The canine teeth are small in the horse, and rudimentary in the mare ; the unworn crown is remarkable for the folding in of the anterior and posterior margins of enamel, which here includes an extremely thin layer of dentine. The representative of the first premolar in the first set of teeth is a very small and simple rudiment, and is soon shed. The three normal premolars are as large and complex as the true molars, the anterior one being usually the largest of the series in the upper jaw.
Salivary The salivary apparatus in the Solipeda is very' extensive, perhaps more so than in any other class of quadrupeds, con sisting of large glandular masses divided into numerous lobes and lobules of a pale colour, and but loosely connected tog,ether by cellular tissue.
The Parotid Glands in the horse constitute a secreting apparatus, the bulk of which is extreraely remarkable. Each of these glands extends from the external meatus auditoreus along the side of the head and of the lower jaw, as far forwards as the masseter inuscle, and at the same time stretching deeply in wards as far as the side of the trachea. This enormous glandular organ may be considered as composed of three principal portions : each furnishing its excretory duct, which, however, soon unite to forrn a common canal, which at first descends within the angle of the jaw, whence, winding round the anterior edge of the masseter, it mounts up externally as far as the buccinator muscle, which it perforates nearly opposite the fourth molar tooth of the upper jaw, its internal orifice being situated in the centre of a prominent papilla.
The Submaxillary Glands are much smaller than the parotids. Posteriorly they consist of a thick globular portion, which is adherent to the inner surface of the parotid, but as they advance forwards, they become consider ably attenuated, each terminating in its appro priate duct. The latter is of considerable length, and, after passing the sublingual gland. with which it contracts some attachnients, opens into the mouth at a little distance be hind the canine tooth, its opening being in the immediate vicinity of a papilla that seeins to form a kind of valve at its orifice.
The Sublingual Glands are smaller than the preceding, and are of an oblong shape : they pour the saliva that they secrete into the cavity of the mouth through numerous orifices arranged in several rows on each side of the tongue.
In addition to the above large glandular organs, there remain to be noticed the Molar Glands, consisting of numerous detached granu lar-looking bodies of a lenticular shape situated beneath the mucous membrane that lines the buccinator muscle, and the inner surface of the superior maxillary bone behind which they mount up into the zygomatic fossa to within a little distance of the abductor muscle of the eye.
Pharynx. — The. pharynx in quadrupeds generally presents a structure very similar to that of the human race, and may be said to be composed of analogous muscles : nevertheless its horizontal position in these animals renders the necessity for muscular exertion during deglutition greater than in man ; and, accord ingly, these fibres are not only stronger in quadrupeds than in our own persons, but sometimes additional muscles are met with, by the aid of which the action of swallowing is facilitated.
In the horse, the muscle which represents the middle constrictor of the pharynx might more properly be called the pharyngeus#, its fibres descending from the pterygoid and palate bones, along the sides of the pharynx, around which they wind obliquely, uniting in the middle line upon its posterior surface, where they form a thick muscular layer.
The inferior constrictor, or thyro-pharyngeus, is equally broad and strong, its fleshy fibres taking nearly the same direction as they pro ceed towards the back of the pharynx, where they join by a median raphe.
In addition to the above, there is a crico pharyngeus, arising from the posterior and inferior margin of the cricoid cartilage, whence its fibres extend obliquely upwards along the sides of the pharynx.
The analogue of the stylo-pharyngeus is, in the Solipeds, a cylindrical muscle derived from the styloid bone, and, running from be hind forwards upon the sides and upper part of the pharynx, mixes its fibres with those of the superior constrictor —its action is to raise the commencement of the pharyngeal sac, which it at the same time dilates and draws back ward.