172 PROCEEDINGS OF THE ACADEMY OP [1894. 



THE DIFFERENTIAL ACTION OF CERTAIN MUSCLES PASSING 

 MORE THAN ONE JOINT. 



BY THOMAS EAKINS. 



It is not without diffidence, that I, a painter, venture to communi- 

 cate with a scientific l)ody upon a scientific subject ; yet I am en- 

 couraged by thinking that Nature is so nmny sided that the humblest 

 observer may, from his point of view, offer suggestions worthy of 

 attention. I am greatly indebted to Dr. Harrison Allen, who has 

 kindly added some explanatory notes. 



I have long been dissatisfied with the account in standard works 

 of the muscular action in animal locomotion. The muscles are clas- 

 sified principally as flexors and extensors, working and resting 

 alternately. Wishing to apply this system during my early dissec- 

 tions to the leg of the living horse, I was surprised to observe in the 

 strain of starting a horse car, that the so-called flexors and extensors 

 were in strong action at the same time. 



The classification was still farther from satisfactory when applied 

 to muscles passing over two or more joints, flexing perhaps one joint, 

 while extending another. In trying to understand the significance 

 of these last named muscles, I came to believe it to be very important 

 to discover if the one joint was extended more rapidly than the other 

 was flexed. This investigation demanded a consideration of the 

 amount and kind of leverage, and was extended from the muscles to 

 tendons ' which pass over the two or more joints. I next constructed 

 a model of the entire limb with flat pieces of half-inch pine board, 

 cut to the outline of the bones, the pieces pivoted together, having 

 catgut for tendons and ligaments, and rubber bands for muscles, all 

 attached to their places and proi>erly restrained. 



I had then the satisfaction of seeing this mechanism imitate in 

 many ways the action of the real leg, and was enabled to establish 

 two important principles, thus : First, the hoof-pieces properly set 

 upon the ground, the leg stood firm, all tendency to collapse being 

 prevented by the leverage of tendons passing joints. Secondly, the 



1 The use of the word " tendons " in the sense hero oniployed, does not refer to 

 the tendons in eonncction with muscular bellies, but to ligament- like structures 

 which ar(^ homologous with tlie muscles as these bodies are uniformly assigned 

 by authors to the musculature oftln^ limits. H. Allen. 



