Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/191

179 ANATOMY.] HORSE 170 of which becomes partially filled up with crusta petrosa or cementum. As the tooth wears, the surface, besides the external enamel layer as in an ordinary simple tooth, shows in addition a second inner ring of the same hard substance surrounding the pit, which of course adds greatly to the efficiency of the tooth as an organ for biting tough, fibrous substances. This pit, generally filled in the living animal with particle? of food, is conspicuous from its dark colour, and constitutes the &quot;mark&quot; by which the age of the horse is judged, as in consequence of its only extending to a certain depth in the crown it becomes obliterated as the crown wears away, and then the tooth assume? the charac ter of that of an ordinary incisor, consisting only of a core of dentine, surrounded by the external enamel layer. It is not quite so deep in the lower as in the upper teeth. c. Yin. 6. Longitudinal and transverse section of upper incis ir of horse, p, pulp cavity; &amp;lt;i. dent me or ivory; e, enamel; ~c, outer layer of cementum or crusta petrosa ; e, inner layer of cementum, lining a, the pit or cavity of the crown of the tooth. The canines are either quite rudimentary or entirely absent in the female. In the male they are compressed, pointed, and smaller than the incisors, from which they are separated by a slight interval. The teeth of the molar series are all in contact with each other, but separated from the canines by a considerable toothless space. The anterior premolars are quite rudimentary, sometimes not developed at all, and generally fall by the time the animal attains maturity, so that there are but six functional grinding teeth, three that have predecessors in the milk dentition, and hence are considered as pramolars, and three true molars, but other wise, except the first and last of the series, not distinguish able in form or structure. These teeth in both upper and lower jaws are extremely long-crowned or hypsidont, suc cessive portions being pushed out as the surface wears awny, a process which continues until the animal becomes advanced in age. The enamelled surface is infolded in a complex manner (a modification of that found in other perissodactyles), the folds extending quite to the base of the crown, and the interstices being filled and the surface covered with a considerable mass of cement, which binds together and strengthens the whole tooth. As the teeth wear, the folded enamel, being harder than the other con stituents, the dentine and cement, forms projecting ridges on the surface arranged in a definite pattern, which give it great efficiency as a grinding instrument (see fig. 1, I and r). The free surfaces of the upper teeth are quad rate, except the first and last, which are nearly triangular. Thelow^r teeth are much narrower than the upper. The milk dentition consists of i. f, c., m. = 24, the canines and first or rudimentary premolars having appar ently no predecessors. In form and structure they much resemble the permanent teeth, having the same characteris tic enamel foldings. Their eruption commences a few days after birth, and is complete before the end of the first year, the upper teeth usually appearing somewhat earlier than those of the lower jaw. The first teeth which appear are the first and second milk molars (about five days), then the central incisor (from seven to ten days) ; this is followed by the second incisor (at one month), then the third molar, and finally the third incisor. Of the permanent teeth the first true molar appears a little after the end of the first year, followed by the second molar before the end of the second year. At about two and a half years the first premolar replaces its predecessor. Between two and a half and three years the first incisor appears. At three years the second and third premola-s, and the third true molar have appeared, at from three and a half to four years the second incisor, at four to four and a half years the canine, and, finally, at five years, the third incisor, completing the permanent dentition. Up to this period the age of the horse is clearlv shown by the condition of dentition, and for some time longer indications can be obtained from the wear of the incisor teeth, though this depends to a certain extent upon the hardness of the food or other accidental circumstances. Asa general rule, the depression caused by the infolding of the surface of the incisor (the &quot; mark &quot;) is obliterated in i the first or central incisor at six years, in the second at seven years, and in the third at eight years. In the upper teeth, as the depressions are deeper, this obliteration does i not take place until about two years later. After this
 * period no certain indications can be obtained of the age of
 * the horse from the teeth.

Digestive Organs. The lips are flexible and prehensile. The membrane that lines them and the cheeks is quite surface has seventeen pairs of not very sharply defined oblique ridges, extending as far back as the last molar tooth, beyond which the velum palati extends for about 3 inches, having a soft corrugated surface, and ending posteriorly in an arched border without uvula. This embraces the base of the epiglottis, and, except while swallowing food, shuts off all communication between the cavity of the mouth and the pharynx, respiration being, under ordinary circum stances, exclusively through the nostrils. Between the mucous membrane and the bone of the hard palate is a dense vascular and nervous plexus. The membrane lining the fauces is soft and corrugated. An elongated raised glandular mass, 3 inches long and 1 inch from above down wards, extending backwards from the root of the tongue along the side of the fauces, with openings on the surface leading into crypts with glandular walls, represents the tonsil. The tongue, corresponding to the general form of the mouth, is long and narrow. It consists of a compressed intermolar portion with a flat upper surface, broad behind and becoming narrower in front, and of a depressed anterior part rather shorter than the former, and which is narrow behind and widens towards the evenly rounded apex. The dorsal surface generally is very soft and smooth. There are two large circumvallate papillae near the ba?c, rather irregular in form, about a quarter of an inch in diameter and half an inch apart. The conical papilla&quot; 1 are very small and close set, though longer and more filament ous on the intermolar portion. There arc no fungiform papilla} on the dorsum, but a few not very conspicuous ones scattered along the sides of the organ. Of the salivary glands the parotid is by far the largest, and elongated in the vertical direction, and narrower in the middle than at either upper or lower extremity. Its upper extremity embraces the lower surface of the cartilaginous ear-concli ; its lower end reaches the level of the inferior margin of the mandible, along the posterior margin of which
 * smooth. The palate is long and narrow ; its mucous