Comprehensive Veterinary Splanchnology Study Guide

Splanchnology Overview and Definitions

• Definition of Splanchnology:

  • Splanchnology is the branch of anatomy dealing with the study of the internal organs (viscera) belonging to the digestive, respiratory, and urogenital systems.
  • The term originates from the Greek word Splanchnon, meaning "internal organ" or "entrails".
  • Splanchnology focuses on organs vital for maintaining individual life and ensuring species continuity.

• Viscera (Singular: Viscus):

  • Rooted in Latin, where viscus means "an internal organ, entrails". Used historically to describe organs within body cavities such as the thoracic and abdominal cavities.
  • Viscera consist of organs with or without a cavity, surrounded by special layers. Hollow viscera are lined internally by mucous membranes.
  • Visceral tracts form continuous tubes communicating with the exterior at one or both ends.
  • Major Categories of Viscera:
  • Hollow Viscera: Organs containing internal cavities (e.g., stomach, intestines).
  • Solid Viscera: Organs lacking internal cavities (e.g., liver, pancreas).

• Systemic Classification and Related 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 and intestines.
  • Respiratory System: Study = Splanchnology; Chief Structures = Lungs and airways.
  • Urinary System: Study = Splanchnology; Chief Structures = Kidneys and urinary bladder.
  • Reproductive System: Study = Splanchnology; Chief Structures = Ovaries and testes.
  • Endocrine System: Study = Endocrinology; Chief Structures = Ductless glands.
  • Nervous System: Study = Neurology; Chief Structures = Brain, spinal cord, and nerves.
  • Circulatory System: Study = Cardiology; Chief Structures = Heart and vessels.
  • Sensory System: Study = Esthesiology; Chief Structures = Eye and ear.

• Common Viscera Categorized by System:

  • Digestive System: Stomach, liver, intestines, pancreas.
  • Respiratory System: Lungs.
  • Urogenital System: Kidneys, reproductive organs.

Body Cavities and Anatomical Boundaries

• Body Cavity Classification by Size:

  • The body trunk and vertebrae contain three primary body cavities:
  1. Abdominal Cavity (Abdomen): Largest in size.
  2. Thoracic Cavity (Thorax): Second largest in size.
  3. Pelvic Cavity (Pelvis): Smallest in size.

• Thoracic Cavity:

  • The space within the chest housing the lungs, heart, pleura (lung membranes), pleural cavities, pericardium (cardiac sac), and pericardial cavity.
  • Anatomical Boundaries:
  • Cranial: Thoracic inlet (cranial thoracic opening).
  • Dorsal: Thoracic vertebral bodies.
  • Ventral: Sternum.
  • Caudal: Diaphragm.
  • Thoracic Inlet (Cranial Thoracic Opening):
  • Bounded dorsally by the first thoracic vertebra ($\text{T1}$), laterally by the first pair of ribs, and ventrally by the manubrium (the rostral segment of the sternum).
  • Diaphragm:
  • Dome-shaped muscle located at the caudal end of the thoracic cavity, separating it from the abdominal cavity.
  • Acts as the primary muscle of respiration; innervated by the phrenic nerve.
  • Composed of a sternal part, costal part, lumbar part, right crus, left crus, and a central tendinous center.
  • Diaphragmatic Openings (Hiatuses):
    1. Aortic Hiatus: The most dorsal opening; transmits the aorta, azygous vein, and thoracic duct.
    2. Oesophageal Hiatus: Transmits the esophagus as well as the dorsal and ventral vagal trunks.
    3. Caval Foramen: Located within the central tendinous region; transmits the caudal vena cava. The vessel wall is fused to the diaphragm, preventing movement.

• Pelvic Cavity (Pelvis):

  • Bounded dorsally by the sacrum and first two caudal vertebrae, and laterally/ventrally by the two hip bones (ilium, ischium, pubis) joining at the pelvic symphysis.
  • Contains the rectum, anal canal, and pelvic portions of the reproductive and urinary systems.
  • Obstetric and Functional Importance:
  • Serves as the birth canal during parturition; adequate pelvic diameter prevents dystocia.
  • Pelvic Inlet (Cranial Pelvic Aperture):
  • Bounded dorsally by the sacral promontory, laterally by the arcuate line, and ventrally by the pecten of the pubis.
  • Pelvic Outlet (Caudal Pelvic Aperture):
  • Bounded dorsally by the $3\text{rd}\text{--}4\text{th}$ caudal vertebrae, laterally by the sacrotuberous ligament and muscles, and ventrally by the ischiatic arch.
  • Pelvic Diameters:
  • Conjugate Diameter: Distance measured from the sacrum to the pubis.
  • Transverse Diameter: Measurement across the widest transverse width of the pelvic inlet.
  • Fetal Presentations in Canine Obstetrics:
  1. Anterior Presentation: Head born first with head and forepaws extended; ideal presentation for natural whelping.
  2. Posterior Presentation: Tail and hind legs delivered first; normal variant in the bitch, requires monitoring.
  3. Breech Presentation: Rump and tail presented with hind legs extended forward alongside the body; abnormal presentation. If the bitch experiences continuous straining without progress, immediate veterinary intervention is required.
  4. Two Front Legs Presentation: Head retained or misdirected; requires veterinary assistance.

Digestive System Structure and Physiology

• Classification of Animals by Food Habits:

  • Herbivores: Consume plant matter exclusively.
  • Omnivores: Consume both plant and animal materials.
  • Carnivores: Consume animal tissues exclusively.

• Sequential Segments of the Alimentary Tract:

  • Mouth $\rightarrow$ Oropharynx $\rightarrow$ Esophagus $\rightarrow$ Stomach $\rightarrow$ Small Intestine $\rightarrow$ Large Intestine $\rightarrow$ Anus.

