ear (auricular base). Its duct runs across the masseter to open into the upper buccal vestibule opposite the upper 4th premolar.
  2. Zygomatic Gland: Specific to carnivores. Located medial to the zygomatic arch in the orbit; duct opens into the upper buccal vestibule opposite the last upper molar.
  3. Mandibular Gland: Located caudal to the angle of the mandible; duct runs rostrally to open on the sublingual caruncle (a papilla lateral to the frenulum).
  4. Sublingual Gland: Situated beneath the tongue mucosa.
  • Monostomatic Sublingual Gland: Single major duct opens alongside the mandibular duct on the sublingual caruncle.
  • Polystomatic Sublingual Gland: Multiple minor ducts opening directly into the sublingual recess of the oral cavity proper.
• Minor Salivary Glands: Buccal glands (prominent caudal to the last lower molar in cats), labial, molar, lingual, and palatine glands.

Digestive System: Pharynx & Dentition

Pharynx (Throat)

• Musculo-membranous passage common to both the digestive and respiratory systems.
• Subdivisions:
  1. Nasopharynx: Respiratory portion located dorsal to the soft palate, extending from the choanae to the palatopharyngeal arch. Contains the pharyngeal openings of the auditory (Eustachian) tubes (ostium pharyngeum tubae auditivae).
  2. Oropharynx: Digestive portion located ventral to the soft palate, extending from the isthmus faucium (bounded by the palatoglossal arches) to the base of the epiglottis.
  3. Laryngopharynx: Shared segment caudal to the soft palate and dorsal to the larynx, extending from the epiglottis to the esophageal entrance. The caudal boundary is marked by the pharyngoesophageal limen.
• Five Openings of the Pharyngeal Cavity:
  1. Choanae: Rostrodorsal paired openings connecting the nasal cavities to the nasopharynx.
  2. Pharyngeal Openings of the Auditory Tubes: Dorsolateral openings connecting the nasopharynx to the middle ear cavities.
  3. Isthmus Faucium: Rostral orifice connecting the oral cavity proper to the oropharynx.
  4. Aditus Laryngis: Caudoventral opening into the larynx (closed by the epiglottis during deglutition).
  5. Aditus Esophagi: Caudal opening leading into the esophagus.

Deglutition (Swallowing)

• Stage 1: Voluntary Initiation: Tongue acts as a plunger to force the masticated bolus into the oropharynx.
• Stage 2: Involuntary Reflex Completion:
  • Bolus contacts pharyngeal mucosa, triggering reflex contractions.
  • Palatopharyngeal arch shortens, closing the opening to the nasopharynx.
  • Larynx is pulled cranially, causing the epiglottis to tilt caudally over the aditus laryngis to seal the respiratory tract.
  • Pharyngeal constrictor muscles contract in sequence, pushing the bolus into the relaxed upper esophageal sphincter.

Pharyngeal Musculature

• Rostral Pharyngeal Constrictors:
  • Pterygopharyngeus: Arises from the hamulus of the pterygoid bone to insert on the dorsal pharyngeal raphe. Constricts and shortens the rostral pharynx.
  • Palatopharyngeus: Arises from the soft palate to insert on the pharyngeal raphe. Constricts, shortens, and elevates the rostral pharynx.
• Middle Pharyngeal Constrictor:
  • Hyopharyngeus: Subdivided into Chondropharyngeus (from thyrohyoid bone) and Ceratopharyngeus (from ceratohyoid bone). Inserts on the dorsal pharyngeal raphe to constrict the mid-pharynx.
• Caudal Pharyngeal Constrictors:
  • Thyropharyngeus: Arises from the thyroid cartilage to insert on the dorsal pharyngeal raphe. Constricts the caudal pharynx.
  • Cricopharyngeus: Arises from the cricoid cartilage to insert on the pharyngeal raphe; caudal fibers blend with the esophageal origin to form the upper esophageal sphincter. Constricts the caudal pharynx.
• Pharyngeal Dilator:
  • Stylopharyngeus: Arises from the stylohyoid bone and inserts on the dorsolateral wall of the pharynx. Dilates, elevates, and draws the pharynx forward.

Canine Dentition

• Dentition Type: Brachydont (short-crowned teeth with fully developed roots that cease growing once erupted) and Diphyodont (possessing two sequential sets of teeth: deciduous and permanent).
• Dental Arcades: Upper dental arcade (implanted in incisive and maxillary bones) and Lower dental arcade (implanted in the mandible).
• Gross Tooth Regions:
  • Crown: Visible portion projecting above the gingival margin.
  • Neck (Cervix): Constricted junction between crown and root at the cementoenamel junction.
  • Root: Segment anchored within the bony alveolus below the gingival margin.
• Microscopic Layers:
  • Enamel: Pearly-white outer covering of the crown. Extremely hard (96% inorganic hydroxyapatite crystals). Incapable of regeneration.
  • Dentine (Dentinum): Bulk of the tooth surrounding the central cavity. Composition similar to bone; composed of mineralized tubules. Capable of secondary regeneration.
  • Cementum: Thin calcified tissue covering the dentine of the root; anchors the fibers of the periodontal ligament to alveolar bone.
  • Pulp Cavity: Central chamber containing soft, vascular, and nerve-rich gelatinous tissue. Termed pulp chamber in the crown and root canal in the root, opening at the apical foramen.

Tooth Morphological Types

1. Incisors (I): Rostral teeth used for nibbling, scraping, and cutting. Feature a V-shaped ridge on the lingual surface called the cingulum.
2. Canines (C): Long, pointed teeth used for grasping, holding, and tearing prey.
3. Premolars (PM): Cheek teeth with shearing edges used for crushing and shredding food.
4. Molars (M): Flat, caudal cheek teeth found only in permanent dentition; used for grinding food.
5. Carnassial (Sectorial / Shearing) Teeth: Largest specialized shearing teeth. Comprises the 4th upper premolar ($PM4$) and the 1st lower molar ($M1$).

Root Structure of Permanent Canine Teeth

• Upper Jaw (Maxilla):
  • Incisors ($I1, I2, I3$): 1 root each.
  • Canine ($C$): 1 long root.
  • 1st Premolar ($P1$): 1 root.
  • 2nd & 3rd Premolars ($P2, P3$): 2 roots each.
  • 4th Premolar ($P4$ - Carnassial): 3 roots (2 buccal, 1 palatal).
  • 1st & 2nd Molars ($M1, M2$): 3 roots each.
  • 3rd Molar ($M3$): Absent.
• Lower Jaw (Mandible):
  • Incisors ($I1, I2, I3$): 1 root each.
  • Canine ($C$): 1 long root.
  • 1st Premolar ($P1$): 1 root.
  • 2nd, 3rd, 4th Premolars ($P2, P3, P4$): 2 roots each.
  • 1st Molar ($M1$ - Carnassial): 2 roots.
  • 2nd & 3rd Molars ($M2, M3$): 2 roots each.

