OSTEOLOGY AND ARTHROLOGY 127 



and drawn backwards, or carried to the right or left side 

 by lateral movements. Owing to the different modes of 

 nutrition of animals, with which the shape of the teeth 

 is clearly correlated, being more specialized than in the 

 human species, the lower jaw is moved in a fashion less 

 varied and in the direction most suitable for the mastication 

 of the foods which form the aliment of the species considered. 

 Moreover, this is plainly shown in the skeleton by the shape 

 of the condyle of the lower jaw (see p. 122, different forms of 

 this condyle). In the carnivora, whose teeth, as we have 

 seen, are all cutting ones, the jaw rises and falls ; the food 

 then is, if we consider the two jaws, cut as by the blades 

 of a pair of scissors. In the ruminants, the incisors exist 

 only in the lower jaw, but the molars are thick and well 

 developed ; the food is ground by these latter as by mill- 

 stones, and the movements which favour this action are, 

 above all, the lateral. As for the rodents, in which the 

 incisors are formed for filing down and cutting through hard 

 resisting bodies, their lower jaw moves in the antero-posterior 

 direction, in such a way that the inferior incisors alternately 

 advance and recede beneath those of the upper jaw. The 

 free cutting border of these teeth effectively fulfils the func- 

 tion to which they are destined ; their constant wear pre- 

 serves and revivifies the chisel edge which characterizes 

 them, without leading to their destruction, for the incisors 

 in rodents are of continuous growth. 



THE SKULL OF BIRDS 



The Skull of Birds (Fig. 65). — If, because it is less 

 important from the artistic point of view, we do not con- 

 sider it necessary to describe in detail the skull of birds, 

 we yet think it useful to indicate, in their general lines, the 

 peculiarities it presents. 



In this group the skull is generally pear-shaped ; to the 

 cranium, of which the bones are arranged in such a way 

 as to give it a form more or less spherical, succeeds a face 

 more or less elongated, according as the bill is more or less 

 developed. 



In general, the bones of the skull coalesce very early, with 