• General Digestive Functions:

  • Responsible for mechanical breakdown (churning and mixing by gastric/intestinal smooth muscle) and chemical breakdown (via hydrochloric acid, pepsin, and specific enzymes) of ingesta for cellular energy, growth, and tissue repair.

• Subdivisions of the Digestive System:

  • Digestive Tube: Continuous tract extending from mouth to anus (mouth, pharynx, esophagus, small intestine, large intestine).
  • Digestive Glands: Exocrine glands providing enzymatic and mucosal secretions (salivary glands, liver, pancreas, gallbladder).
  • Digestive Accessory Organs: Structures aiding physical breakdown and prehension (lips, tongue, cheeks, palate, teeth).

• Salivary Physiology in Carnivores:

  • In humans, salivary amylase converts starch to maltose. In dogs and cats, saliva lacks significant enzymatic activity.
  • Functions of canine/feline saliva: Lubricating food passage, moistening oral mucosa, assisting evaporative thermoregulation during panting.

• Pathology and Clinical Conditions of Digestive Organs:

  • Lips & Oral Cavity: Stomatitis, oral ulcers, trauma. Signs: Excessive salivation, foul odor, dysphagia, oral pain. Treatment: Oral antiseptics, antibiotics, analgesics, foreign body removal.
  • Teeth: Periodontal disease, dental caries (rare), tooth fracture. Signs: Halitosis, loose teeth, gingivitis, pain. Treatment: Scaling, extraction, antibiotics, analgesia.
  • Tongue: Glossitis, foreign body injury, neoplasia. Signs: Swelling, drooling, anorexia, dysphagia. Treatment: Antibiotics, anti-inflammatories, surgical excision.
  • Salivary Glands: Sialadenitis, salivary mucocele (sialocele). Signs: Ventral jaw swelling, dysphagia, soft fluctuant mass. Treatment: Surgical excision of affected gland, drainage, antibiotics.
  • Pharynx: Pharyngitis, foreign body obstruction. Signs: Gagging, coughing, dysphagia, hypersalivation. Treatment: Obstruction removal, antibiotics, anti-inflammatories.
  • Esophagus: Megaesophagus, esophagitis, esophageal obstruction. Signs: Regurgitation, weight loss, aspiration pneumonia. Treatment: Upright feeding, treating underlying cause, antibiotics.
  • Stomach: Gastritis, gastric ulcers, Gastric Dilatation-Volvulus (GDV). Signs: Vomiting, abdominal pain, anorexia, bloating, shock. Treatment: Fluid therapy, gastroprotectants, antiemetics, emergency surgery for GDV.
  • Small Intestine: Enteritis, intestinal parasites, intussusception. Signs: Diarrhea, vomiting, dehydration, abdominal pain. Treatment: IV fluids, anthelmintics, antibiotics, surgery.
  • Large Intestine: Colitis, constipation, rectal prolapse. Signs: Tenesmus, dyschezia, mucoid/bloody diarrhea. Treatment: Dietary management, laxatives, fluid therapy, surgery.
  • Liver: Hepatitis, hepatic lipidosis, cirrhosis, liver failure. Signs: Jaundice (icterus), vomiting, lethargy, ascites. Treatment: Hepatoprotectants, fluids, nutritional support.
  • Gallbladder: Cholecystitis, cholelithiasis, gallbladder mucocele. Signs: Vomiting, icterus, abdominal pain. Treatment: Antibiotics, cholecystectomy, supportive care.
  • Pancreas: Pancreatitis, Exocrine Pancreatic Insufficiency (EPI). Signs: Vomiting, severe abdominal pain, steatorrhea, weight loss. Treatment: IV fluids, analgesia, low-fat diet, pancreatic enzyme replacement for EPI.
  • Peritoneum: Peritonitis. Signs: Fever, acute abdominal pain, distension, septic shock. Treatment: Emergency surgery, peritoneal lavage, aggressive antibiotics, IV fluids.

Oral Cavity and Associated Anatomy

• Boundaries and Subdivisions of the Oral Cavity (Cavum Oris):

  • Subdivided into the oral vestibule (space between lips/cheeks and teeth) and oral cavity proper.
  • Dorsal Boundary: Hard palate and soft palate.
  • Lateral Boundary: Cheeks and dental arches.
  • Ventral Boundary: Body of the mandible and hyoid musculature.

• Osteology of the Oral Cavity:

  • Upper Jaw: Formed by the incisive bone and maxilla.
  • Roof: Formed by the palatine bone.
  • Lower Jaw: Formed by paired mandibles joined at the midline by the mandibular symphysis.
  • Temporomandibular Joint (TMJ): Articulation between the condylar process of the mandible and the temporal bone of the skull, covered by muscles of mastication (masseter, temporalis, pterygoids).

• Diastema:

  • The interdental space or gap between the canine teeth and cheek teeth. Narrow in dogs compared to herbivores (cattle and horses).

• Functional Mechanisms of the Oral Cavity:

  1. Prehension: Grasping food using lips, tongue, and incisors.
  2. Mastication: Chewing and breaking food into boluses using teeth, tongue, and cheeks.
  3. Lubrication: Secretion of mucus and saliva to facilitate deglutition.
  4. Chemical Digestion: Minimal to absent in carnivores due to transient oral retention times.

• Lips (Labia Oris) and Cheeks (Buccae):

  • Lips form the rostral boundary of the vestibule, meeting at the angles of the mouth (angulus oris) to bound the oral fissure (rima oris).
  • Canine upper lips are pendulous, termed flews. Hound breeds, bulldogs, and bloodhounds possess long, low-hanging flews.
  • The upper lip features a hairless central groove termed the philtrum.
  • Cheeks form the lateral wall of the vestibule, running medial to the masseter muscle to attach along the buccinator muscle.