Dental Formulas

• Expressed for one side of the upper and lower arcades:
  • Deciduous Dental Formula (28 total teeth):     2 \times \n    \begin{pmatrix}\n    \text{I} \frac{3}{3}, & \text{C} \frac{1}{1}, & \text{PM} \frac{3}{3}\n    \end{pmatrix}\n    = 28     (Note: No deciduous 1st premolars or molars exist. Deciduous $PM4$ mimics adult $M1$ crown shape; deciduous $PM3$ mimics adult $PM4$ shape).
  • Permanent Dental Formula (42 total teeth):     2 \times \n    \begin{pmatrix}\n    \text{I} \frac{3}{3}, & \text{C} \frac{1}{1}, & \text{PM} \frac{4}{4}, & \text{M} \frac{2}{3}\n    \end{pmatrix}\n    = 42

Eruption Times in Dogs

• Deciduous Teeth:
  • Incisors: 3 to 4 weeks (eruption begins around day 20, completed by day 35).
  • Canines: 3 weeks.
  • Premolars: 4 to 12 weeks.
• Permanent Teeth:
  • Central & Intermediate Incisors: 2 to 5 months.
  • Corner Incisors: 4 to 5 months.
  • Canines: 4 to 6 months (5 to 6 months in some records).
  • Premolar 1: 4 to 5 months.
  • Premolars 2, 3, 4: 4 to 6 months (P4: 4-5 months; P2/P3: 6 months).
  • Molar 1: 5 to 6 months.
  • Molars 2 & 3: 6 to 7 months.

Modified Triadan Numbering System

• Standardized 3-digit system for clinical identification:
  • Quadrants (Permanent): 100 = Right Upper; 200 = Left Upper; 300 = Left Lower; 400 = Right Lower.
  • Quadrants (Deciduous): 500 = Right Upper; 600 = Left Upper; 700 = Left Lower; 800 = Right Lower.
  • Individual Teeth: Numbered mesial to distal from 01 to 10/11.
  • Central Incisor = 01; Canine = 04; 4th Premolar = 08; 1st Molar = 09.
  • Example: Permanent left upper canine = Tooth 204; Deciduous left upper canine = Tooth 604.

Directional and Surface Terminology

• Vestibular (Labial / Buccal): Surface facing lips or cheeks.
• Lingual: Surface facing the tongue (mandibular arcade).
• Palatal: Surface facing the hard palate (maxillary arcade).
• Contact Surfaces:
  • Mesial: Surface facing toward the rostral midline of the dental arch.
  • Distal: Surface facing caudally away from the midline.
• Occlusal: Masticatory contact surface with opposing teeth.
• Coronal: Direction toward the crown tip.
• Apical: Direction toward the root tip.

Digestive System: Alimentary Canal & Gastrointestinal Tract

Esophagus (Gullet)

• Muscular transport tube (~30 cm long and 2 cm diameter collapsed in a medium dog) connecting the laryngopharynx to the stomach.
• Anatomical Course:
  1. Begins dorsal to the cricoid cartilage of the larynx.
  2. Shifts to the left side of the trachea as it traverses the cervical region.
  3. Re-enters the dorsal position at the tracheal bifurcation within the thorax.
  4. Passes through the esophageal hiatus of the diaphragm (at the level of $T10$).
  5. Terminates at the cardia of the stomach.
  • Clinical Note: Esophageal feeding tube placement requires right lateral recumbency so the exposed left side of the neck provides direct access to the esophagus.
• Histological Differentiation by Species:
  • Canine Esophagus: Tunica muscularis consists of striated muscle throughout its entire length.
  • Feline Esophagus: Cranial two-thirds is striated muscle; distal (caudal) one-third transitions to smooth muscle.
• Esophageal Sphincters:
  • Upper Esophageal Sphincter (UES): Formed by the cricopharyngeus muscle; opens during deglutition and prevents aerophagia.
  • Lower Esophageal Sphincter (LES / Cardiac Sphincter): Smooth muscle tone at the gastric junction preventing gastroesophageal acid reflux.
• Concentric Layers (Coats) of Esophagus Wall:
  1. Tunica Adventitia (Fibrous Coat): Dense connective tissue anchoring the organ in the neck and thorax.
  2. Tunica Muscularis: Outer longitudinal and inner circular muscle layers (striated in dogs), anchored cranially by the cricoesophageal tendon.
  3. Tela Submucosa: Loose connective tissue containing blood vessels, nerves, lymphatics, and tubuloalveolar esophageal glands.
  4. Tunica Mucosa: Innermost lining featuring stratified squamous epithelium, lamina propria, and muscularis mucosae, formed into deep longitudinal folds (limen pharyngoesophageum at origin) allowing luminal expansion.

Stomach (Ventriculus / Gaster)

• Simple, J-shaped/V-shaped, musculoglandular expansion of the digestive tract situated in the cranial abdominal cavity immediately behind the diaphragm and liver.
• Functions: Storage of ingested food, mechanical churning/mixing, chemical and enzymatic digestion via gastric juice (acidic mixture containing $HCl$, pepsinogen/pepsin, and gastrin).
• Capacity: Empty stomach weighs ~100 g in a 15-kg dog. Highly distensible; capacity ranges from 100 to 250 cc per kg of body weight when full.
• Peritoneal Attachments:
  • Lesser Omentum: Peritoneal fold connecting the lesser curvature of the stomach and initial duodenum to the porta of the liver.
  • Greater Omentum: Extensively folded apron-like sheet connecting the greater curvature of the stomach to the dorsal abdominal wall, covering the intestinal mass.
• Anatomical Subdivisions:
  1. Cardia (Pars Cardiaca): Small region surrounding the esophageal inlet.
  2. Fundus (Fundus Ventriculi): Large, blind, dome-shaped outpocketing lying dorsal and to the left of the cardia.
  3. Body (Corpus Ventriculi): Large central region extending from fundus to the pyloric part; primary site of gastric churning.
  4. Pyloric Part (Pars Pylorica): Distal third of the stomach. Begins at the angular incisure (incisura angularis, a 50–70 degree angle along the lesser curvature). Subdivided into thin-walled pyloric antrum, narrowing into the pyloric canal, ending at the pylorus (thick circular muscular sphincter regulating chyme outflow into the duodenum).
• Surfaces & Curvatures:
  • Parietal Surface: Faces cranially/ventrally against the diaphragm and liver (creates gastric impression on liver).
  • Visceral Surface: Faces caudodextrally against the intestine, pancreas, and left kidney.
  • Greater Curvature: Long convex border (~30 cm) giving origin to the greater omentum.
  • Lesser Curvature: Short concave border giving origin to the lesser omentum; accommodates the papillary process of the liver.
• Gastric Rugae: Mucosal folds lined with simple columnar epithelium and gastric pits/glands, allowing massive stomach expansion upon ingestion.
• Coats of Stomach Wall (Outside to Inside):
  1. Tunica Serosa: Visceral peritoneum with subserous connective tissue.
  2. Tunica Muscularis: Smooth muscle organized into three layers: outer longitudinal, middle circular (forms sphincters), and inner oblique (unique layer providing churning strength).
  3. Tela Submucosa: Connective tissue support layer.
  4. Tunica Mucosa: Mucous membrane containing deep gastric glands.
• Gastric Dilatation-Volvulus (GDV): Life-threatening emergency in deep-chested dogs. Stomach fills with gas/fluid (dilatation) and rotates along its long axis (volvulus), occluding esophageal and duodenal ends, compressing the caudal vena cava, and leading to obstructive shock.

