This muscle acts especially during mastication; it serves to press back again under the molar teeth the portions of food which fall outside the dental arch.
In the pig, the ox, and the horse, a muscle which is considered as supplemental to the buccinator is placed along the inferior border of the latter.
This muscle, which we describe separately under the names of maxillo-labialis ([Fig. 91], 10; [Fig. 92]) and depressor of the lower lip, is clearly distinct from the buccinator, especially in the horse. It arises, behind, with the deep layer of the muscle to which it is annexed, from the anterior border of the ramus of the lower jaw; in front it terminates in the thickness of the lower lip.
In the ox, it is more intimately united with the buccinator.
It depresses the lip to which it is attached, and displaces it laterally when it acts on one side only.
In the human species, the pinna of the ear being generally immobile, the muscles which belong to it are, very naturally, considerably atrophied. Accordingly, the auricular muscles, anterior, superior, and posterior, are reduced to pale and thin fleshy lamellæ, whose action is revealed in certain individuals, only in a way which may be said to be abnormal.
It is not the same in quadrupeds. The pinna of the ear is extremely mobile, and its displacements have a real value from the point of view of physiognomical expression. It is therefore necessary to review the muscles which move this pinna without giving them, at the same time, more importance than they merit, since in themselves they do not determine the formation of surface reliefs, which are sufficiently apparent.
Notwithstanding that for certain of these muscles it is possible to trace their analogy with those of the auricular region of man, it is very difficult, because of their complexity, to trace this analogy for all. This is why we shall not be able here, as we have done for the other muscles of the subcutaneous layer, to give at the head of each paragraph the name of a human muscle, and then to group in the same paragraph the muscles which correspond to it in different quadrupeds. Therefore the nomenclature and the divisions adopted for these latter must serve us as a base or starting-point.
Because the pinna of the horse’s ear is so very mobile, we will first begin with a study of its auricular muscles.
Zygomatico-auricularis ([Fig. 92], 11).—This muscle, which is formed of two small bands of fleshy fibres, arises from the zygomatic arch in blending with the orbicular muscle of the eyelids; thence it is directed towards the base of the pinna of the ear, and is inserted into this base, and also into the cartilaginous plate situated in front of and internal to this, and resting on the surface of the temporal muscle; this is the scutiform cartilage.