Lab 3: Median section of the head

Median Section of the Head (Dog)

Dissection Preparations

  • Required Reading: Pages 229-33 (oral cavity onwards) in Guide to Dissection of the Dog 7th ed.

  • Required Study: Figures 5-23, 5-24, and 5-25.

Sagittal Section of the Head (Dog) - Medial Side Structures

  • Tongue and Lingual Muscles: Muscular organ essential for taste, mastication, and deglutition.

  • Paranasal Sinuses: Air-filled cavities within skull bones, lined with mucous membrane.

  • Ethmoidal Turbinates: Delicate, scroll-like bones covered by olfactory epithelium, part of the ethmoid labyrinth.

  • Larynx: Cartilaginous organ at the top of the trachea, involved in breathing, sound production, and protecting the airway.

  • Cribriform Plate: Perforated bony plate forming part of the ethmoid bone, allowing passage of olfactory nerve filaments.

  • Soft Palate: The muscular, soft posterior part of the palate.

  • Oesophagus: Muscular tube connecting the pharynx to the stomach.

  • Mandible: The lower jaw bone.

  • Conchae (Turbinates): Scroll-like bones in the nasal cavity (dorsal, ventral, and middle).

  • Trachea: The windpipe, a cartilaginous tube connecting the larynx to the bronchi.

  • Hyoid Apparatus: Bones and cartilages supporting the tongue and larynx.

  • Hard Palate: The bony, rigid anterior part of the palate, forming the roof of the oral cavity.

  • Epiglottis: A flap of cartilage at the root of the tongue, which folds over the opening of the larynx during swallowing.

  • Nasal Septum: The cartilaginous and bony wall dividing the nasal cavity into two.

Boundaries of Oral Cavity

  • Rostral: Lips

  • Caudal: Palatoglossal arch

  • Ventral: Mylohyoid muscles (floor of the mouth)

  • Dorsal: Hard and soft palates

  • Lateral: Cheeks

Boundaries of Nasal Cavity

  • Rostral: External nares

  • Caudal: Cribriform plate

  • Ventral: Hard palate

  • Dorsal: Nasal bones, frontal bones

  • Lateral: Maxillary and incisive bones, lateral nasal cartilages

Palate Tissues and Epithelium

  • Hard Palate: Formed by palatine, maxillary, and incisive bones.

  • Soft Palate: Formed by muscle and connective tissue.

  • Oral Surface Epithelium: Both hard and soft palates are lined by stratified squamous epithelium on their oral surface.

Relative Palate Size (Horse vs. Dog)

  • Horse: Has a significantly longer soft palate compared to the dog.

  • Dog: Has a relatively shorter soft palate.

Equine Soft Palate Position

  • The equine soft palate is positioned ventral to the epiglottis, making it difficult for horses to open-mouth breathe, as the epiglottis is normally above the soft palate. This structural arrangement is crucial for horses as obligate nasal breathers.

Salivary Glands (Dog)

Major Salivary Glands and Associated Structures (Lateral Side)

  • Parotid Salivary Gland:

    • Apex: Penetrated by the maxillary vein, which must be removed to displace the gland from the base of the ear.

    • Parotid Duct: Originates at the gland apex, grooves the surface of the masseter muscle, passes between the dorsal and ventral buccal branches of the facial nerve, and terminates in the upper cheek vestibule.

  • Mandibular Salivary Gland:

    • Thickly encapsulated.

    • Its lateral surface is deeply grooved by the maxillary vein.

  • Sublingual Salivary Gland:

    • Composed of two parts:

      • I Monostomatic part: Exposed by removing the linguofacial vein and the mandibular lymph nodes. This part surrounds the proximal end of the mandibular gland duct.

      • II Polystomatic part: This gland is scattered along the path of the mandibular and sublingual gland ducts.

    • Sublingual Caruncle & Sublingual Fold: The sublingual fold on the inside of the oral cavity marks the course of the mandibular and sublingual ducts, which open into the oral cavity at the sublingual caruncle.

