ON DESIGN IN ANIMAL BODIES. 203 



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 hol- 

 low 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 accidental, 

 as some have imagined. But let us first demon- 

 strate the advantage which, in the structure of the 

 bones, is derived from the cylindrical form, or a 

 form approaching to that of a cylinder. If a piece 

 of timber supported on two points, thus — 



bear a weight upon it, it sustains this weight by 

 different qualities in its different parts. For ex- 

 ample, divide it into three equal pai ts (A, B, C) : 

 the upper part A supports the weight by its soli- 

 dity and resistance to compression : the lowest 

 part B, on the other hand, resists by its tough- 

 ness or adhesive quality. Between the portions 