Small Intestine (Intestinum Tenue)

• Longest portion of the alimentary canal (average length in life is 3.5 times body length). Extends from pylorus to ileocolic orifice. Suspended from the dorsal abdominal wall by the mesentery (supplied by the cranial mesenteric artery).
• Subdivisions:
  1. Duodenum: Shortest, most fixed proximal segment (~25 cm / 10 inches long). Bound to the right dorsal wall by the mesoduodenum.
  • Four Portions: Cranial portion, Descending portion (contacts right parietal peritoneum and right pancreatic lobe), Transverse (caudal) portion, Ascending portion.
  • Two Flexures: Cranial duodenal flexure and Caudal duodenal flexure (ends at duodenojejunal flexure).
  1. Jejunum: Longest, highly mobile middle segment forming extensive sagittal and transverse coils occupying the ventrocaudal abdominal cavity from liver/stomach to urinary bladder. Suspended by mesojejunum.
  2. Ileum: Short (~10 cm), thick, highly muscular terminal segment. Marked proximally by the extent of the ileocecal fold. Courses cranially on the right side of the mesenteric root to terminate at the ileocolic orifice into the ascending colon.

Large Intestine (Intestinum Crassum)

• Simple, short, non-sacculated tube lacking haustra, tenia, or complex flexures. Function: water reabsorption (dehydration of fecal material) and electrolyte storage.
• Subdivisions:
  1. Cecum: Blind-ending, S-shaped/spiral pouch situated on the right side of the abdomen ventral to the caudal right kidney. In dogs, the cecum is a diverticulum of the ascending colon and communicates via the cecocolic orifice (guarded by the cecocolic sphincter). It does not directly receive the ileum.
  2. Colon: Suspended by the mesocolon; shaped like a question mark (?):
  • Ascending Colon: Short segment on the right side of the mesenteric root.
  • Right Colic Flexure: Bend between ascending and transverse colons.
  • Transverse Colon: Crosses the abdominal cavity cranial to the root of the mesentery.
  • Left Colic Flexure: Bend between transverse and descending colons.
  • Descending Colon: Longest segment, running caudally along the left abdominal wall to terminate at the pelvic inlet (~25 cm long, 2 cm diameter).
  1. Rectum: Straight pelvic segment (~5 cm long, 3 cm diameter) extending from pelvic inlet to anal canal. Suspended dorsally from the sacrum by the mesorectum (terminates at 2nd caudal vertebra).
  2. Anal Canal (Canalis Analis): Terminal 1-cm segment lined by cutaneous stratified squamous epithelium.

Anal Musculature, Sacs, and Defecation

• Internal Anal Sphincter: Thickened caudal coat of smooth muscle (involuntary control; parasympathetic innervation via pelvic nerve).
• External Anal Sphincter: Outer ring of striated muscle (voluntary control; motor innervation via pudendal/caudal rectal nerves).
• Anal Sacs (Sinus Paranalis): Paired, hazelnut-sized cutaneous invaginations positioned ventrolateral to the anus between the internal and external anal sphincters (at the 4 o'clock and 8 o'clock positions).
  • Lined with glandular epithelium containing sebaceous and apocrine sweat glands secreting a malodorous fluid.
  • Expressed mechanically during defecation to act as a territorial scent marker.
• Defecation Reflex Sequence:
  1. Fecal accumulation distends the rectum, activating sacral stretch receptors.
  2. Afferent impulses transmit to sacral spinal cord segments ($S1 - S3$).
  3. Parasympathetic signals via pelvic nerve cause contraction of rectal smooth muscle and involuntary relaxation of internal anal sphincter.
  4. Voluntary somatic relaxation of external anal sphincter via pudendal nerve accompanied by increased abdominal wall pressure expels feces.

Digestive System: Accessory Organs

Liver (Hepar)

• Largest gland in the body, performing dual exocrine (bile production) and endocrine/metabolic functions (carbohydrate, lipid, protein metabolism, plasma protein synthesis, detoxification).
• Physical Properties: Deep reddish-brown, friable tissue. Located in the cranial abdominal cavity immediately caudal to the diaphragm (primarily right of the median plane).
• Surfaces:
  • Diaphragmatic (Parietal) Surface: Convex; conforms to the concavity of the diaphragm (extends from 6th to 12th intercostal space on the right, 10th intercostal space on the left).
  • Visceral Surface: Concave; faces caudoventrally, featuring gastric, duodenal, pancreatic, and deep renal impressions.
• Porta Hepatis: Central hilus on the visceral surface where the portal vein, hepatic artery, and autonomic nerves enter, and hepatic bile ducts exit.

Lobes and Processes of the Canine Liver

1. Left Hepatic Lobe (Lobus Hepatis Sinister): Forms 1/3 to 1/2 of total liver mass. Deeply divided into:
   • Left Lateral Hepatic Lobe.
   • Left Medial Hepatic Lobe.
2. Right Hepatic Lobe (Lobus Hepatis Dexter): Divided into:
   • Right Medial Hepatic Lobe: Contains a deep fossa housing the gallbladder.
   • Right Lateral Hepatic Lobe: Smaller lobe adjacent to the caudate lobe.
3. Caudate Lobe (Lobus Caudatus): Divided into two distinct processes:
   • Papillary Process (Processus Papillaris): Tongue-shaped process projecting into the lesser curvature of the stomach (visible through the lesser omentum).
   • Caudate Process (Processus Caudatus): Most caudal extension (reaches 12th intercostal space/last rib on the right); caps the cranial pole of the right kidney, forming the deep renal impression.
4. Quadrate Lobe (Lobus Quadratus): Undivided median wedge bounded laterally by the gallbladder fossa and left medial lobe.

