Its posterior border is in relation with the parotid gland (Fig. 90, 14; Figs. 91, 92), this gland being situated between the corresponding border of the lower jaw bone and the transverse process of the atlas. Such are the general characters; the following are the particular ones: In the carnivora it is thick and convex. In the horse it is flat, but more expanded; it forms the _flat of the cheek_. In the ox it is flat, as in the latter; but, while being less thick, it is more prolonged in the vertical direction. The form of the osseous parts which give it origin is, besides, in relation with these differences, and explains the peculiar characters which the masseter presents. [Illustration: FIG. 89.--MYOLOGY OF THE DOG: MASTICATORY MUSCLES (A DEEPER DISSECTION THAN THAT SHOWN IN FIG. 90). 1, Zygomatic arch; 2, masseter; 3, temporal exposed by the suppression of the auricular and occipital muscles and the pinna of the ear; 4, auditory canal; 5, inferior maxillary bone; 6, digastric.] Indeed, in the dog and the cat the zygomatic arch, strongly convex, springs up in a marked manner from the plane of the lateral aspects of the skull. In the horse the same arch, less prominent externally, is prolonged by a rectilinear crest on the superior maxillary bone, where it is continued in forming the zygomatic or maxillary spine. In the ox the same crest ascends a good way towards the inferior margin of the orbit in a curved direction with the concavity inferior, to redescend afterwards on the external surface of the superior maxilla. The masseter is an elevator of the lower jaw. It acts, above all, as in the human species, in the process of mastication. =Temporal Muscle= (Fig. 89, 3).--The development of the temporal is in proportion to the energy of the movements of elevation which the lower jaw has to execute. It arises from the temporal fossa, and is inserted into the coronoid process of the inferior maxilla. Its development, enormous in the carnivora, is such that the muscle projects beyond its fossa. It is less voluminous in the horse, and still less so in the ox. In the latter, indeed, the temporal fossa, although deep, is of small extent (see Fig. 62, p. 119); the frontal bone being large, it is found to be thrown back on the lateral walls of the cranium, below the osseous processes which support the horns and overhang the fossa in question, as well as the muscle which it contains. It is covered by the auricular muscles, and by the base of the pinna of the ear. Like the masseter, the temporal is an elevator of the lower jaw. Cutaneous Muscles of the Head =Occipito-Frontalis.=--The epicranial aponeurosis is extremely thin. In the dog the occipital muscle occupies the superior part of the head; it overlies the temporal muscle.