• Gums (Gingiva):

  • Dense, pink mucous membrane covering the alveolar margins of the maxilla and mandible, forming a protective seal around the cervix of each tooth.
  • Anatomical Subdivisions:
  • Free (Marginal) Gingiva: Unattached margin surrounding the tooth neck, forming the gingival sulcus.
  • Attached Gingiva: Firmly bound to underlying alveolar periosteum; resists mechanical forces of mastication.
  • Interdental Gingiva (Interdental Papilla): Mucosal extension filling space between adjacent teeth.
  • Clinical Color Indicators in Canine/Feline Gums:
  • Bubblegum Pink: Healthy tissue perfusion.
  • Bright Red: Gingivitis, local inflammation, infection.
  • Cherry Red: Carbon monoxide, toxin ingestion, heatstroke, hypertension.
  • Yellow / White: Icterus/jaundice (liver/hemolytic), severe anemia, shock, hypovolemia.
  • Purple / Grey / Blue: Cyanosis, hypoxia, methemoglobinemia, cardiovascular collapse.
  • Brown / Black Spots: Melanotic pigmentation (normal in specific breeds); acquired dark macules require evaluation for melanoma.

• Palate (Palatum):

  • Hard Palate (Palatum Durum):
  • Bony rostral partition formed by palatine processes of incisive, maxillary, and palatine bones.
  • Widest at the level of the 4th upper cheek tooth (carnassial tooth).
  • Features a central longitudinal median palatine raphe (formally a crest in dogs; a groove in horses and ruminants) flanked by $6\text{--}10$ transverse palatine rugae.
  • Possesses an incisive papilla rostrally, containing orifices for incisive ducts communicating with the vomeronasal organ (Jacobson's organ) for pheromone and sexual olfactory reception.
  • Soft Palate (Palatum Molle):
  • Musculomembranous caudal continuation of the hard palate separating the nasopharynx from the oropharynx.
  • Brachycephalic Airway Syndrome: Congenital elongation of the soft palate in short-muzzled breeds (English Bulldog, Pug, Pekingese, Shar-Pei) where tissue extends over the epiglottis into the laryngeal lumen, causing airway obstruction, stertor, stridor, reduced thermoregulatory capacity, and heatstroke risk. Corrected via laser soft palate resection (palatoplasty).

Anatomy and Physiology of the Tongue

• Structure and Regions of the Tongue (Lingua):

  • Muscular organ composed of interwoven intrinsic and extrinsic striated muscle bundles.
  • Anatomical Divisions:
  • Root (Radix linguae): Caudal third attached to hyoid apparatus.
  • Body (Corpus linguae): Slender middle portion featuring a dorsal median sulcus.
  • Apex (Apex linguae): Free, mobile, spatula-shaped rostral tip.

• Lingual Papillae Classification:

  • Mechanical Papillae (Lacking Taste Buds):
  1. Filiform Papillae: Most numerous, small, cornified structures covering the rostral two-thirds of the dorsal surface. Function to grasp food and groom fur. In felines, filiform papillae feature sharp, backward-pointing keratinized spines used to scrape meat from bones and groom coats.
  2. Conical Papillae: Larger than filiform papillae, located on the caudal third (root) of the tongue, directing ingesta toward the pharynx.
  3. Marginal Papillae: Present along the rostral margins of newborn puppies; facilitate suckling and involute upon transition to solid food.
  • Gustatory Papillae (Containing Taste Buds):
  1. Fungiform Papillae: Mushroom-shaped projections interspersed among filiform papillae on the apex and body.
  2. Vallate (Circumvallate) Papillae: Largest papillae, numbering $4\text{--}6$ in dogs, arranged in a V-shaped line rostral to the root, bounded by a deep circular trench containing serous gustatory glands.
  3. Foliate Papillae: Leaf-like parallel folds located on the lateral caudal borders of the tongue.

• Ventral Lingual Structures:

  • Lingual Frenulum: Ventral median mucosal fold anchoring the tongue to the floor of the oral cavity.
  • Lyssa (Lyssa Body):
  • A elongated, fusiform, rod-like fibrous structure located ventrally in the midline of the canine apex.
  • Composed of a dense connective tissue capsule enclosing adipose tissue (ventrally), striated muscle fibers (dorsally), cartilage, and innervated by branches of the hypoglossal nerve ($\text{CN XII}$).

• Musculature of the Tongue:

  • Intrinsic Muscle: Propria linguae (subdivided into longitudinalis dorsalis, longitudinalis ventralis, transversus linguae, and verticalis linguae). Alters tongue shape, produces local movements, and prevents tongue injury during mastication.
  • Extrinsic Muscles (Paired):
  1. Genioglossus: Arises from mandibular symphysis; depresses tongue; caudal fibers protrude the tongue, rostral fibers retract the apex.
  2. Hyoglossus: Arises from thyrohyoid and basihyoid; depresses and retracts the tongue.
  3. Styloglossus: Arises from stylohyoid bone via three heads (short, rostral, long); retracts and elevates the tongue.
  • Innervation:
  • Motor: Hypoglossal nerve ($\text{CN XII}$).
  • Sensory/Taste: Trigeminal nerve ($\text{CN V}$), Facial nerve ($\text{CN VII}$), Glossopharyngeal nerve ($\text{CN IX}$), Vagus nerve ($\text{CN X}$).

Salivary Glands and Pharyngeal Anatomy

• Salivary Glands Classification:

  • Major Salivary Glands:
  1. Parotid Gland: Located ventral to ear base; duct runs across masseter muscle to open in upper buccal vestibule opposite the upper 4th premolar.
  2. Zygomatic Gland: Specific to carnivores; located medial to zygomatic arch; duct opens near last upper molar.
  3. Mandibular Gland: Located caudal to ramus of mandible; duct opens at the sublingual caruncle.
  4. Sublingual Gland: Located beneath mucosa of tongue; divided into a monostomatic portion (duct opens at sublingual caruncle) and a polystomatic portion (multiple ducts opening into oral cavity proper).
  • Minor Salivary Glands:
  • Includes buccal glands (prominent in cats caudal to last lower molar), labial, molar, lingual, and palatine glandular tissues.
  • Microscopic Secretory Characteristics:
  • Gustatory glands (von Ebner's) at vallate/foliate bases are predominantly serous.
  • Lingual submucosal glands in caudal third are mucoserous; lateral margin glands are seromucous.