Peritoneal Attachments of the Liver

1. Coronary Ligament: Reflection of peritoneum around the bare area (area nuda) connecting the liver to the diaphragm surrounding the caudal vena cava.
2. Right Triangular Ligament: Peritoneal fold extending from the diaphragm to the dorsal right lateral lobe.
3. Left Triangular Ligament: Fold extending from the diaphragm to the left lateral lobe (may contain atrophic liver tissue called appendix fibrosa hepatis).
4. Falciform Ligament: Remnant of the embryonic ventral mesentery running between quadrate and left medial lobes to the diaphragm and abdominal wall down to the umbilicus. Contains the round ligament of the liver (ligamentum teres hepatis, remnant of umbilical vein).
5. Hepatorenal Ligament: Peritoneal fold connecting the caudate process to the right kidney.

Gallbladder (Vesica Fellea) & Bile Ducts

• Storage organ for hepatic bile, located in a fossa between the quadrate and right medial liver lobes. Reaches the diaphragmatic wall when distended.
• Functions: Concentrates and stores bile secreted continuously by hepatocytes, absorbing water and electrolytes, and secreting mucin.
• Biliary Duct System: Microscopic bile canaliculi combine into hepatic ducts. These join the cystic duct coming from the gallbladder to form the common bile duct (ductus choledochus).
• Common bile duct traverses the mesoduodenum to open into the descending duodenum alongside the major pancreatic duct at the major duodenal papilla.

Pancreas

• Elongated, coarsely lobulated, V-shaped gland (pinkish-gray in life, yellowish-gray preserved) located caudal to the liver in the dorsal abdomen.
• Dual Function:
  • Exocrine: Acinar tissue secretes alkaline pancreatic juice rich in digestive enzymes (trypsinogen/pepsin for proteins, pancreatic lipase for fats, amylase for carbohydrates).
  • Endocrine: Pancreatic islets (Islets of Langerhans) secrete hormones directly into blood (insulin to lower blood glucose, glucagon to raise it; absence or dysfunction causes diabetes mellitus).
• Anatomical Subdivisions:
  • Body (Pancreatic Angle): Central junction located caudomedial to the pylorus.
  • Right Lobe: Slender, long lobe running dorsomedial to the descending duodenum within the mesoduodenum (ventral to right kidney).
  • Left Lobe: Shorter, broader lobe extending leftward between peritoneal leaves of the greater omentum.
• Pancreatic Ducts (Canine Pattern):
  1. Pancreatic Duct (Ventral Duct): Smaller duct (sometimes absent); opens at the major duodenal papilla alongside the common bile duct.
  2. Accessory Pancreatic Duct (Dorsal Duct / Duct of Santorini): Main functional conduit in dogs; always present and larger; opens into the descending duodenum at the minor duodenal papilla (located ~2-3 cm caudal to the major papilla).

Summary of Gastrointestinal Pathology & Clinical Signs

• Lips/Oral Cavity: Stomatitis, ulcers, trauma $\rightarrow$ Ptvalism (drooling), halitosis, dysphagia $\rightarrow$ Antiseptics, analgesics, foreign body removal.
• Teeth: Periodontal disease, fractures $\rightarrow$ Halitosis, gingivitis, pain $\rightarrow$ Dental scaling, extraction, antibiotics.
• Tongue: Glossitis, neoplasia $\rightarrow$ Macroglossia, anorexia, ptyalism $\rightarrow$ Anti-inflammatories, surgery.
• Salivary Glands: Sialadenitis, Salivary Mucocele (Sialocele) $\rightarrow$ Fluctuant ventral cervical swelling, dysphagia $\rightarrow$ Surgical sialoadenectomy, drainage.
• Pharynx: Pharyngitis, obstruction $\rightarrow$ Gagging, coughing, dysphagia $\rightarrow$ Removal of obstruction, anti-inflammatories.
• Esophagus: Megaesophagus, esophagitis, obstruction $\rightarrow$ Regurgitation, weight loss, aspiration pneumonia $\rightarrow$ Upright feeding (Bailey chair), prokinetics.
• Stomach: Gastritis, Ulcers, GDV $\rightarrow$ Vomiting, hematemesis, abdominal pain, shock $\rightarrow$ Fluids, gastroprotectants, emergency gastropexy for GDV.
• Small Intestine: Enteritis, Intussusception, Parasites $\rightarrow$ Diarrhea, vomiting, dehydration, melena $\rightarrow$ Anthelmintics, fluids, surgery.
• Large Intestine: Colitis, Constipation, Prolapse $\rightarrow$ Tenesmus, hematochezia, mucus in stool $\rightarrow$ Dietary fiber, laxatives, prolapse repair.
• Liver: Hepatitis, Hepatic Lipidosis, Cirrhosis $\rightarrow$ Jaundice, ascites, lethargy, encephalopathy $\rightarrow$ Hepatoprotectants, nutritional support.
• Gallbladder: Cholecystitis, Mucoceles, Calculi $\rightarrow$ Jaundice, cranial abdominal pain $\rightarrow$ Cholecystectomy, antibiotics.
• Pancreas: Pancreatitis, EPI $\rightarrow$ Severe abdominal pain, steatorrhea, weight loss $\rightarrow$ Aggressive IV fluids, analgesia, enzyme replacement (EPI).
• Peritoneum: Peritonitis $\rightarrow$ Rigid abdomen, fever, septic shock $\rightarrow$ Emergency lavage, abdominal drainage, broad-spectrum antibiotics.

Respiratory System

Physiology and Functions of Respiration

• Definition: Respiration is the biochemical and physiological process of moving oxygen ($O_2$) from the atmosphere into living cells for metabolic phosphorylation, and removing waste carbon dioxide ($CO_2$).
• Difference Between Breathing and Respiration:
  • Breathing (Ventilation): Mechanical expansion and contraction of the thoracic cavity changing pulmonary gas volumes.
  • Respiration: Cellular exchange of gases across biological membranes.
• Major & Secondary Functions:
  1. Gas exchange ($O_2$ uptake and $CO_2$ clearance).
  2. Maintenance of acid-base balance (pH regulation of body fluids via $CO_2$ retention/elimination).
  3. Thermoregulation (heat dissipation via evaporative panting).
  4. Elimination of water vapor.
  5. Vocalization (phonation).
• Two Stages of Respiration:
  • External Respiration: Gas exchange occurring in the lungs between alveolar air sacs and blood flowing through pulmonary capillaries.
  • Internal Respiration: Systemic gas exchange occurring in capillaries throughout body tissues between systemic blood and metabolizing cells.

