Week 3 Dissection

Guide to the Dissection of the Dog

Anatomical Structures of the Skull

Orbital Fissure and Associated Nerves

• Orbital Fissure Formation: The orbital fissure is formed by an articulation between the basisphenoid and presphenoid bones.

• Optic Nerve Passage: The optic nerve passes through the optic canal.

• Nerves and Vessels Through Orbital Fissure:

  • The following cranial nerves and vessels pass through the orbital fissure:

  • Oculomotor nerve (cranial nerve III)

  • Trochlear nerve (cranial nerve IV)

  • Abducent nerve (cranial nerve VI)

  • Ophthalmic nerve (a branch of trigeminal nerve, cranial nerve V1)

Vascular Supply to the Maxilla

• Rostral Alar Foramen: Emerging from this foramen are the maxillary artery and maxillary nerve.

• Pterygopalatine Fossa Foramina:

  • Several foramina are present in the rostral part of the pterygopalatine fossa.

  • Caudal Palatine Foramen and Sphenopalatine Foramen:

  • These are closely related openings of approximately equal size located on the rostromedial wall of the pterygopalatine fossa.

  • The sphenopalatine foramen is dorsal to the caudal palatine foramen.

  • Major Palatine Structures:

  • Major palatine artery, vein, and nerve enter the palatine canal through the caudal palatine foramen, coursing toward the hard palate.

  • Sphenopalatine artery and vein along with the caudal nasal nerve enter the nasal cavity via the sphenopalatine foramen.

Maxillary Foramen

• The maxillary foramen serves as the caudal opening of the infraorbital canal.

• The infraorbital artery, vein, and nerve ascend rostrally through the infraorbital canal.

• Open Defect in Rostromedial Wall:

  • Just caudal to the maxillary foramen, this deficient area is typically occupied by a thin plate of bone, which acts as the origin of the ventral oblique eye muscle.

• Alveolar Canals:

  • Numerous small openings caudal to the maxillary foramen serve small nerves and vessels passing through alveolar canals to the roots of the molar teeth and the caudal root of the last premolar.

• Lacrimal Bone:

  • Above the maxillary foramen is a shallow fossa for the lacrimal sac, which continues rostrally via the nasolacrimal canal for the nasolacrimal duct.

Ventral Surface of the Skull

• Cranial Bones: The ventral aspect of the cranium comprises:

  • Basioccipital Bone

  • Tympanic and Petrosal Parts of the Temporal Bone

  • Basisphenoid Bone

  • Presphenoid Bone

• Basioccipital Bone:

  • Forms the caudal third of the cranial base.

  • Articulates laterally with the tympanic and petrosal parts of the temporal bone.

  • Articulates rostrally with the body of the basisphenoid.

  • The occipital condyle articulates with the atlas (the first cervical vertebra).

  • The paracondylar process projects ventrally and articulates with the caudolateral part of the tympanic bulla.

• Digastricus Muscle: This muscle arises from the paracondylar process.

• Tympanic Part of Temporal Bone:

  • Contains a bulbous enlargement called the tympanic bulla, which encloses the middle ear cavity and its ossicles.

  • The external acoustic meatus is located on the lateral side of the bulla.

  • The tympanic membrane typically covers the opening of the external acoustic meatus, with annular cartilage attached around the periphery.

• Petrosal Part of Temporal Bone:

  • Houses the membranous and bony labyrinths of the inner ear.

  • When the tympanic bulla is removed, a barrel-shaped eminence known as the promontory becomes visible, which contains the cochlear window (closed by a membrane in life).

  • The vestibular window is dorsal to the promontory, housing the footplate of the stapes.

• Ossicular Chain:

  • The stapes articulates with the incus; the incus in turn articulates with the malleus, which is attached to the medial side of the tympanic membrane.

• Mastoid Process: Small structure that is the only external part of the petrosal portion of the temporal bone, located caudal to the external acoustic meatus.

• Muscle Attachments: The cleidocephalicus, sternocephalicus, and splenious muscles insert onto the mastoid process.

Basisphenoid and Related Openings

• Articulations: The basisphenoid articulates caudally with the basioccipital and rostrally with the presphenoid and pterygoid bones.

• Foramina in Basisphenoid: Includes the oval foramen, round foramen, and alar canal.

• Presphenoid Articulations:

  • Articulates caudally with the basisphenoid and pterygoid bones, and laterally with the perpendicular part of the palatine.

  • Only a minor part of the presphenoid bone is exposed on the ventral surface.

  • Optic Canals: Pass through the orbital wing of the presphenoid.

