The orbit is the conical cavity that contains the eyeball and the ocular adnexa. The orbital margin outlines the base of the cone, which is directed rostrolaterally. The shape of the base approximates a rounded trapezoid more than a true circle. The axis of the orbit, a line passing from the center of the base to the apex at the optic canal, is directed obliquely caudal and ventral. In mesaticephalic dogs, the axis of the orbit forms an angle of approximately 30 degrees with a median plane and 30 degrees with a dorsal plane. In the foreshortened skulls of the brachycephalic breeds, the axis of the orbit deviates as much as 50 degrees from the median plane. The eyeball occupies the base of the orbit and projects a variable distance rostral to the orbital margin. The average dog’s visual field encompasses approximately 250 degrees of arc. Binocular vision is confined to the central 60 degrees.
The orbital margin is bony for approximately four-fifths of its circumference. The caudolateral one-fifth of the margin is completed by the orbital ligament (ligamentum orbitale). In the brachycephalic dog the ligament forms a larger proportion of the circumference. The ligament is a thick fibrous band that unites the zygomatic process of the frontal bone with the frontal process of the zygomatic bone. The ligament serves as the lateral attachment of the m. orbicularis oculi and the lateral palpebral ligament. The dorsal and medial segments of the orbital margin are formed by the frontal bone. In most dogs, the lacrimal bone forms a small portion of the ventromedial orbital margin. In some brachycephalic skulls, however, the lacrimal bone is confined to the medial orbital wall and does not contribute to the orbital margin. In these cases only, the medioventral orbital margin is formed by the maxillary bone. The ventrolateral orbital margin is the orbital border of the zygomatic bone.
In humans the confines of the orbit are entirely bony and readily discernible from a study of the skull. In the dog, only the medial wall and part of the roof of the orbit are osseous. The lateral wall and floor are formed by soft tissue. Consequently, the orbit of the dog cannot be properly appreciated from study of a skeletal preparation. The anatomic difference between the orbit of the dog and that of the human is manifested by marked differences in diseases that affect the orbit and in surgical approaches to this area.
The medial wall of the orbit is formed primarily by the orbital part of the frontal bone. The orbital wing of the presphenoid forms the caudal part of the medial wall and contains the optic canal. The lacrimal bone contributes to a small portion of the rostroventral medial wall and contains the fossa for the lacrimal sac and the caudal orifice of the nasolacrimal canal.
Five foramina are found in the medial wall of the orbit. At the apex of the orbit are the optic canal and orbital fissure. The optic nerve and internal ophthalmic artery leave the cranial cavity through the optic canal. The orbital fissure between the basisphenoid and presphenoid bones gives passage to the oculomotor, trochlear, abducent, and ophthalmic nerves, the anastomotic branch of the external ophthalmic artery, and the ophthalmic venous plexus. The retractor bulbi muscle originates within the orbital fissure. Rostrodorsal to the orbital fissure are the two small ethmoidal foramina that transmit the external ethmoidal artery and the ethmoidal nerve. Rostrally, the fossa for the lacrimal sac occupies the center of the orbital face of the lacrimal bone. This fossa is continued rostromedially as the lacrimal canal, which contains the nasolacrimal duct.
The dorsally convex ventral orbital crest (crista orbitalis ventralis) of the frontal bone demarcates the boundary between the orbit dorsally and the more ventral pterygopalatine fossa. The crest is not prominent, and in unfleshed skulls the orbit appears to extend much farther ventrally than is actually the case. The ventral orbital crest is the dorsal boundary of the origin of the medial pterygoid muscle, which forms the medial third of the orbital floor. The zygomatic salivary gland rests on the dorsolateral surface of the medial pterygoid muscle. Its dorsal surface forms most of the floor of the orbit, from the orbital margin to nearly the optic canal. The maxillary artery and nerve cross the floor of the orbit near its apex. The pterygopalatine ganglion is just dorsal to the maxillary nerve on the dorsal surface of this muscle.
The medial aspect of the roof of the orbit is formed by the zygomatic process of the frontal bone. A very small foramen is often found in the midorbital face of the process, through which a small artery passes dorsally. In some skulls a palpable depression, the fossa for the lacrimal gland, is present on the ventral surface of the zygomatic process of the frontal bone at the origin of the orbital ligament.
The orbit is bounded dorsolaterally and laterally by the medial surface of the temporalis muscle and the orbital ligament. The ramus of the mandible is embedded in the masseter and temporal muscles immediately caudal to the orbit. When the mouth is opened, the dorsal aspect of the ramus of the mandible moves rostrally, compressing the orbital contents. Thus pain on opening the mouth is a cardinal sign of retrobulbar abscesses. Denervation atrophy of the masseter and temporal muscles effectively enlarges the orbit, and a sinking of the eye into the orbit (enophthalmos) results. Conversely, swelling of the muscles of mastication, as in masticatory myositis, results in exophthalmos. Periorbital fat cushions the globe within the orbit.
Because the floor of the orbit is composed entirely of soft tissue, a retroorbital abscess can be drained into the oral cavity by blunt dissection caudal to the last molar tooth. Similarly, the lateral aspect of the orbit can be explored surgically without osseous resection.
The relationship of the orbit to surrounding structures is well illustrated in Hamon’s Atlas of the Head of the Dog (1977).
The zygomatic salivary gland (formerly the orbital or dorsal buccal gland) forms the lateral two-thirds of the floor of the orbit. When diseased it may produce ocular manifestations.
The zygomatic gland lies dorsal and lateral to the pterygoid muscles and ventral to the periorbita and the ventral orbital margin. Rostrally and laterally it is bounded by the orbital surface of the zygomatic bone. The gland extends caudally almost to the optic canal. It is roughly pyramidal, tapering caudomedially. The surface of the gland is lobulated and covered by a thin capsule and a layer of fat.