Temporalis muscle

The temporalis muscle is located in the temporal fossa, the shape of which it follows.

Conformation: It is a large muscle, very thick and springy in carnivores, weaker in herbivores. As the shape of the temporal fossa suggests, it is thin in equines, thicker and narrower in ruminants and rabbits. Its fibers converge on a very strong tendinous blade which continues to the coronoid process of the mandible, receiving the fleshy fascicles on these two faces. In many species, a more or less distinct accessory part is added to the main part described earlier. It consists of a large bundle attached to the infratemporal crest of the sphenoid bone and the adjacent part of the temporal fossa. Very distinct in rabbits, this bundle is much less distinct in ungulates and humans; it is not discernible in carnivores, which on the other hand have a superficial accessory fasciculus, originating from the temporal crest.

Insertions: The temporalis muscle originates in most of the temporal fossa, as well as the temporal line, external sagittal crest, and infratemporal crest. In addition, its most superficial fleshy fascicles attach to the deep surface of the temporal fascia. It ends with its tendinous blade on the coronoid process of the mandible, which appears to be an ossified part, and on the rostral edge of the branch of this bone.

Relations: The temporalis muscle is covered by the temporal fascia which converts the temporal fossa into an almost completely closed compartment. This fascia, thinner in domestic mammals than in humans, originates all around the temporal fossa, that is to say the orbital ligament, the caudal edge of the zygomatic process of the frontal bone, the temporal line , the nuchal crest and the temporal crest, with the specificity in humans of the interval between the upper and lower temporal lines, in equines and carnivores the external sagittal crest.

It splits ventrally into a superficial layer which is worn on the lateral side of the zygomatic arch and tends to be in continuity with the masseteric fascia and a deep layer which is worn on the medial side of the zygomatic arch; between these two layers circulate vascular and nervous branches within a connective tissue that is much less abundant in animals than in humans.

The temporal compartment delimited by this fascia is open ventrally under the zygomatic arch.

Through this opening, the temporalis muscle is placed on the rostral edge of the mandibular ramus. It is accompanied by a voluminous orbito-temporal adipose body which, through its diffusion, promotes the movements of the temporomandibular joint and the movements of neighboring organs during mastication. Variously developed depending on the species but always present, this adipose bundle is delimited by a thin connective envelope. It begins in the sub zygomatic region, on the deep side of the masseter muscle, goes up behind the alveolar tuberosity between the end of the temporalis muscle and the orbital sheath and ends at the rostral part of the temporal fossa. in equines this extremity is very wide.

The superficial surface of the temporalis muscle, attached by its dorsal part to the temporal fascia, is separated by the latter from the scutiform cartilage and the auricular muscles as well as the branches of the superficial temporal vessels and their satellite nerves. The deep surface is related to the bones of the temporal fossa as well as the deep temporal vessels and nerves. The part attached to the infratemporal crest exchanges bundles with the deep part of the masseter muscle. It is also related to the fibrous sheath of the orbit and on the medial side with the pterygoid muscles and branches of the maxillary vessels and nerve.

Functions: The temporalis muscle contributes to the elevation of the mandible, which it tilts around the temporomandibular joint; it thus acts on an inter-powerful lever. The infratemporal fasciculus further contributes to the adduction of the mandible.

Vessels and nerves: Blood is supplied by the deep temporal arteries and collaterals of the maxillary artery. The veins anastomose with the pterygoid plexus. The deep temporal nerves originate from the mandibular nerve, a branch of the trigeminus.

In equines, the temporalis muscle is flat, relatively wide, covered with a pearly aponeurosis which narrows but increases at the level of the ending. Its infratemporal bundle is paler in color than the rest of the muscle, but no less distinct. The highly developed orbito-temporal adipose body forms a fluctuating mass behind the orbit which occupies the so-called “salt shaker” depression and is perceptible through the skin, the rostral auricular muscles and the temporal fascia.