356 THE ANATOMY OF THE HORSE 



presenting its broad base forwards, and is connected with the temporal, 

 posterior maxillary, and lachrymal bones. The lachrymal hones form the 

 internal corners of the orbits, where they are very thin, and are hollowed 

 out to receive the lachrymal sac, and give passage to its duct. The pa^oYe 

 hones are of a very irregular shape, and are connected with the frontal, 

 jethmoid, sphenoid, posterior maxillaiy, inferior turbinated bones, and 

 vomer. The turhinated hones, two in each nasal cavity, are attached to 

 its outer walls one above the other, and appear to be intended merely for 

 the purpose of extending the surface for the expansion of the olfactory 

 nerve. The vomer resembles in shape the share of the plough, after which 

 it is named, and rises up in the middle line of the back of the division 

 between the nostrils. 



THE LOWER JAW 



The lower jaw resembles in general shape the letter V, the point being 

 rounded off forwards and receiving the incisor teeth, while the two branches 

 are broad, thin, and slightly cux'ved, being surmounted by the condyles for 

 articulation with the glenoid cavity of the temporal bone, and giving 

 lodgment to the tushes and grinders. The two lines of alveolar cavities are 

 not so wide as those of the upper jaw, and consequently the two sets of 

 teeth do not exactly correspond. 



THE TEETH 



The teeth are developed within their appropriate cavities or sockets, 

 which are found exactly corresponding with their number in the upper 

 and lower jaws, being narrower in the lower than in the upper. Before 

 birth they are nearly all in a state of incomplete growth, covered and con- 

 cealed by the gums, but soon afterwards they rise through it in pairs, the 

 first set, or milk teeth, being in course of time superseded by the permanent 

 teeth as in all the mammalia. The following is the formula of the com- 

 plete dentition of the horse : — 



Incisors f , canine #, molars yi. 



Each tooth is developed within its corresponding cavity in the jaw, and 

 is made up of three distinct substances — cement, enamel, and dentine. 

 The cement of the horse's tooth (sometimes called crusta petrosa) closely 

 corresponds in texture with his bone, and, like it, is traversed by vascular 

 canals. The enamel is the hardest constituent of the tooth, and consists of 

 earthy matter arranged in the animal matrix, but contained in canals, so as 

 to give the striated appearance which it presents on splitting it open. 

 Dentine has an organized animal basis, presenting extremely minute tubes 

 and cells, and containing earthy particles, which are partly blended v/ith the 

 animal matter in its interspaces, and partly contained in a granular state 

 within its cells. These three substances are shown in the annexed sections 

 of an incisor tooth, one of which, Fig. 4, is of the natural size, while the 

 other. Fig. 5, is shown under the microscope. 



In the molar teeth the arrangement of these three substances is the 