• Pharynx (Throat):

  • Subdivisions:
  1. Nasopharynx: Respiratory region located dorsal to soft palate, extending from choanae to palatopharyngeal arch.
  2. Oropharynx: Digestive region located ventral to soft palate, extending from isthmus faucium to epiglottis base.
  3. Laryngopharynx: Shared respiratory/digestive cavity extending from epiglottis base to esophageal entrance.
  • Five Openings of the Pharyngeal Cavity:
  1. Choanae (rostrodorsal, connecting to nasal cavity).
  2. Pharyngeal openings of auditory (Eustachian) tubes (dorsolateral, connecting to middle ear).
  3. Isthmus faucium (rostral opening from oral cavity proper).
  4. Aditus laryngis (laryngeal entrance, closed by epiglottis during deglutition).
  5. Aditus esophagi (caudal entrance into esophagus).
  • Pharyngeal Musculature and Actions:
  • Rostral Constrictors: Pterygopharyngeus (origin: pterygoid hamulus) and Palatopharyngeus (origin: palatine raphe). Action: Constrict and shorten rostral pharynx.
  • Middle Constrictor: Hyopharyngeus (divided into chondropharyngeus and ceratopharyngeus). Action: Constrict middle pharynx.
  • Caudal Constrictors: Thyropharyngeus (origin: thyroid cartilage) and Cricopharyngeus (origin: cricoid cartilage). Action: Constrict caudal pharynx.
  • Pharyngeal Dilator: Stylopharyngeus (origin: stylohyoid bone). Action: Dilates, elevates, and draws pharynx forward.

• Deglutition (Swallowing):

  • Stage 1 (Voluntary): Tongue forces food bolus caudally into oropharynx.
  • Stage 2 (Reflex): Bolus elevates soft palate; palatopharyngeal arch shortens closing nasopharynx; hyoid/larynx pulled cranially, causing epiglottis to fold caudally over aditus laryngis; pharyngeal constrictors contract sequentially, driving bolus into relaxed upper esophageal sphincter.

Canine Dentition and Triadan System

• Dental Characteristics:

  • Canines possess brachydont (short-crowned) and diphyodont (two sets: deciduous and permanent) dentition.
  • Deciduous eruption begins at day $20$ and finishes by day $35$ ($3\text{--}5$ weeks).
  • Teeth cease growth once fully erupted.

• Microscopic and Gross Structure of the Tooth:

  1. Pulp: Vascular, neural, gelatinous core occupying pulp chamber (crown) and root canal (root).
  2. Dentine (Dentinum): Mineralized tubule network forming the main structural mass; synthesized by odontoblasts; sensitive to hydraulic pressure shifts.
  3. Enamel: Extremely dense, white outer crown surface; $96\%$ inorganic hydroxyapatite crystals; hardest bodily substance; incapable of cellular regeneration.
  4. Cementum: Thin calcified tissue covering root dentine; provides anchoring site for periodontal ligament fibers.
  5. Cervix (Neck): Cementoenamel junction transitioning crown to root.

• Tooth Surfaces and Directions:

  • Vestibular (Labial/Buccal): Surface facing lips or cheeks.
  • Lingual: Surface facing tongue (Palatal on maxillary teeth).
  • Contact Surfaces: Mesial (towards rostral/medial midline) and Distal (towards caudal/lateral aspect).
  • Occlusal: Masticatory surface contacting opposing dental arcade.
  • Directional Terms: Coronal (towards crown apex) and Apical (towards root tip).

• Permanent Tooth Root Morphology:

  • Incisors ($\text{I1, I2, I3}$): 1 root (Maxilla & Mandible).
  • Canines ($\text{C}$): 1 long, robust root (Maxilla & Mandible).
  • 1st Premolar ($\text{P1}$): 1 root (Maxilla & Mandible).
  • 2nd & 3rd Premolars ($\text{P2, P3}$): 2 roots (Maxilla & Mandible).
  • Upper 4th Premolar ($\text{P4}$ - Upper Carnassial): 3 roots (2 buccal, 1 palatal).
  • Lower 4th Premolar ($\text{P4}$): 2 roots.
  • Upper 1st & 2nd Molars ($\text{M1, M2}$): 3 roots each.
  • Lower 1st Molar ($\text{M1}$ - Lower Carnassial): 2 roots.
  • Lower 2nd & 3rd Molars ($\text{M2, M3}$): 2 roots each (Upper $\text{M3}$ is absent).
  • Carnassial (Sectorial / Shearing) Teeth: Upper $\text{P4}$ and Lower $\text{M1}$.

• Dental Formulas:

  • Permanent Canine Formula:     $2 \times \left( \text{I}\frac{3}{3}, \text{C}\frac{1}{1}, \text{PM}\frac{4}{4}, \text{M}\frac{2}{3} \right) = 42$
  • Deciduous Canine Formula:     $2 \times \left( \text{I}\frac{3}{3}, \text{C}\frac{1}{1}, \text{PM}\frac{3}{3} \right) = 28$(Note: Deciduous $\text{P1}$ and all molars are non-existent. Permanent molars and $\text{P1}$ are non-successional).

• Permanent Tooth Eruption Schedule:

  • Central & Intermediate Incisors: $2\text{--}5$ months.
  • Corner Incisors: $4\text{--}5$ months.
  • Canines: $4\text{--}6$ months.
  • First Premolars: $4\text{--}5$ months.
  • Second, Third, Fourth Premolars: $4\text{--}6$ months.
  • First Molars: $5\text{--}6$ months.
  • Second & Third Molars: $6\text{--}7$ months.