  • Zygomatic Salivary Gland: Will be dissected later, along with the eye.

Salivary Gland Questions

  • Parotid Duct Termination: The parotid duct terminates opposite the caudal aspect of the last upper premolar tooth.

  • Types of Secretion:

    • (a) Parotid Gland: Produces serous secretion (watery, enzyme-rich).

    • (b) Mandibular Gland: Produces mixed seromucous secretion (both watery and mucous components).

Oral Cavity and Tongue

Examination of Vestibule and Oral Cavity

  • Observe the space between the lips/cheeks and teeth (vestibule) and the main oral cavity proper.

Structures to Identify

  • Palatoglossal Arch: Accentuated by pulling the tongue to one side. These arches are pillars of the soft palate and mark the caudal border of the oral cavity.

  • Palate:

    • Hard Palate: Characterized by transverse ridges. Composed of components from the palatine, maxillary, and incisive bones.

    • Soft Palate: Muscle and connective tissue extending caudally from the hard palate.

  • Parotid Papilla: An eminence marking the opening of the parotid gland duct into the cheek. Located just rostral to the zygomatic gland duct.

  • Teeth: Identify incisor, canine, premolar, and molar teeth.

Dissection of Oral Cavity and Tongue Structures

  • Lingual Frenulum: Carefully dissect to observe its attachment, anchoring the tongue to the floor of the oral cavity.

  • Intrinsic Muscles: Cut a transverse section halfway along the length of the tongue. Observe that the extrinsic muscles continue within the tongue itself in an ordered arrangement, contributing to its complex movements.

Nerves of the Tongue

  • Hypoglossal Nerve XII: By carefully probing caudal and medial to the digastricus muscle, follow this large rostrally-running nerve as it enters the tongue, responsible for intrinsic and extrinsic tongue muscle innervation.

Comparative Anatomy (Using Specimens, Skulls, and Radiographs)

  • Hypsodont Dentition (Horse vs. Cow):

    • Horse: Possesses hypsodont teeth, which have a long crown that erupts throughout life, an infundibulum filled with cementum, and a characteristic wear pattern. The occlusal surface is complex.

    • Cow: Also possesses hypsodont teeth, but they are generally less complex in their folding and have a somewhat different pattern of enamel and dentin on the occlusal surface, adapted for rumination.

  • Tongue (Horse vs. Dog):

    • Horse: Generally less mobile and less prehensile than the dog's tongue. It's often thicker and less specialized for fine manipulation of food.

    • Dog: Highly mobile and agile, used extensively for grooming, lapping water, and manipulating food in the oral cavity. Possesses a prominent median sulcus.

Nasal and Vomeronasal Structures

Nasal Cavity Examination

  • Examine the sectioned surface of the head to appreciate the nasal cavity and associated structures.

Nasal Septum

  • Complete/Partial/Absent: In your specimen, determine if the nasal septum is complete, partially intact, or absent.

  • Cartilaginous and Bony Portions: Examine the extent of these different tissue types within the septum.

  • Removal: Carefully remove the septum (piece by piece) to reveal the underlying nasal mucosa and conchae.

Structures Revealed After Septum Removal

  • Nasal Conchae (Turbinate Bones): These bony projections extend into the nasal cavity.

    • Function: Their primary function is to increase the surface area of the nasal cavity, warm and humidify inspired air, and filter out particulate matter.

  • Nasal Mucosa: Peel off a small portion from the underlying bone to reveal its thickness. This mucosa is rich in blood vessels and glands.

Ethmoturbinates

  • Attachment: Attached to the cribriform plate, they are extremely delicate and covered by olfactory epithelium.

Paranasal Sinuses

  • Frontal Sinus: An air-filled cavity within the frontal bone.

  • Maxillary Recess: A caudal extension of the nasal cavity into the maxilla, often considered a rudimentary sinus in dogs.

Radiographic Identification

  • Canine Head Radiographs: Practice identifying the cribriform plate, nasal conchae, frontal sinus, and maxillary recess on radiographs of the canine head.