Normal Resting Respiratory Rates across Species

• Dog: 18 to 34 breaths per minute (Normal mean resting rate = 24 breaths/min; rates >40 breaths/min at rest indicate pathology).
• Cat: 16 to 40 breaths per minute.
• Horse: 10 to 14 breaths per minute.
• Dairy Cow: 26 to 50 breaths per minute.
• Sheep: 16 to 34 breaths per minute.
• Pig: 32 to 58 breaths per minute.

Upper Respiratory Tract (URT) Structures

• External Nose & Nasal Cavity (Cavum Nasi):
  • Bounded dorsally by nasal bones, laterally by maxillae, ventrally by incisive and palatine bones, and caudally by the cribriform plate of the ethmoid bone.
  • Planum Nasale: Hairless, non-glandular apical region of the muzzle surrounding the comma-shaped nostrils (nares) in dogs. Kept moist by secretions from lateral nasal glands.
  • Philtrum: Deep median vertical furrow dividing the nose and upper lip.
  • Warms, humidifies, and filters incoming air, providing olfactory input via sensory conchae.
• Pharynx: Shared passage (detailed under Digestive System).
• Larynx (Voice Box):
  • Short musculo-cartilaginous organ connecting the pharynx to the trachea, acting as a valve guarding the respiratory airway.
  • Three Primary Functions: Regulates airflow volume, prevents aspiration of foreign matter, acts as the organ of voice.
  • Nine Laryngeal Cartilages:
  • Unpaired Cartilages (3):
    1. Epiglottis: Leaf-shaped elastic cartilage; closes over the laryngeal inlet (aditus laryngis) during deglutition.
    2. Thyroid Cartilage: Largest shield-shaped hyaline cartilage; forms floor/sides of larynx; anchors vocal folds.
    3. Cricoid Cartilage: Complete signet ring-shaped hyaline cartilage; articulates with thyroid and arytenoid cartilages; connects larynx to trachea.
  • Paired Cartilages (3 pairs = 6):
    4. Arytenoid Cartilages: Paired pyramid-shaped cartilages controlling the vocal folds and glottic opening.
    5. Corniculate Cartilages: Nodules atop the arytenoids.
    6. Cuneiform Cartilages: Small rods within the aryepiglottic folds.

Lower Respiratory Tract (LRT) Structures

• Trachea (Windpipe):
  • Non-collapsible cartilaginous/membranous tube extending from the cricoid cartilage of the larynx down the cervical region (left of esophagus) into the thorax, terminating at the tracheal bifurcation (bifurcatio tracheae) over the heart base.
  • Tracheal Rings: Composed of hyaline cartilage, incomplete dorsally (C-shaped).
  • Canine Tracheal Rings: 40 to 45 incomplete rings closed dorsally by the smooth trachealis muscle located on the external/retromucosal aspect of the ring.
  • Comparative Tracheal Ring Characteristics:
  • Dog: 40–45 rings; C-shaped/incomplete dorsally; trachealis muscle situated retromucosally/externally.
  • Pig: 32–36 rings; free ends overlap.
  • Sheep: 48–60 rings; oval cross-section; ends overlap.
  • Goat: 48–60 rings; U-shaped; gap between ends.
  • Horse: 48–60 rings; flattened dorsally; ends overlap.
• Bronchial Tree & Alveoli:
  • Trachea splits into right and left primary bronchi, branching into secondary (lobar) and tertiary (segmental) bronchi, conducting bronchioles, respiratory bronchioles, alveolar ducts, alveolar sacs, and pulmonary alveoli (primary microscopic site of gas exchange).

Lungs (Pulmones)

• Paired, light, spongy respiratory organs housed within the pleural cavities of the thorax.
• Generalized Lobation Pattern:
  • Left Lung (2 Lobes):
  1. Left Cranial (Apical) Lobe: Subdivided into cranial and caudal parts.
  2. Left Caudal (Diaphragmatic) Lobe.
  • Right Lung (4 Lobes):
  1. Right Cranial Lobe.
  2. Middle (Cardiac) Lobe.
  3. Right Caudal Lobe (Largest lobe).
  4. Accessory (Azygos/Intermediate) Lobe: Fits in the mediastinal recess between heart and diaphragm.
• Comparative Characteristics: Normal vs. Fetal Lungs:
  • Normal Post-Natal Lung: Soft, light, pink, spongy, crepitates upon palpation (contains trapped air in alveoli); floats in water.
  • Fetal (Non-Breathed) Lung: Small, firm (liver-like consistency), pale gray/red, non-crepitant (alveoli unexpanded); sinks in water (used in forensic post-mortem testing to confirm live birth vs. stillbirth).

Circulatory System

Heart (Cor) Topography & Anatomy

• Location: Situated within the middle mediastinum of the thoracic cavity between the 3rd and 6th intercostal spaces (ICS). Overlapped by the triceps muscle anteriorly.
• Orientation: Base directed dorsocranially; Apex directed caudoventrally and shifted to the left (formed exclusively by the left ventricle).
• Heart Wall Layers:
  1. Pericardium: Fibroserous outer protective envelope.
  2. Epicardium: Visceral layer of serous pericardium attached to myocardium.
  3. Myocardium: Middle muscular layer (substantially thickest in the left ventricle to overcome systemic vascular resistance).
  4. Endocardium: Smooth inner endothelial lining.

Pericardium

• Fibroserous sac enclosing the heart and proximal root of great vessels.
• Structural Components:
  • Fibrous Pericardium: Tough, non-distensible outer fibrous sac. Anchored to surrounding chest structures by:
  • Sternopericardiac Ligament: Connects pericardium to the sternum.
  • Phrenopericardiac Ligament: Connects pericardium to the diaphragm.
  • Serous Pericardium: Closed serous membrane bag lining the inside of the fibrous sac (parietal layer) and reflecting onto the heart muscle (visceral layer / epicardium).
  • Pericardial Cavity: Potential space containing ~1 mL of clear serous pericardial fluid (liquor pericardii) that reduces friction during cardiac contraction.

