240 THE ANIMALS AND MAN 



tebrae their usual number is about thirty. The mammalian 

 skull is very firm and rigid, all the bones composing it, ex- 

 cepting the lower jaw, the tiny auditory ossicles, and the 

 slender bones of the hyoid arch, being immovably articu- 

 lated together. The correspondence between the bones of 

 the two sets of limbs is very apparent. 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 

 masticating 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 over as a thin layer 

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

 of the crown. The teeth in most forms are of three groups: 

 (a) the incisors, with sharp cutting edges and simple roots, 

 situated in the centre of the jaw; (b) the canines, often con- 

 ical 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 mam- 

 mals, their variations being much used in classification. 

 The number and arrangement of the teeth is expressed by 

 a dental formula, as, for example, in the case of man: 



2 — 2 I — I 2 — 2 3 — 3 



i , c , p , m = 32. 



2 — 2 I — I 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 papil- 



