In the ox, it arises from the maxillary spine, and then divides into three parts; the superior passes under the internal portion of the internal elevator of the upper lip and the alæ of the nose, and goes into the nostril; whilst the two others, situated lower down, terminate in the upper lip. In the pig, it is formed of two superimposed fasciculi, which arise from the spine of the maxilla and the impressions in front of it. These two fasciculi terminate in the snout, which they move laterally. In the horse, it is situated at a certain distance from the external elevator; in the preceding animals it is in contact with the latter. Arising behind from the external surface of the maxilla, in front of the maxillary spine, it is directed towards the anterior part of the face, passes under the external portion of the internal elevator (it is the opposite of this in the ox), and proceeds, on expanding, to terminate in the skin of the nostril. By its contraction it dilates the latter. =Orbicularis Oris= (Fig. 91, 8; Fig. 92).--This muscle, very fleshy in the solipeds and the ruminants, is arranged as a ring round the buccal orifice, in the thickness of the lips, where it is blended with the other muscles of this region. Having for its function the narrowing of the orifice it surrounds, it acts during suction and in the prehension of food. =Triangularis Oris.=--This muscle does not exist in domestic quadrupeds. =Quadratus Menti.=--In the pig and the carnivora, it arises from the anterior part of the body of the inferior maxillary bone, and passes at the other end to terminate in the corresponding portion of the lower lip, which it depresses by its contraction. In the ox and the horse this muscle does not exist; it is replaced for the depression of the lower lip, which it affects in other animals, by supplemental fibres of the buccinator. =The Prominence of the Chin.=--Below the lower lip in the horse is situated the so-called _prominence of the chin_, limited posteriorly by the _beard_, a depressed region which gives point to the curb of the bridle. The prominence, which also exists in the ox, is a fibro-muscular pad which blends with the orbicular muscle of the lips, and receives on its superior aspect the insertion of the two muscles (_levator menti_) by which it is suspended. These arise, above, on each side of the symphysis of the inferior maxillary bone. They raise the lower lip with force, and they are the agents which, as we can sometimes observe in the horse, make it click against the upper lip, suddenly projecting it upwards. This action sometimes becomes a habit, and its continuance constitutes a vice. A corresponding structure is found in the pig and in the carnivora, but in them it does not produce an external prominence such as we have described.