• Modified Triadan System:

  • Quadrant Identification:
  • Right Upper Maxilla: 100 (Deciduous: 500).
  • Left Upper Maxilla: 200 (Deciduous: 600).
  • Left Lower Mandible: 300 (Deciduous: 700).
  • Right Lower Mandible: 400 (Deciduous: 800).
  • Tooth numbering begins at central incisor (01) extending distally to last molar (10 or 11).
  • Examples: Permanent left upper canine = 204; Deciduous left upper canine = 604.

Anatomy of the Esophagus and Stomach

• Esophagus:

  • Muscular tube transporting ingesta from laryngopharynx to cardiac orifice of stomach via primary peristalsis.
  • Dimensions: Approximately $30\,\text{cm}$ long and $2\,\text{cm}$ diameter when collapsed in a medium-sized dog.
  • Anatomical Course:
  • Begins dorsal to cricoid cartilage (limen pharyngoesophageum demarks pharyngeal transition).
  • Courses down cervical region inclining to the left of the trachea.
  • Enters thoracic inlet, crosses trachea to regain dorsal position at tracheal bifurcation.
  • Passes through esophageal hiatus of diaphragm at level of $\text{T10}$.
  • Terminates at gastric cardia via short abdominal portion.
  • Sphincters:
  • Upper Esophageal Sphincter (UES): Cricopharyngeus muscle.
  • Lower Esophageal Sphincter (LES): Cardiac sphincter.
  • Histology and Species Differences:
  • Composed of 4 coats: Adventitia/Serosa, Muscularis, Submucosa, Mucosa (folded stratified squamous epithelium).
  • Muscularis in dogs: Striated muscle throughout entire length.
  • Muscularis in cats: Striated muscle in cranial two-thirds; smooth muscle in caudal third.

• Stomach (Gaster / Ventriculus):

  • Simple, highly distensible, musculoglandular organ located in the cranial abdominal cavity.
  • Attached to liver by lesser omentum (lesser curvature) and to dorsal body wall by greater omentum (greater curvature).
  • Anatomical Regions:
  1. Cardia (Pars cardiaca): Smallest region surrounding esophageal entry.
  2. Fundus (Fundus ventriculi): Large blind outpocketing located left and dorsal to cardia.
  3. Body (Corpus ventriculi): Central region where mechanical churning occurs.
  4. Pyloric Part (Pars pylorica): Subdivided into pyloric antrum, pyloric canal, and pylorus (thickened circular smooth muscle sphincter).
  • Curvatures:
  • Greater Curvature: Convex border (~$30\,\text{cm}$ long filled); serves as attachment for greater omentum.
  • Lesser Curvature: Short concave border angled at $50\text{--}70^{\circ}$ at the angular incisure (incisura angularis), embracing the papillary process of the liver.
  • Capacity and Weight:
  • Empty stomach weight (15-kg dog): ~$100\,\text{g}$.
  • Distended capacity: $100\text{--}250\,\text{cc/kg}$ body weight.
  • Histological Layers of Stomach Wall (Outside to Inside):
  1. Serosa (Tunica Serosa): Visceral peritoneum.
  2. Muscularis (Tunica Muscularis): Composed of three smooth muscle layers: outer longitudinal, middle circular (forms cardiac and pyloric sphincters), and inner oblique (characteristic feature enhancing churning capacity).
  3. Submucosa (Tela Submucosa): Connective tissue containing vessels, Meissner's plexus, and glands.
  4. Mucosa (Tunica Mucosa): Simple columnar epithelium forming internal rugae (gastric folds) containing gastric pits and glands secreting $\text{HCl}$, pepsinogen, and gastrin.
  • Gastric Dilatation-Volvulus (GDV):
  • Acute, life-threatening condition where the stomach fills with gas/fluid (dilatation) and rotates along its longitudinal axis (volvulus), compromising venous return, causing tissue necrosis, and requiring emergency surgical intervention.

Intestinal Tract Anatomy and Defecation Physiology

• Small Intestine (Intestinum Tenue):

  • Longest segment of alimentary tract; average living length is $3.5 \times$ total body length. Suspended by mesentery.
  • Subdivisions:
  1. Duodenum: Proximal fixed loop (~$25\,\text{cm}$ long). Attached by mesoduodenum. Consists of cranial portion, cranial duodenal flexure, descending portion, caudal duodenal flexure, and ascending portion ending at duodenojejunal flexure. Features major duodenal papilla (bile and pancreatic ducts) and minor duodenal papilla (accessory pancreatic duct).
  2. Jejunum: Longest, highly mobile middle segment forming sagittal and transverse coils in ventral abdomen, supplied by cranial mesenteric artery.
  3. Ileum: Short (~$10\,\text{cm}$), firm, thick terminal segment passing cranially on right side of mesenteric root to enter ascending colon at ileocolic orifice. Demarcated by ileocecal fold.

• Large Intestine (Intestinum Crassum):

  • Simple, non-sacculated tube lacking haustra or tenia. Primary function is fecal dehydration.
  • Subdivisions:
  1. Cecum: S-shaped, spiral, blind-ending pouch on right side; diverticulum of ascending colon communicating via cecocolic orifice/sphincter.
  2. Colon: Shaped like a shepherd's crook / question mark (?). Suspended by mesocolon. Subdivided into short ascending colon (right side), transverse colon (cranial to mesenteric root), and long descending colon (left side, ~$25\,\text{cm}$ long, $2\,\text{cm}$ diameter). Features right colic flexure and left colic flexure.
  3. Rectum: Straight pelvic segment (~$5\,\text{cm}$ long, $3\,\text{cm}$ diameter) suspended by mesorectum attached to sacrum.
  4. Anal Canal (Canalis Analis): Short terminal section (~$1\,\text{cm}$) lined by stratified squamous epithelium.

