504 THE ANATOMY OF VERTEBRATED ANIMALS. 



are tliree permanent molars. Consequently the dental formula 

 of the adult Horse is 



• ^'^ 1—1 3-3 3-3 , „ 



i. 3 . 3 C. 1 — I ^^7?1. 3 . 3 171, 3 . 3 4:U. 



The permanent canines are the last teeth to be fully de- 

 veloped, and, in the mare, they do not often make their appear- 

 ance. 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 interval, 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 produced. 

 As the incisors wear down, the mark changes its form in con- 

 sequence of the differences in the transverse section of the 

 valley at different 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 indica- 

 tion of age. The structure and patterns of the grinding sur- 

 faces 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 imjDortant 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 succeeds 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 sharpl}'^ distin- 

 guished by the dense epithelium which lines the inner surface 

 of the former. 



The cascum is enormous, having fully twice the volume of 

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

 veloped in the septum of the heart. There is no Eustachian 

 valve, and only one anterior cava remains. The aorta divides 

 immediately after its origin into an anterior and a posterior 

 trunk ; the latter becomes the thoracic aorta ; the former is 

 the source of the arteries for the head and the anterior ex 

 tremities, giving off first the left subclavian, and then as 

 an " innominata " supplying the right subclavian and the 

 carotids. 



The trachea divides into only two bronchi, no accessory 

 bronchus being given off to the right lung. In the brain the 

 following points are worthy of notice : The medulla oblongata 



