CHAPTER XVI 

 THE SKELETON 



We have seen how foods furnish the body with heat and 

 act as a source of human energy in general; we may now give 

 attention to the more permanent forms which foods take as 

 body tissues, studying especially the skeleton and the mus- 

 cles, which form the largest part of the body in bulk. 



The Functions of the Skeleton. The function of the skele- 

 ton is two-fold. 



1. It gives firmness to the body; without such a frame- 

 work, the other, softer tissues would form but a shapeless 

 mass, with no means of bracing itself for doing work. 



2. It furnishes attachments for muscles, and thus makes 

 motions possible; muscles are so fastened to the bones as to 

 move them, thus producing movements of the body. 



All large animals, except a few living in the ocean (jelly- 

 fishes, etc.), have skeletons of one sort or another; it is the 

 only method nature has devised for holding together large 

 masses of soft tissues and making it possible for them to 

 work to purposive ends. In some animals, e. g. lobsters, in- 

 sects, clams, etc., the skeleton is a mere crust on the outside, 

 and thus is called an exoskeleton ; but the circumstances are the 

 reverse with all "backboned" animals, including man, which 

 are spoken of as possessing an endoskeleton. In the adult 

 human, the skeleton makes up about 16% of the weight of the 

 entire body, and comprises about two hundred separate bones ; 

 the child's skeleton contains more because during the process 

 of growth some bones fuse together. In spite of their number, 

 the bones are so strongly bound together by tough, stringy 

 liaments and by muscles that the whole skeleton forms a very 



fiirm. solid unit; Fig. 120. 



250 



