THE MUSCULAR SYSTEM 195 



thus necessary to study all possible individual as well as specific 

 variations of a muscle or a muscle group before laying down 

 the outlines of its history. 



When the study of comparative myology was in its infancy, 

 muscles were homologized and named from their general 

 location, appearance and .use, without regard to their develop- 

 mental history, a method which, while fairly safe when ap- 

 plied to closely allied forms, was apt to be very misleading 

 when applied to animals as different, for example, as members 

 of the different vertebrate Classes. Thus, if the starting point 

 were the human subject, as was the former universal custom, it 

 would be quite easy to recognize such a muscle as the deltoid 

 in the apes and monkeys, which possess prehensile arms of a 

 similar shape and used in a similar way, but it would be much 

 more difficult to determine the same muscle in the ox, which 

 uses its fore legs so differently, and the difficulty might be- 

 come insurmountable in a form as different as a turtle or a 

 frog. 



Somewhat more reliable as criterions for homology than 

 position or use are the origin and insertion, the points at which 

 the muscle fibers are attached, although these are altered by 

 increase or decrease in volume or by a slight change in use; 

 and, if the change is marked, may attain quite different re- 

 lationships to the skeletal parts. There is, however, some dif- 

 ference in the relative value of these two points, the origin 

 being the more constant in certain regions, the insertion in 

 others. It has long been considered an axiom of comparative 

 myology to give the credit for greater constancy to the origin 

 in all cases, but in the muscles of the appendicular skeleton the 

 reverse seems to be the case, since here the insertions are at 

 the distal end and are effected through narrow tendons, the 

 precise attachment of which is a matter of great importance 

 in the action of the muscle, while the origins occupy large and 

 rather indefinite areas, the extent of which is relative to the 

 degree of development in each case. 



Undoubtedly the most reliable criterion for muscular ho- 

 mology is that based upon the constant relation between a given 