Cardiac Chambers and Valves

• Chambers:
  • Right Atrium (RA): Receives deoxygenated systemic blood from cranial and caudal vena cavae, and cardiac blood from the coronary sinus.
  • Right Ventricle (RV): Pumps blood through the pulmonary trunk into the lungs.
  • Left Atrium (LA): Receives oxygenated blood from the pulmonary veins.
  • Left Ventricle (LV): Thickest chamber; pumps oxygenated blood into the systemic aorta.
• Valves:
  • Right Atrioventricular (AV) Valve (Tricuspid Valve): Prevents backflow into RA during ventricular systole. In dogs, features 2 primary major cusps (parietal and septal).
  • Left Atrioventricular (AV) Valve (Bicuspid / Mitral Valve): Prevents backflow into LA during ventricular systole; heavier construction due to high LV pressure.
  • Pulmonary Semilunar Valve: Three-cusped valve at junction of RV and pulmonary trunk; prevents diastolic backflow into RV.
  • Aortic Semilunar Valve: Three-cusped valve at junction of LV and ascending aorta; prevents diastolic backflow into LV.
• Supporting Structures: Valve cusps are anchored by fibrous cords called chordae tendineae ("heartstrings") attached to muscular projections called papillary muscles extending from the ventricular walls.

Clinical Cardiac Auscultation Landmarks (Anatomical Intercostal Spaces)

• Mitral Valve: Left side, 5th intercostal space at the costochondral junction.
• Aortic Valve: Left side, 4th intercostal space just above the costochondral junction.
• Pulmonic Valve: Left side, 2nd to 4th intercostal space near the sternum.
• Tricuspid Valve: Right side, 3rd to 5th intercostal space near the costochondral junction.
• Heart Sounds:
  • First Heart Sound ($S1$): Closure of AV valves (mitral and tricuspid) at onset of systole.
  • Second Heart Sound ($S2$): Closure of semilunar valves (aortic and pulmonic) at onset of diastole.

Cardiac Conduction System

• Specialized network of modified cardiac myocytes generating and distributing action potentials:
  1. Sinoatrial (SA) Node: Primary pacemaker located in the right atrial wall near the cranial vena cava opening. Intrinsic firing rate in dogs = 90 to 160 beats/min.
  2. Atrioventricular (AV) Node: Located in the interatrial septum near the coronary sinus opening. Delays electrical impulse (~0.1 sec) to allow complete atrial emptying before ventricular contraction.
  3. Bundle of His (AV Bundle): Conducts impulses through the fibrous heart skeleton.
  4. Purkinje Fiber Network: Subendocardial network in ventricles driving rapid apex-to-base ventricular depolarization. (Less extensive penetration in dogs compared to horses).

Coronary Circulation

• Left Coronary Artery: Main supply, branches into the paraconal interventricular branch.
• Right Coronary Artery: Supplies right myocardium, branches into the subsinuosal branch.
• Venous blood returns via cardiac veins into the coronary sinus opening directly into the Right Atrium.

Major Blood Vessels and Vascular Pathways

• Arteries vs. Veins vs. Capillaries:
  • Arteries: Deep muscular vessels, thick tunica media, no valves (except semilunar outflow), carry oxygenated blood away from heart (except pulmonary artery).
  • Veins: Superficial, thin walls, low pressure, contain internal valves, carry deoxygenated blood toward heart (except pulmonary veins).
  • Capillaries: Microscopic single-layer endothelial vessels in tissues for nutrient/gas diffusion.
• Blood Vessel Wall Layers (Tunics):
  1. Tunica Intima: Endothelium and internal elastic membrane.
  2. Tunica Media: Smooth muscle and elastic fibers (substantially thicker in arteries).
  3. Tunica Externa (Adventitia): Collagen and fibrous sheath.
• Systemic Circulation Pathway:   Left Ventricle $\rightarrow$ Aorta $\rightarrow$ Arteries $\rightarrow$ Arterioles $\rightarrow$ Systemic Capillaries $\rightarrow$ Venules $\rightarrow$ Veins $\rightarrow$ Vena Cavae $\rightarrow$ Right Atrium.
• Pulmonary Circulation Pathway:   Right Ventricle $\rightarrow$ Pulmonary Trunk/Arteries $\rightarrow$ Lung Capillaries $\rightarrow$ Pulmonary Veins $\rightarrow$ Left Atrium.
• Hepatic Portal Circulation:   Vascular bed draining blood from the GI tract via the hepatic portal vein directly into the liver parenchyma for detoxification and nutrient processing before reaching systemic circulation.
• Fetal Shunts:
  • Foramen Ovale: Shunts blood from Right Atrium to Left Atrium.
  • Ductus Arteriosus: Shunts pulmonary artery blood into the aorta (remnant = ligamentum arteriosum; failure to close = Patent Ductus Arteriosus / PDA).
  • Umbilical vein and arteries.

Clinical Injection Sites in Dogs

• Intravenous (IV) Sites: Cephalic vein (forelimb), Lateral Saphenous vein (hindlimb outer aspect), Medial Saphenous vein, Jugular vein (neck).
• Intramuscular (IM) Sites: Hamstring group (Biceps femoris, Semitendinosus, Semimembranosus), Epaxial muscles (along spine), Quadriceps, Triceps.
• Subcutaneous (SQ) Vaccination Sites:
  • Right Rear Leg: Rabies Vaccine.
  • Right Shoulder: DHPP (Distemper, Hepatitis, Parvovirus, Parainfluenza).
  • Left Rear Leg: Lyme / Rattlesnake Vaccine.
  • Left Shoulder: Injectable Bordetella / Influenza.

Genitourinary System: Urinary Organs

Overview and Functions of the Urinary System

• Comprises the paired kidneys, paired ureters, urinary bladder, and urethra.
• Primary Functions:
  1. Homeostasis: Maintenance of stable internal fluid and electrolyte environment.
  2. Osmoregulation: Regulation of water and salt balance.
  3. Excretion: Elimination of metabolic waste products (urea, creatinine).
  4. Hormone Secretion: Production of erythropoietin (stimulates RBC production in bone marrow) and renin (regulates blood pressure via RAAS pathway).
  5. Vitamin D Hydroxylation: Conversion of vitamin D to its active form.

