MECHANISM OF THE BONES. 



greatest possible lightness at the same time, as in the 

 limb bones, for example, the hollow form is the best. 

 Every boy is sensible of the superior strength of a cane 

 pole beyond that of any other rod of the same weight. 

 The reason is, it is hollow. If he should split the cane 

 into strips, and glue the strips together ever so strong- 

 ly, but without leaving any hollow, though he would 

 have the same wood, he would find the rod hardly ca- 

 pable of supporting its own weight. A tin tube has 

 very considerable strength; but flattened together, 

 that is, no hollow remaining, it bends with ease. We 

 see, therefore, it was not accident, but the skill of a 

 wise Mechanician which has assigned the tubular fonn 

 to the bones of the limbs, in which strength and light- 

 ness are particularly needed. What is more remark- 

 able, this form is the most conspicuous in the bones 

 of the bird. These animals have the most occasion, 

 plainly, for light bones. Their bones, that is, the 

 hollow bones, differ in three properties. First, the 

 hollows are much larger in proportion to the w r eight of 

 the bone than in those of men or of four footed ani- 

 mals. Secondly, these hollows are empty, or con- 

 tain only air. They have a direct communication also 

 with the lungs ; the air which is received into the 

 lungs, escaping and returning alternately by a hole in 

 some of the bones; whereas, in man, &ic., they are 

 filled with marrow. Thirdly, the shell is of a firmer 

 texture than is the substance of other bones ; and 

 therefore, less bone is required to furnish the same de- 

 gree of strength. 



