ANIMAL MECHANICS. 



75 



instead of the cross sections of the muscles themselves. As the 

 cross sections of both muscles and tendons must be measured 

 after death, and as it is not always convenient to kill subjects 

 suitable for observation, it is obvious that the tendons, which 

 are less liable to be wasted by natural disease than the muscles, 

 are capable of affording most valuable relative measure- 

 ments, if we are allowed to assume that the cross sections 

 of the tendons are proportional to the forces resisted by 

 them. 



It may be readily shown, a priori, that this must be the 

 case, by means of the following considerations : — 



The principle of economy of force, or of material, in nature, 

 would lead necessarily to the principle that each tendon con- 

 veying the effect of a force to a distant point should have the 

 exact strength required, and neither more nor less ; for, accord- 

 ing to the doctrine of Final Causes, it was originally contrived 

 by a perfect Architect, and according to Lamarckian views it 

 must have perfectly accommodated itself to the uses to which 

 it is applied. According, therefore, to either view, if the ten- 

 don be too strong, it will become atrophied down to the proper 

 limit ; and if too weak, it must either break, or be nourished 

 up to the requisite degree of strength. It seemed to me de- 

 sirable to prove this fundamental proposition in animal mecha- 

 nics by direct observation ; and I selected for this purpose 

 the tendons in the leg of several of the large running birds 

 (Struthionidce), with the result, that the cross sections of any 

 two muscles tending to produce a similar effect, and subjected 

 to similar friction in their tendons, are directly proportional 

 to the cross sections of those tendons. 



I shall select as an example the case of the Flexor hallucis 

 longus and Flexor digitorum communis perforans of the Khea, 

 whose tendons unite into a common tendon half way down 

 the posterior side of the cannon bone of the bird. 



The areas of the cross sections of these muscles were found 