Kidneys (Renes)

• Paired, bean-shaped, smooth, reddish-brown organs situated in the retroperitoneal space of the cranial abdomen (attached to lumbar muscles, covered ventrally by parietal peritoneum).
• Topographical Location:
  • Right Kidney: Located below the last ribs and bodies of $L1 - L3$ vertebrae (caps caudate process of liver; more cranial).
  • Left Kidney: Located below bodies of $L3 - L5$ vertebrae (more mobile).
• Dimensions & Weight (Medium Dog): 6 to 9 cm long, 4 to 5 cm wide, 3 to 4 cm thick; weight = 25 to 35 g.
• Gross Features: 2 Surfaces (convex dorsal/ventral), 2 Borders (convex lateral, concave medial with hilus where renal artery, renal vein, nerves, and ureter enter/exit), 2 Poles (cranial and caudal).
• Gross Internal Layers (Longitudinal Section):
  1. Fibrous Capsule: Outer dense connective tissue sheath (peels easily from healthy kidney).
  2. Cortex: Dark red outer layer containing renal corpuscles, proximal convoluted tubules (PCT), and distal convoluted tubules (DCT).
  3. Medulla: Paler inner layer containing renal pyramids, loops of Henle, and collecting ducts.
  4. Renal Pelvis: Whitish, funnel-shaped fibrous basin collecting urine from ducts and channeling it into the ureter.

Microscopic Anatomy of the Nephron

• Nephron: Functional microscopic unit (~1,000,000 per kidney in dogs).
• Components:
  1. Renal Corpuscle: Glomerulus (capillary tuft filtering blood plasma) surrounded by double-walled Bowman's Capsule.
  2. Proximal Convoluted Tubule (PCT): Lined with dense microvilli brush border; primary site of solute (glucose, amino acids, ions) and water reabsorption.
  3. Loop of Henle: Descending and ascending limbs extending into medulla; generates osmotic concentration gradient.
  4. Distal Convoluted Tubule (DCT): Lined with simple cuboidal cells; site of active secretion and hormonal ion regulation.
  5. Collecting Duct: Final tubular segment regulating water reabsorption under Antidiuretic Hormone (ADH) influence.

Renal Vascular Circulation Sequence

Abdominal Aorta $\rightarrow$ Renal Artery (receives ~20% of cardiac output) $\rightarrow$ Interlobar Arteries (between pyramids) $\rightarrow$ Arcuate Arteries (arch over pyramids) $\rightarrow$ Interlobular Arteries (pass into cortex) $\rightarrow$ Afferent Arterioles $\rightarrow$ Glomerulus (Filtration occurs) $\rightarrow$ Efferent Arterioles $\rightarrow$ Peritubular Capillaries / Vasa Recta $\rightarrow$ Interlobular Veins $\rightarrow$ Arcuate Veins $\rightarrow$ Interlobar Veins $\rightarrow$ Renal Vein $\rightarrow$ Caudal Vena Cava.

Ureters & Urinary Bladder

• Ureters: Muscular tubes (0.6 to 0.9 cm diameter distended, 12 to 16 cm length; right longer than left) suspended by mesoureter. Transport urine via smooth muscle peristalsis; lined by expandable transitional epithelium.
• Urinary Bladder (Vesica Urinaria):
  • Hollow, expandable musculomembranous sac located in the pelvic cavity when contracted, extending onto the abdominal floor when filled (capacity ~100–120 mL in a 25-lb dog).
  • Anatomical Regions: Apex/Fundus (blunt cranial extremity), Body (central part), Neck (constricted caudal end continuous with urethra).
  • Detrusor Muscle: Interlaced smooth muscle wall layer contracting under parasympathetic control to empty bladder.
  • Trigone of the Bladder: Smooth triangular mucosal region at the inner neck bounded by the two ureteral orifices and the internal urethral orifice.
  • Sphincters:
  • Internal Urethral Sphincter: Smooth muscle (involuntary autonomic control).
  • External Urethral Sphincter: Striated skeletal muscle ring (voluntary control).

Genitourinary System: Male Reproductive Tract

Scrotum & Testicular Thermoregulation

• Cutaneous sac located in the inguinal region of dogs (perineal region in cats) divided by a median septum into two compartments housing the testicles.
• Anatomical Comparison (Dog vs. Cat):
  • Dog: Inguinal location, spherical/oval shape, thin pigmented skin with fine hair, oblique testicular orientation, active cremaster muscle.
  • Cat: Perineal location (below anus), small rounded shape, thick hairy skin, horizontal testicular orientation, lacks active cremaster muscle (possesses levator scroti muscle originating from external anal sphincter).
• Scrotal Wall Layers (Outside to Inside):
  1. Skin.
  2. Tunica Dartos: Smooth muscle layer intermixed with elastic fibers beneath skin; contracts/wrinkles scrotum in cold to reduce surface area and pull testes closer to trunk.
  3. Cremaster Muscle: Striated muscle derived from internal abdominal oblique passing along external spermatic fascia; raises/lowers testes.
  4. Spermatic Fascia (External and Internal components).
  5. Vaginal Tunic (Parietal layer lining fascia; Visceral layer covering testicle).
• Pampiniform Plexus: Complex countercurrent vascular meshwork of convoluted testicular veins wrapping around the testicular artery within the spermatic cord, cooling warm arterial blood before it reaches the testicle to preserve spermatogenesis.

Testes (Testes / Gonads)

• Paired ellipsoidal male gonads (~3 cm long, 2 cm wide, weight = 8 g in a 25-lb dog) situated obliquely in the scrotum.
• Functions:
  1. Spermatogenesis: Production of haploid spermatozoa within seminiferous tubules.
  2. Hormone Secretion: Testosterone production by interstitial Leydig cells.
• Testicular Descent (Descensus Testis): Migration of testes from the lumbar region through the inguinal canal into the scrotum guided by the gubernaculum testis. Occurs shortly after birth in carnivores. Failure of one or both testes to descend is called cryptorchidism (heritable condition; affected animals should not be bred).
• Internal Microscopic Architecture:
  • Encapsulated by thick fibrous tunica albuginea.
  • Septa (Septula Testis) divide tissue into lobules, converging at the central mediastinum testis.
  • Path of Spermatozoa: Contorted Seminiferous Tubules (tubuli seminiferi contorti) $\rightarrow$ Straight Tubules (tubuli seminiferi recti) $\rightarrow$ Rete Testis $\rightarrow$ 8 to 12 Efferent Ductules (ductuli efferentes) $\rightarrow$ Head of Epididymis.

Epididymis & Spermatic Cord

• Epididymis: Convoluted tubule attached to the testis (length = 5 to 8 meters in dogs, 4 to 6 m in cats). Site of sperm maturation and storage.
  • Subdivisions: Head (Caput), Body (Corpus), Tail (Cauda - primary storage site).
  • Ligaments: Proper ligament of the testis (anchors tail to caudal testis); Ligament of the tail of the epididymis (anchors tail to vaginal tunic).
• Deferent Duct (Ductus Deferens): Continuation of epididymal tail traversing the spermatic cord through the inguinal canal, arching over the ureter to enter the pelvic urethra at the colliculus seminalis. Terminal region forms the dilated ampullary gland.
• Spermatic Cord (Funiculus Spermaticus): Structures wrapped in mesofuniculus passing through the inguinal canal: Deferent duct, testicular artery, testicular vein (pampiniform plexus), nerves, lymphatics, and visceral vaginal tunic.