• Anal Sphincters and Glands:

  • Internal Anal Sphincter: Thickened caudal circular smooth muscle layer; involuntary autonomic control.
  • External Anal Sphincter: Striated skeletal muscle ring surrounding internal sphincter; voluntary control via pudendal nerve.
  • Anal Sacs (Sinus Paranalis): Paired cutaneogenous invaginations situated ventrolateral to anus between internal and external sphincters at 4 o'clock and 8 o'clock positions. Lined by sebaceous and apocrine glands secreting foul-smelling fluid for territorial marking.

• Defecation Reflex Sequence:

  1. Rectal distension activates wall stretch receptors.
  2. Sensory impulses travel along sacral afferent nerves ($\text{S1--S3}$).
  3. Parasympathetic efferents via pelvic nerve stimulate rectal muscular contraction and internal anal sphincter relaxation.
  4. Voluntary external sphincter relaxation via pudendal nerve ($\text{S1--S3}$).
  5. Increased intra-abdominal pressure (Valsalva maneuver) expels feces.

Anatomy of the Liver, Gallbladder, and Pancreas

• Liver (Hepar):

  • Largest abdominal gland; combined exocrine (bile production) and endocrine (metabolic) functions.
  • Deep red color, firm yet friable texture. Located in cranial intrathoracic abdomen caudal to diaphragm.
  • Surfaces:
  • Diaphragmatic Surface: Strongly convex, fitting against diaphragm ($6\text{th}\text{--}12\text{th}$ intercostal space right side; $6\text{th}\text{--}10\text{th}$ left side).
  • Visceral Surface: Concave, displaying gastric, duodenal, pancreatic, and renal impressions.
  • Hepatic Lobes and Processes:
  1. Left Hepatic Lobe: Forms $1/3\text{--}1/2$ total mass; divided into Left Lateral and Left Medial sublobes.
  2. Right Hepatic Lobe: Divided into Right Medial (contains gallbladder fossa) and Right Lateral sublobes.
  3. Caudate Lobe: Composed of Papillary Process (tongue-shaped, resting in lesser gastric curvature) and Caudate Process (caps cranial pole of right kidney, bearing deep renal impression).
  4. Quadrate Lobe: Narrow median wedge forming left boundary of gallbladder fossa.
  • Peritoneal Attachments (Ligaments):
  • Coronary Ligament: Reflects around bare area (area nuda) onto diaphragm.
  • Right & Left Triangular Ligaments: Connect lateral lobes to diaphragm (left contains appendix fibrosa hepatis).
  • Falciform Ligament: Ventral remnant containing round ligament (ligamentum teres hepatis).
  • Hepatorenal Ligament: Connects caudate process to right kidney.
  • Lesser Omentum: Extends from porta hepatis to lesser curvature and cranial duodenum.

• Gallbladder (Vesica Fellea) and Bile Passages:

  • Located in fossa between quadrate and right medial liver lobes. Stores and concentrates hepatic bile.
  • Bile pathway: Hepatic ducts + Cystic duct $\rightarrow$ Common Bile Duct $\rightarrow$ Major duodenal papilla.

• Pancreas:

  • V-shaped, coarsely lobulated gland (yellowish-gray preserved; pinkish-gray living).
  • Anatomy: Composed of Body (pancreatic angle), Right Lobe (enclosed in mesoduodenum), and Left Lobe (within deep leaf of greater omentum).
  • Duct System:
  • Pancreatic Duct (Ventral Duct): Smaller, opens on major duodenal papilla with bile duct (may be absent in dogs).
  • Accessory Pancreatic Duct (Dorsal Duct / Duct of Santorini): Larger, main conduit in dogs; opens on minor duodenal papilla.

Anatomy and Physiology of the Respiratory System

• Respiration vs. Breathing:

  • Breathing: Mechanical ventilation changing chest volume.
  • Respiration: Biochemical gas exchange ($\text{O2}$ pickup, $\text{CO2}$ expulsion).

• Primary & Secondary Functions:

  • Primary: Supplying $\text{O2}$, removing $\text{CO2}$.
  • Secondary: Acid-base balance (pH regulation), thermoregulation, water elimination, phonation.

• Stages of Respiration:

  • External Respiration: Alveolar-capillary gas exchange in lungs.
  • Internal Respiration: Systemic capillary-tissue gas exchange.

• Resting Respiratory Rates (Breaths/min):

  • Dog: $18\text{--}34$ (Normal average = $24$; $>40$ abnormal).
  • Cat: $16\text{--}40$.
  • Horse: $10\text{--}14$.
  • Dairy Cow: $26\text{--}50$.
  • Sheep: $16\text{--}34$.
  • Pig: $32\text{--}58$.

• Upper Respiratory Tract (URT):

  • External Nose & Planum Nasale: Hairless, non-glandular apex featuring nostrils (nares) separated by the philtrum. Kept moist by lateral nasal gland secretions.
  • Larynx (Voice Box): Musculocartilaginous valve guarding trachea. Composed of 9 cartilages:
  • Unpaired: Epiglottis (leaf-shaped, covers laryngeal inlet), Thyroid (shield-shaped, largest), Cricoid (complete ring).
  • Paired: Arytenoid (pyramid-shaped, controls glottis), Corniculate, Cuneiform.

• Lower Respiratory Tract (LRT):

  • Trachea: Non-collapsible tube containing $40\text{--}45$ C-shaped hyaline cartilage rings in dogs (dorsally incomplete, bridged by trachealis muscle). Length: $15\text{--}20\,\text{cm}$.
  • Comparative Tracheal Rings: Equine ($48\text{--}60$), Bovine ($48\text{--}60$), Swine ($32\text{--}36$), Ovine/Caprine ($40\text{--}50$).
  • Lungs (Pulmones):
  • Paired spongy respiratory organs divided by interlobar fissures.
  • Left Lung (2 Lobes): Cranial lobe (subdivided into cranial and caudal parts), Caudal lobe.
  • Right Lung (4 Lobes): Cranial lobe, Middle (cardiac) lobe, Caudal lobe, Accessory lobe.
  • Fetal vs. Postnatal Lung Differentiation:
  • Fetal Lung: Unexpanded, solid liver-like density, pale gray, firm, sinks in water.
  • Postnatal Lung: Expanded with surfactant, pink, light, spongy, crepitates on palpation, floats in water.

