MOTION AND LOCOMOTION 45 



that of circulating both crude and manufactured food- 

 materials to all parts of the body. 



What, then, is the nature of this skeleton ? In animals 

 there is an infinite variety of substances ; it may consist, 

 as in many of the lower forms, of a thin horn-like sub- 

 stance with or without an additional impregnation of 

 various salts, while in the higher animals it consists 

 mainly of cartilage and bone. In plants the skeleton 

 consists mostly of wood. 



A chemical examination shows that about two-thirds 

 of the weight of the animal skeleton consists of mineral 

 matter, while only about two to five per cent, of the 

 wood consists of inorganic salts. 



In structure there is also a great difference. Wood 

 consists of overlapping spindle-shaped fibres, while bone 

 is built up of concentric lamellae, which are, as it were, 

 firmly fixed together by projecting processes, which often 

 penetrate into several of the other lamellae. 



Another point of great interest in connection with the 

 skeleton is its disposition throughout the body. In 

 plants where the chief strain to be withstood is the 

 tearing caused when the trunk is bending before the 

 wind, we find that the skeleton of the stem is near the 

 outside, while the roots which have to bear a rectilineal 

 strain and are seldom if ever subjected to bending, 

 have the vascular and skeletal tissue aggregated in the 

 centre. This difference in the disposition of the skeletal 

 tissue is entirely in accordance with elementary physical 

 laws. 



On the other hand, the chief force to be contended 

 with in animals is the crushing force, and on this account 

 we have the skeleton arranged somewhat differently. 



