Week 8 Dissection

Guide to the Dissection of the Dog Paranasal Sinuses

• There are three frontal sinuses located between the outer and inner tables of the frontal bone:

  • Lateral Frontal Sinus:

  • Much larger than the other two sinuses.

  • Only one of clinical relevance, size and shape varies with the skull type.

  • Occupies the zygomatic process, extends caudally.

  • Boundaries: laterally by the temporal line, medially by the median septum.

  • May be partially divided by bony septa.

  • Ethmoidal ectoturbinates project into the rostral floor.

  • Rostral Frontal Sinus:

  • Small size, located between the median plane and the orbit.

  • The ethmoid labyrinth bulges into this sinus.

  • Medial Frontal Sinus:

  • Very small, lies between the median septum and the walls of the other two sinuses.

  • May be absent entirely.

• Communication with Nasal Cavity:

  • All three sinuses connect with the nasal cavity.

Maxillary Recess

• Communication: Opens into the nasal cavity.

• Location: Opening in a transverse plane through the rostral roots of the superior fourth premolar tooth.

• Continuation: Extends caudally through a plane that runs through the last molar tooth.

• Formation of Walls:

  • Laterally and ventrally formed by maxilla.

  • Medially formed by the orbital lamina of the ethmoid bone.

• Lateral Nasal Gland:

  • Occupies the rostral portion of the maxillary recess.

  • Duct opens rostrally into the dorsal vestibule.

  • Secretion prevents desiccation from nasal panting.

Tympanic Cavity

• Definition: Cavity of the middle ear.

• Functions: Houses auditory ossicles.

• Communication: Connects with the nasopharynx via the auditory tube.

• Boundaries:

  • Ventrally by the tympanic bulla.

  • Dorsally by the petrosal part of the temporal bone.

  • Laterally closed by the tympanic membrane.

Joints of the Head

• Atlanto-Occipital Joint:

  • Described alongside the vertebrae.

• Temporomandibular Joint:

  • Between the condylar process of the mandible and the mandibular fossa of the zygomatic process of the squamous part of the temporal bone.

  • Articulation is elongated transversely.

  • Includes a thin cartilaginous articular disc dividing the joint capsule into two compartments.

  • Lateral and caudal ligaments strengthen the joint capsule.

• Intermandibular Articulation:

  • A synchondrosis that features an interdigitating surface, persists throughout life in most dogs.

Palpation of Dog's Skull Features

• Palpation Points:

  • Widest part: palpable zygomatic arch.

  • Zygomatic process of the frontal bone (widest point).

  • Locate orbital ligament between the zygomatic bone and frontal process.

• Follow the Skull:

  • Frontal bone caudally to the temporal line and sagittal crest.

  • Feel the external occipital protuberance and nuchal crest down to the mastoid process.

• Atlanto-Occipital Joint Movement:

  • Flex and extend; follow frontal bones to the nose and maxillary bones.

• Additional Palpation:

  • Mental foramina on the lateral side of the mandible.

  • Hyoid bones and infra-orbital foramen at the fourth premolar level.

Structures of the Head for Dissection

• Preparation for Dissection:

  • Head should be removed and divided on the median plane.

  • Complete transection of the neck at the level of the fourth cervical vertebra using a handsaw.

  • Sections should be thoroughly washed to remove bone dust and hair.

  • Skin both halves while preserving muscle positions, leaving narrow rim around eyelids and lips.

• Storage:

  • At the end of dissection, wrap in cheesecloth moistened with 1% phenol water or 2% phenoxyethanol, cover with a plastic bag to prevent desiccation.

Muscles of the Face

• Function: Open, close, or move lips, eyelids, nose, and ear.

• Innervation: All except an elevator of the upper eyelid are innervated by the facial (seventh cranial) nerve.

• Facial Anatomy:

  • Superior (upper) and Inferior (lower) Lips (Labia Oris):

  • Form lateral wall of vestibule, bounded medially by teeth and gums.

  • Cheek (Buccae):

  • Caudal to lips, forms lateral wall of vestibule.

  • Nose:

  • Region rostral to orbits, forms rostral portion of the respiratory passage.

Detailed Structure of Facial Muscles

• Platysma:

  • Cutaneous muscle from dorsal median raphe of the neck to the angle of the mouth.

  • Superficial, covering the ventrolateral surface of the face.

  • Action: Reflect to expose underlying structures.

• Orbicularis Oris:

  • Around the lips, lets the lips close; fibers from both sides end at median plane.

• Buccinator Muscle:

  • Thin, wide structure forming foundation of the cheek, attaches to alveolar margins of mandible/maxilla and adjacent mucosa.

  • Functions to return food from vestibule to occlusal surface of teeth.

• Levator Nasolabialis:

  • Flat muscle from maxillary bone to edge of superior lip and naris.

  • Actions: Dilation of nostril and raising superior lip.

Eyelid Anatomy and Muscles

• External Features:

  • Palpebrae: Upper (superior) and lower (inferior) eyelids surround the palpebral fissure.

  • Join at medial and lateral palpebral commissures with associated ligaments.

  • Medial Palpebral Ligament: Attaches medial commissure to frontal bone (near nasomaxillary suture).

  • Lateral Palpebral Ligament: Poorly developed, attaches to zygomatic bone.

• Inner Surface:

  • Covered by mucous membrane, known as palpebral conjunctiva, reflecting onto the globe of the eye as bulbar conjunctiva (Figs. 5-36).

  • Fornix: Angle formed at the reflection, bounded by the bulbar and palpebral conjunctiva.

  • Lacrimal Caruncle: Triangular prominence at medial commissure, with small openings called lacrimal punctum that begin the lacrimal ducts.

• Lacrimal Gland:

  • Located ventral to the zygomatic process of the frontal bone.

  • Secretes fluid into the dorsolateral part of conjunctival sac.

  • Fluid flow: through lacrimal duct -> lacrimal sac -> nasolacrimal duct -> nasal cavity for evaporation.

• Other Glands Contributing to Tear Secretion:

  • Gland of the third eyelid, conjunctival goblet cells, and tarsal glands of eyelids.

• Plica Semilunaris (Third Eyelid):

  • Concave fold of conjunctiva and cartilage, surrounds serous secretion that enters conjunctival sac under third eyelid.

External Ear Muscles

• Rostral Auricular Muscles:

  • Located on the forehead caudal to the orbit, converge toward auricular cartilage.

  • Must be transected at origins to explore underlying structures.

• Scutiform Cartilage:

  • Small, boot-shaped cartilaginous plate in the auricular muscles, isolated between muscle attachments.

• Caudal Auricular Muscles:

  • Largest group, arise from median raphe of the neck, attach to auricular cartilage.

Oral Cavity

• Division:

  • Vestibule: cavity outside teeth and gums, inside lips and cheeks, where ducts of salivary glands enter.

  • Oral Cavity Proper: bounded dorsally by hard palate and soft palate, laterally and rostrally by dental arches, ventrally by tongue.

• Communication:

  • Free connection to vestibule by interdental spaces, continues caudally into oropharynx.

Tongue Anatomy

• Structure:

  • Muscular organ with intrinsic and extrinsic muscles.

  • Divided into three parts:

  • Root: Caudal third of tongue.

  • Body: Long, slender rostral part.

• Dissection: Will be detailed later; acknowledgment of connection and structure is vital for further anatomical study.