Rostral Head Examination

  • Incisive Papilla: A rounded eminence located a little caudal to the upper central incisors.

  • Incisive Duct: Slit-like openings found on each side of the incisive papilla. These ducts communicate with the oral, nasal, and vomeronasal organ cavities.

  • Vomeronasal Organ (Jacobson's Organ):

    • Description: Paired tubes of olfactory epithelium, enclosed by a scroll of cartilage, located in the rostroventral part of the septum.

    • Opening: They open into the incisive duct.

    • Role: Plays a crucial role in sexual behavior and recognition, particularly through the 'flehmen reaction' where animals curl back their lips to draw air into the organ.

Canine Skull Identification

  • Bony Foramen: On the canine skull, locate the bony foramen through which the incisive duct passes.

  • Vomer Bone: Identify the vomer bone, which gives the vomeronasal organ its name and forms part of the nasal septum.

Nasal Cavity and Paranasal Sinuses (Horse)

  • Comprehensive Familiarization: Using skulls, radiographs, and models, thoroughly understand the sinus positions in the skull, their borders, interconnections, and communications with the nasal cavity.

  • Tracing Ducts/Canals: Trace the nasolacrimal duct and infraorbital canal as they traverse the maxillary sinus in the horse. These are important for clinical procedures.

  • Comparative Paranasal Sinuses: Compare the paranasal sinuses of the horse, cow, and dog. Horses have extensively developed paranasal sinuses, impacting their respiratory health, while cows and dogs have different configurations and extents.

Relationships within Paranasal Sinuses

  • Tooth Roots in Sinuses: Identify which paranasal sinuses hold the roots of the premolar and molar teeth. This is particularly relevant in horses for dental disease.

Anatomy of Passing an Endoscope or Stomach Tube in the Horse

Purposes of Procedures

  • Endoscopy: Used in equine practice to examine internal regions of the head, such as the larynx and guttural pouch (a diverticulum of the auditory tube).

  • Stomach Tube: Passed for the detection and relief of obstruction of the oesophagus and to introduce liquid medication into the stomach.

Instrument Pathway for Endoscope/Stomach Tube

  • 1. External Naris:

    • The instrument should be inserted ventrally to enter the true nostril.

    • It is crucial to avoid entering the nasal diverticulum (false nostril), which is a blind-ended sac.

  • 2. Ventral Meatus:

    • The conscious horse tolerates entry into the ventral meatus far more than into the middle meatus due to anatomical sensitivity.

    • The ventral meatus is appreciably ventral to the floor of the nostril.

    • To enter the ventral meatus, the instrument must be firmly directed ventrally by inserting a forefinger into the nasal cavity above the instrument to guide it.

  • 3. Nasopharynx:

    • After passing through the ventral meatus, the instrument enters the nasopharynx (the dorsal part of the pharynx).

  • 4. Common Pharynx:

    • The boundary between the nasopharynx and the common pharynx is the pharyngopalatine arch, located at the caudal end of the soft palate.

    • A stomach tube passes through this arch and the common pharynx, approaching the oesophageal pharynx and the laryngeal entrance.

  • 5. Oesophageal (Laryngo) Pharynx:

    • It is absolutely necessary to ensure that a stomach tube has entered the oesophageal pharynx and not the larynx (airway).

    • Once in the oesophageal pharynx, the instrument is then swallowed by the oesophagus, allowing it to proceed to the stomach.

Diagrammatic Representation (Nasal Cavity)

  • A diagram illustrating the position of the ventral meatus relative to the floor of the nasal cavity on the left side is important for visualization.

  • An arrow in such a diagram typically indicates the degree to which a stomach tube must be deflected ventrally to ensure it enters the ventral meatus, thereby preventing accidental entry into the nasal diverticulum or the middle meatus. The arrow is depicted entering the right nostril.

  • Key structures shown in such a diagram often include: outline of nasal diverticulum, nasal cartilage, dorsal turbinate, middle meatus, ventral turbinate, and ventral meatus.