Anatomy of the Circulatory System

• Heart (Cor) Location and Orientation:

  • Situated in middle mediastinum between $3\text{rd}\text{--}6\text{th}$ intercostal spaces ($\text{ICS}$).
  • Base directed dorsocranially; apex directed caudoventrally and to the left.

• Layers of Heart Wall & Pericardium:

  • Fibrous Pericardium: Tough outer fibrous sac anchored via sternopericardiac and phrenopericardiac ligaments.
  • Serous Pericardium: Parietal layer lines fibrous sac; Visceral layer (Epicardium) adheres to myocardium. Encloses pericardial cavity containing serous lubricant ($\sim 1\,\text{mL}$).
  • Myocardium: Thick muscular layer (thickest in left ventricle).
  • Endocardium: Smooth inner endothelial lining.

• Heart Chambers and Valves:

  • Right Atrium: Receives deoxygenated blood via cranial/caudal vena cavae and coronary sinus.
  • Right Ventricle: Pumps blood through Pulmonary Semilunar Valve to pulmonary trunk. Entrance guarded by Right AV (Tricuspid) Valve ($2$ major cusps in dogs: parietal and septal).
  • Left Atrium: Receives oxygenated blood via pulmonary veins.
  • Left Ventricle: Pumps blood through Aortic Semilunar Valve into aorta. Entrance guarded by Left AV (Bicuspid / Mitral) Valve. Features thickest myocardium.
  • Chordae Tendineae: Fibrous cords extending from papillary muscles to AV valve cusps, preventing eversion during ventricular systole.

• Valve Auscultation Landmarks:

  • Mitral Valve: Left $5\text{th}$ ICS at costochondral junction.
  • Pulmonic Valve: Left $2\text{nd}\text{--}4\text{th}$ ICS above sternum.
  • Aortic Valve: Left $4\text{th}$ ICS above costochondral junction.
  • Tricuspid Valve: Right $3\text{rd}\text{--}5\text{th}$ ICS near costochondral junction.

• Cardiac Conduction System:

  • Sinoatrial (SA) Node: Primary pacemaker in right atrial wall near cranial vena cava opening. Intrinsic rate in dog: $90\text{--}160\,\text{beats/min}$.
  • Atrioventricular (AV) Node: In interatrial septum; delays impulse ~$0.1\,\text{sec}$ to permit ventricular filling.
  • Bundle of His & Purkinje Fibers: Transmit impulses rapidly across subendocardium to initiate apex-to-base ventricular contraction.

• Coronary Circulation:

  • Left coronary artery (paraconal branch) & Right coronary artery (subsinuosal branch). Venous return via cardiac veins into coronary sinus draining into right atrium.

• Systemic Injection & Venipuncture Sites:

  • Intravenous (IV): Cephalic vein, Lateral saphenous vein, Jugular vein, Medial saphenous vein.
  • Intramuscular (IM): Lumbar epaxial muscles, Hamstring group (biceps femoris, semitendinosus, semimembranosus), Quadriceps, Triceps brachii.
  • Subcutaneous (SQ) Vaccination Sites:
  • Rabies = Right rear leg.
  • DHPP = Right shoulder.
  • Lyme / Rattlesnake = Left rear leg.
  • Bordetella = Left shoulder (or intranasal/oral).

Anatomy of the Urinary System

• Kidneys (Renes):

  • Paired, bean-shaped, smooth retroperitoneal organs situated against lumbar hypaxial muscles.
  • Dimensions & Weight (Dog): Length = $6\text{--}9\,\text{cm}$; Width = $4\text{--}5\,\text{cm}$; Thickness = $3\text{--}4\,\text{cm}$; Weight = $25\text{--}35\,\text{g}$.
  • Position:
  • Right Kidney: Located below last rib to $1\text{st}\text{--}3\text{rd}$ lumbar vertebrae ($\text{T13--L3}$); more cranial, fitting into liver renal impression.
  • Left Kidney: Located below $3\text{rd}\text{--}5\text{th}$ lumbar vertebrae ($\text{L3--L5}$).
  • Macroscopic Cross-Section Layers:
  1. Capsule: Fibrous outer protective membrane.
  2. Cortex: Outer red layer containing renal corpuscles and convoluted tubules.
  3. Medulla: Paler inner layer containing renal pyramids, loops of Henle, and collecting ducts.
  4. Pelvis: Basin-shaped fibrous collecting space draining into ureter.
  • Microscopic Structure (Nephron):
  • Functional unit (~$1,000,000$ per kidney).
  • Renal Corpuscle: Glomerulus (capillary tuft) + Bowman's capsule (filtrate collection).
  • Tubular System: Proximal Convoluted Tubule (PCT) $\rightarrow$ Loop of Henle $\rightarrow$ Distal Convoluted Tubule (DCT) $\rightarrow$ Collecting Duct.
  • Renal Blood Flow Sequence:
  • Renal Artery $\rightarrow$ Interlobar Arteries $\rightarrow$ Arcuate Arteries $\rightarrow$ Interlobular Arteries $\rightarrow$ Afferent Arterioles $\rightarrow$ Glomerulus $\rightarrow$ Efferent Arterioles $\rightarrow$ Peritubular Capillaries $\rightarrow$ Interlobular Veins $\rightarrow$ Arcuate Veins $\rightarrow$ Interlobar Veins $\rightarrow$ Renal Vein $\rightarrow$ Caudal Vena Cava.