Male Accessory Sex Glands & Penis

• Species Comparison of Accessory Glands:
  • Dog: Possesses only the Ampullary Gland and Prostate Gland (compact part is large and globular, completely surrounding the pelvic urethra).
  • Cat: Possesses Ampullary, Prostate, and Bulbourethral glands.
  • Stallion & Bull: Possess Ampullary, Vesicular, Prostate, and Bulbourethral glands.
  • Boar: Possesses Vesicular, Prostate, and Bulbourethral glands.
• Penile Types:
  • Musculocavernous Penis (Dog, Cat, Stallion): High proportion of erectile vascular spaces; enlarges significantly in both length and diameter during erection.
  • Fibroelastic Penis (Ruminants, Pig): High connective tissue content, features a sigmoid flexure.
• Canine Penile Anatomy:
  • Root (Radix Penis): Attached to ischial arch; formed by paired crura and urethral bulb.
  • Body (Corpus Penis): Contains paired corpora cavernosa dorsally and unpaired corpus spongiosum ventrally surrounding the penile urethra.
  • Glans Penis: Composed of two distinct parts:
  1. Bulbus Glandis: Proximal expanded erectile bulb that swells during coitus to form the "tie" or lock with the bitch's vestibule.
  2. Pars Longa Glandis: Elongated distal portion of the glans.
  • Os Penis (Penile Bone): Splanchnic bone ossification within the distal cavernous tissue. Features a ventral groove housing the urethral spongy body; common site for urethral calculus obstruction.
• Prepuce: Cutaneous sheath covering the non-erect glans penis.

Genitourinary System: Female Reproductive Tract

Overview and Ligaments of the Female Tract

• Reproductive Type: Bicornuate uterus (two long horns, short body) adapted for multiparous litter-bearing animals (bitch and queen).
• Broad Ligament (Ligamentum Latum): Peritoneal folds suspending the tract from the dorsal body wall:
  1. Mesovarium: Suspends the ovary; contains ovarian vessels.
  2. Mesosalpinx: Suspends the uterine tube (forms walls of ovarian bursa).
  3. Mesometrium: Suspends the uterine horns and body.
• Specialized Ligaments:
  • Suspensory Ligament of the Ovary: Attaches the cranial pole of the ovary to the dorsal body wall near the kidney. Must be broken/ruptured during ovariohysterectomy (spay) to exteriorize the ovary.
  • Proper Ligament of the Ovary: Anchors the ovary to the cranial extremity of the uterine horn.
  • Round Ligament of the Uterus: Runs within the mesometrium through the inguinal canal to the vulva.

Ovaries (Ovaria)

• Paired female gonads situated in the dorsal abdominal cavity caudal to the kidneys, encapsulated within a fat-laden ovarian bursa.
• Internal Zones:
  • Medulla (Zona Vasculosa): Central vascular and neural core.
  • Cortex (Zona Parenchymatosa): Outer functional shell containing follicles and corpora lutea.
• Follicular Maturation Stages:
  1. Primordial Follicle $\rightarrow$ 2. Primary Follicle $\rightarrow$ 3. Secondary Follicle $\rightarrow$ 4. Tertiary Follicle $\rightarrow$ 5. Graafian (Mature Ovulatory) Follicle.
• Ovulation & Corpus Luteum Formation:
  • Luteinizing Hormone (LH) surge induces follicle rupture at the stigma, releasing the oocyte into the infundibulum.
  • Ruptured cavity fills with blood (corpus hemorrhagicum), transforming under LH into a progesterone-secreting corpus luteum (CL).
  • If non-pregnant, CL regresses into a fibrous scar (corpus albicans).
  • Note: Dogs are spontaneous ovulators; cats are induced ovulators (requiring coitus to trigger ovulation).

Uterine Tubes (Oviducts / Fallopian Tubes)

• Convoluted tubes (~4 to 7 cm long, 1 to 3 mm diameter) suspended by the mesosalpinx.
• Subdivisions:
  1. Infundibulum: Funnel-shaped opening fringed with finger-like fimbriae that sweep over the ovary to capture ovulated oocytes at the abdominal ostium.
  2. Ampulla: Expanded middle segment (primary site of fertilization).
  3. Isthmus: Narrowed caudal segment leading to the uterine ostium at the tubouterine junction.
• Lined by ciliated columnar epithelium propelling oocytes caudally.

Uterus & Cervix

• Uterine Anatomy: Y-shaped organ comprising two long uterine horns (approx. 5 times the length of the body) where embryo implantation occurs, a short uterine body, and the cervix.
• Uterine Wall Layers:
  1. Endometrium: Inner glandular mucosa supporting placental development.
  2. Myometrium: Thick smooth muscle layers producing parturition contractions.
  3. Perimetrium: Outer serous coverage continuous with the mesometrium.
• Cervix: Thick, muscular sphincter connecting uterine body to vagina. Contains a narrow cervical canal tightly sealed by a mucoid plug during pregnancy.
• Ovariohysterectomy (Spay): Surgical excision of ovaries, uterine horns, and uterine body to a point just cranial to the cervix.

Vagina, Vestibule, Vulva, & Mammary Glands

• Vagina: Extends from external uterine orifice to the external urethral orifice (urethral tubercle). Copulatory organ and birth canal lined by expandable stratified squamous epithelium.
• Vestibule: Shared caudal pathway extending from the external urethral orifice to the vulva. Contains constrictor muscles that maintain the coital tie.
• Vulva: External genital opening comprising two vertical lips (labia) meeting at dorsal and ventral commissures surrounding the vulval cleft.
  • Clitoris: Homologue of male penis, situated in the ventral clitoral fossa.
• Blood Supply to Female Tract:
  • Ovarian Artery: Arises directly from abdominal aorta caudal to renal artery.
  • Uterine Artery: Anastomoses with ovarian artery to supply uterine body and horns.
• Mammary Glands: Modified cutaneous glands on ventral trunk:
  • Bitch (Canine): Possesses 5 pairs (10 total glands: 2 cranial thoracic, 2 caudal thoracic, 2 cranial abdominal, 2 caudal abdominal, 2 inguinal).
  • Queen (Feline): Possesses 4 pairs (8 total glands).