20 



THE BOOK OF NATURE STUDY 



The Skeleton. Let us begin with the skull. The skull of the 

 mammal may at once be distinguished from that of all other 



animals except frogs and toads 

 and their kind by the fact 

 that it joins the backbone or 

 spinal column by means of a pair 

 of bony " bosses/' known as the 

 condyles, placed one on each side 

 of the opening through which 

 the spinal cord, or " marrow/' 

 passes from the brain (Fig. 3). 

 Next, it is to be noted, the lower 

 jaw is hinged directly on to the 

 skull, not by means of an inter- 

 mediate and generally loosely 

 slung bone, as in birds and 

 reptiles, for example. Then the 

 lower jaw itself is composed, as 

 to its right and left halves, of 

 a single bone, and not of several 

 pieces, as it is again in birds and 

 reptiles (Fig. 4). Finally, the 

 number and relations of the 

 several bones which form the cranium or brain case, and the 

 upper jaws, differ in many very important particulars from what 

 obtains in other vertebrated animals. But these are details which 

 are of far too technical a character to be described here. 



In nearly all mammals the jaws are armed with teeth which, 

 in some groups, are all of the same pattern, as in the curious 

 sloths and armadillos, and in the dolphins and porpoises ; but 

 as a rule these teeth present considerable difference in size and 

 shape in different regions of the jaw, according to the peculiar 

 food of the animal and the method of feeding. These differences 

 are well seen in the dog, for example (Fig. 4). Here in the front 

 of the jaws, on the right and left sides of the head respectively, 

 will be seen three small teeth, known as the "incisors" or " cut- 



FlG. 3. Back of the skull of a pig to show 

 the bony " bosses " or " condyles " for the 

 attachment of the vertebral column. 

 C, condyle ; F, foramen magnum, the 

 aperture by which the spinal cord passes 

 into the brain. 



ting " teeth (Fig. 4, 

 known as the 



" canine." 



next follows a long and powerful tooth 

 Behind this come four teeth having 