• Rostral Alar Foramen: Acts as the rostral opening to the alar canal, which ends at the caudal alar foramen.

Maxillary Nerve and Alar Canal

• The maxillary nerve from the trigeminal nerve enters the alar canal from the cranial cavity through the round foramen, traverses rostrally, and exits the canal at the rostral alar foramen.

• The maxillary artery travels through the entire length of the alar canal.

• Oval Foramen: A significant opening into the cranial cavity located caudolateral to the caudal alar foramen. It allows passage for the mandibular nerve from the trigeminal nerve.

Foramen Lacerum and Musculotubal Canal

• Foramen Lacerum: Located at the rostromedial edge of the tympanic bulla, it houses a loop of the internal carotid artery linking the carotid canal and cavernous sinus in the cranial cavity.

• Musculotubal Canal: Lies lateral to the foramen lacerum, housing the auditory tube connecting the middle ear to the nasopharynx.

• Tympano-Occipital Fissure: An oblong opening between the basilar part of the occipital bone and the tympanic part of the temporal bone through which the carotid canal transmits the internal carotid artery and the petro-occipital canal transmits the ventral petrosal venous sinus.

Nerves Through Tympano-Occipital Fissure

• Cranial nerves including the glossopharyngeal, vagus, and accessory nerves course from the jugular foramen through the tympano-occipital fissure, along with the internal carotid artery, and venous radicles of the vertebral veins and sympathetic axons from the cranial cervical ganglion.

• Hypoglossal Canal: Lies caudomedial to the tympano-occipital fissure within the occipital bone, accommodating the hypoglossal nerve.

Mandibular Joint and Process

• Mandibular Fossa: Corresponds with the condyle of the mandible to form the temporomandibular joint.

• Retroarticular Process: Forms the medial aspect of the mandibular fossa.

• Retroarticular Foramen: Located caudally and allows for the passage of the emissary vein from the temporal venous sinus.

• Stylomastoid Foramen: Located between the tympanic bulla and mastoid process; serves as the opening of the facial canal that transmits the facial nerve through the petrosal part of the temporal bone.

Facial Bones and Hard Palate

• Ventral Surface of Facial Skull:

  • Characterized by the teeth and hard palate.

  • Two dental arches: superior (maxillary) and inferior (mandibular).

  • Teeth situated in alveoli (sockets) separated by interalveolar septae; those with multiple roots further divided by interradicular septae.

  • Each dental arch has designated alveoli for each tooth type (incisors, canines, premolars, molars):

  • Incisive bones host single alveoli for incisors.

  • Maxilla has single alveoli for canine and first premolar, double for second and third premolars, triplet for molars and fourth premolar.

• Composition of Hard Palate:

  • Hard palate comprises horizontal parts of the palatine, maxillary, and incisive bones, with palatine fissures opening at canine teeth midline.

  • The palatine bones form the caudal third of the hard palate.

• Foramen Structure:

  • Major palatine foramen located medial to the fourth cheek tooth; minor palatine foramen situated caudal to this.

  • Major palatine artery, vein, and nerve emerge through these foramina.

  • Choanae: Openings into the nasopharynx located at the caudal end of the hard palate where the vomer articulates with the palatine bones.

Caudal Surface of the Skull

• Development of Occipital Bone: Occipital bone develops from paired exoccipitals (bearing condyles), a supraoccipital and basioccipital, forming lateral borders as nuchal crests where they meet parietals and squamous temporals.

• External Occipital Protuberance: A mid-dorsal structure formed from the fusion of the interparietal bone with the occipital bone at the caudal end of the sagittal crest.

• Foramen Magnum: A large opening facilitating the passage of the medulla as it transitions into the spinal cord.

• Mastoid Foramen: Located within the occipitotemporal suture, transmitting the caudal meningeal artery and vein; the remaining caudal skull surface is roughened for muscular attachment.

• Skeletal Identification: Disarticulated skull studies enhance spatial understanding of bones aiding in radiographic feature interpretation; overlaps among bones may obscure individual boundaries in situ.

• Identifying Optic Canals: Noting the presphenoid bone outline along with significant adjacent bones on articulated and disarticulated skulls assists in anatomical comprehension.

• Basicranial Axis: The presphenoid, basisphenoid, and basioccipital bones comprise this fundamental axis supporting the brain.

Mandible Structure

• Mandible Composition:

  • Consists of two mandibles forming the lower jaw, each bearing the inferior teeth and articulating with the mandibular fossa of the temporal bone, joining at the intermandibular articulation.

  • Division of Mandible: Each mandible can be classified into the body (horizontal part) and ramus (vertical part).