356 THE ANATOMY OF VEETEBEATED ANIMALS. 



The first deciduous molar usually falls out wben the 

 first premolar appears, and is not replaced; but it is occa- 

 sionally retained. All the other milk teeth have successors, 

 and there are three permanent molars. Consequently the 

 dental formxila of the adult Horse is 



. 3-3 I— I 3.3 3-3 .n 



^•^''•I=I^*^-3-3''^-^ = ^^- 



The permanent canines are the last teeth to be fully 

 developed, and, in the mare, they often do not make their 

 appearance. The upper canines are distant from the outer 

 incisors, while the lower canines are quite close to them. 

 In both jaws there is a wide intei'val, or diastema, between 

 the canines and the premolars. 



The deep valley of the incisor teeth becomes filled up 

 with masticated matter, and thus the dark " mark " is pro- 

 duced. As the incisors wear down, the mark changes its 

 foiTn in consequence of the difierences in the transverse 

 section of the valley at difi'erent points; and eventually, 

 when the wear has extended beyond the bottom of the 

 valley, it disappears. The presence or absence of the 

 " mark " thus serves as an indication of age. The structui-e 

 and patterns of the grinding surfaces of the permanent 

 molars are essentially the same as those of the milk molars ; 

 but the enamel becomes more or less plaited ; and, at an 

 advanced period of life, the development of the long teeth 

 is completed by the formation of roots. It is important to 

 notice that the last molar of the Horse is not more complex in 

 its structure than the other molars, and that the last milk 

 molar is not more complex than the premolar which suc- 

 ceeds it. 



The alimentary canal of the Horse is about eight times 

 as long as the body. The stomach, simple in its form,^ 

 presents a cardiac and a pyloric division, which are sharply 

 distinguished by the dense epithelium which lines the inner 

 surface of the former. 



The coecum is enormous, having fully twice the voliune 

 of the stomach. There is no gall-bladder. A cartilage 

 is developed in the septum of the heart. There is no 



