BONES AND JOINTS OF THE EXTREMITIES 39 



by means (if we may be permitted to say so) still 

 more artificial. Every one must have observed, 

 that the breastbone of a fowl extends along the 

 whole body, and that the body is very large com- 

 pared with the weight : this is for the purpose 

 of rendering the creature specifically lighter and 

 more buoyant in the air ; and that it may have a 

 surface for the attachment of muscles, equal to 

 the exertion of raising it on the wing. This com- 

 bination of lightness with increase of volume is 

 gained by air-cells extending through the body, 

 and communicating by tubes between the lungs 

 and cavities of the bones. By these means, the 

 bones, although large and strong to withstand the 

 operation of powerful muscles upon them, are 

 much lighter than those of quadrupeds. 



The long bones of the human body, being 

 hollow tubes, are called cylindrical, though they 

 are not accurately so, the reason of which we 

 shall presently explain ; and we shall, at the same 

 time, show that their irregularities are not acci- 

 dental, as some have imagined. But let us first 

 demonstrate the advantage which, in the struc- 



O 7 



ture of the bones, is derived from the cylindrical 

 form, or a form approaching to that of the cyl- 

 inder. If a piece of timber supported on two 

 points (Fig. 7) bear a weight upon it, it sustains 

 this weight by different qualities in its different 

 parts. For example, divide it into three equal 



