THE BOOK OF NATURE STUDY 



"centrum," and of the arches 

 which protect the spinal cord. 



The backbone or spinal column differs from that of birds 

 in one or two important particulars. In the first place, with 



a few rare exceptions, the number of 

 the neck bones is always seven. And 

 this is true alike of the giraffe, 

 and the whale wherein in the living 

 animal no visible neck is traceable. In 

 the second place, the bones of the back, 

 at least as far as the hip-girdle, to be 

 described presently, are always sepa- 

 rate. In birds, in quite a large number 

 of groups, more or fewer of the bones 

 are welded together to form asolidmass. 



FIG. S.-A vertebra from the spinal The vertebrae from the base of 

 column of a man, showing the the neck to the middle of the trunk 

 form of the solid body, or SU pp O rt the long curved rods of bone 



known as the ribs, which afford pro- 

 tection to the vital organs of the 

 body the heart and lungs, and digestive organs. The lower 

 ends of these ribs are attached to the breast-bone or sternum, 

 so that the organs just referred to are enclosed within a bony cage. 

 The scapulae or blade-bones (p. 33), which form the sup- 

 ports for the fore-legs, and the pelvic bones, which similarly 

 support the hind-limbs, differ fundamentally from those of birds, 

 but these differences are hardly matters with which children 

 need be troubled. 



The limb bones, though differing markedly from those of 

 birds, are composed of the same elements, as will be shown later. 

 In most mammals the number of the toes has undergone more 

 or less reduction. Originally five in number, as a rule, the fore- 

 or hind-feet, as the case may be, will be found to have fewer, 

 and they may even be reduced to a single toe on each foot, as 

 in the case of the horse. The bony framework of the limbs varies 

 greatly among different groups of animals, as may be seen, for 

 instance, by comparing the skeletons of say a lion, a seal, and 

 a horse ; the differences observable being related to the very 

 different functions which these limbs have to perform. Birds and 

 men alone walk upon the hind-limbs. Such mammals as progress 



