3 8 2 



ELEMENTARY ZOOLOGY 



immovably articulated together. The correspondence 

 between the bones of the two sets of limbs is very ap- 

 parent. The number of digits varies in different mammals, 

 and also in the fore and hind limbs of a single species. 

 Among the Ungulates the reduction in the number of 

 digits is especially noticeable; the forefoot of a pig has 

 four digits, that of the cow two, and that of the horse one. 

 The two short " splint " bones in the horse are remnants 

 of lost digits. The teeth are important structures in 

 mammals, being used not only for tearing and masticat- 

 ing food, but as weapons of offence and defence. A tooth 

 consists of an inner soft pulp (in old teeth the pulp may 

 become converted into bone-like material) surrounded by 

 hard white dentine or ivory, which is covered by a thin 

 layer of enamel, the hardest tissue known in the animal 

 body. A hard cement sometimes covers as a thin layer 

 the outer surface of the root, and may also cover the 

 enamel of the crown. The teeth in mcst forms are of 

 three groups: (a) the incisors, with sharp cutting edges 

 and simple roots, situated in the centre of the jaw; (&) the 

 canines, often conical and sharp-pointed, next to the 

 incisors; (c) next the molars, broad and flat-topped for 

 grinding, and divided into premolars and true molars. 

 There is great variety in the character and arrangement 

 of these structures in mammals, their variations being 

 much used in classification. The number and arrange- 

 ment of the teeth is expressed by a dental formula, as, for 

 example, in the case of man 



2 2 33 



= 32. 



2 - - 2 I - - I J 2 -- 2 3 3 



The mouth is bounded by fleshy lips. On the floor of 

 the mouth is the tongue, which bears the taste-buds or 

 papillae, the organs of taste. The oesophagus is always 

 a simple straight tube, but the stomach varies, greatly, 