• Ureters:

  • Muscular tubes ($12\text{--}16\,\text{cm}$ long; $0.6\text{--}0.9\,\text{cm}$ distended) transporting urine via peristalsis from renal hilus to bladder trigone. Suspended by mesoureter.

• Urinary Bladder (Vesica Urinaria):

  • Musculomembranous sac located on pelvic floor (empty) or ventral abdominal wall (distended; capacity $100\text{--}120\,\text{mL}$ in 25-lb dog).
  • Anatomical Parts: Apex (fundus), Body, Neck.
  • Detrusor Muscle: Smooth muscle coat relaxing for storage and contracting for micturition.
  • Trigone: Internal smooth triangular mucosa bounded by two ureteral orifices and internal urethral orifice.
  • Sphincters: Internal (involuntary smooth muscle) and External (voluntary striated muscle).

• Urethra:

  • Female: Short tube terminating at external urethral orifice on urethral tubercle of vestibule.
  • Male: Long tube divided into pelvic urethra (preprostatic and prostatic) and penile urethra.

Anatomy of the Male Reproductive System

• Scrotum:

  • Cutaneous pouch divided by a median septum enclosing testes, epididymides, and spermatic cords.
  • Scrotal Layers: Skin, Tunica dartos (smooth muscle, bunches skin to regulate heat), Cremaster muscle (striated muscle elevating tunic), Spermatic fascia, Vaginal tunic.
  • Species Differences:
  • Dog: Scrotum in inguinal region; oblique testicular orientation.
  • Cat: Scrotum in perineal region (subanal); horizontal orientation; lacks cremaster muscle, using levator scroti muscle instead.

• Testes (Testes):

  • Male gonads producing spermatozoa and testosterone.
  • Dimensions (25-lb Dog): Length = $3\,\text{cm}$; Width = $2\,\text{cm}$; Weight = $8\,\text{g}$.
  • Internal Architecture: Enclosed by tunica albuginea; septa divide parenchyma into lobules containing contorted seminiferous tubules $\rightarrow$ straight seminiferous tubules $\rightarrow$ rete testis $\rightarrow$ efferent ductules ($8\text{--}12$).
  • Spermatic Cord Contents: Deferent duct, testicular artery, pampiniform plexus (venous network for countercurrent heat exchange), testicular nerves, lymphatics, tunic.
  • Descent of Testes: Testes descend from lumbar region into scrotum via gubernaculum. Cryptorchidism = failure of descent.

• Epididymis:

  • Convoluted duct ($5\text{--}8\,\text{m}$ long in dog; $4\text{--}6\,\text{m}$ in cat) divided into Head (Caput), Body (Corpus), and Tail (Cauda). Functions in sperm maturation and storage.

• Deferent Duct (Ductus Deferens):

  • Direct continuation of epididymal duct ascending spermatic cord through inguinal canal to open at colliculus seminalis of pelvic urethra.

• Accessory Sex Glands:

  • Dog: Possesses only Ampullary Glands and a large, globular Prostate Gland completely encircling the pelvic urethra.
  • Cat: Possesses Ampullary, Prostate, and Bulbourethral glands.

• Penis:

  • Musculocavernous type in dogs and cats (vs. Fibroelastic in pigs/ruminants).
  • Divisions: Root (Radix, 2 crura + bulb), Body (Corpus), Glans (Glans penis).
  • Canine Glans Penis Subdivisions:
  • Bulbus Glandis: Proximal cavernous expansion responsible for the "tie" during coitus.
  • Pars Longa Glandis: Distal elongated portion.
  • Os Penis (Penile Bone):
  • Ossified cavernous modification grooved ventrally to accommodate urethral duct; common site for urinary calculus impaction.

Anatomy of the Female Reproductive System

• Ovaries:

  • Paired female gonads located in lumbar abdomen caudal to kidneys. Suspended by mesovarium and suspensory ligament of the ovary (attaches to dorsal wall near kidney; must be transected during spay).
  • Structure: Outer parenchymatous zone (cortex) containing follicles and inner vascular medulla.
  • Ovarian Follicle Developmental Sequence:
  • Primordial Follicle $\rightarrow$ Primary Follicle $\rightarrow$ Secondary Follicle $\rightarrow$ Tertiary Follicle $\rightarrow$ Graafian Follicle $\rightarrow$ Ovulation $\rightarrow$ Corpus Hemorrhagicum $\rightarrow$ Corpus Luteum $\rightarrow$ Corpus Albicans.

• Uterine Tubes (Oviducts / Fallopian Tubes):

  • Convoluted tubes ($4\text{--}7\,\text{cm}$ long) suspended by mesosalpinx. Subdivided into Infundibulum (bearing fimbriae surrounding abdominal ostium), Ampulla (site of fertilization), Isthmus, and Uterine Ostium.

• Uterus:

  • Bicornuate (Y-shaped) structure comprising two long uterine horns (five times length of body), a short central body, and a muscular neck (cervix).
  • Uterine Wall Layers:
  1. Endometrium: Mucosal/glandular inner lining.
  2. Myometrium: Smooth muscle layer.
  3. Mesometrium: Broad ligament suspension fold.
  • Ovariohysterectomy (Spay): Complete surgical excision of ovaries, uterine tubes, horns, and body cranial to cervix.

• Vagina, Vestibule, and Vulva:

  • Vagina: Extends from cervix to external urethral orifice.
  • Vestibule: Shared urogenital tract extending from urethral orifice to vulva.
  • Vulva: External genitalia consisting of two labia meeting at dorsal and ventral commissures, enclosing the clitoris within the clitoral fossa.

• Mammary Glands:

  • Modified cutaneous apocrine/sebaceous glands.
  • Bitch: 5 pairs ($10$ glands).
  • Queen: 4 pairs ($8$ glands).