THE MUSCULAR SYSTEM 



197 



fibers become divided up and reunited in new combinations; 

 and although in two allied forms the final nerves that emanate 

 from the plexus may be constant in number and position, no 

 one can be quite sure that their make-up in individual fibers 

 is the same, and indeed it is more than likely that they are 

 not. Furthermore, in cases in which a single muscular ele- 

 ment has differentiated into a group of well-separated muscles, 

 each with a specific action, all will be supplied by branches 

 of the same nerve, and the innervation will furnish no clew 

 to homology, beyond that of identifying them as members 

 of the same limited group. 



Thus the criterion of nerve supply, although in theory an 

 accurate and definite method, often fails in its application 

 through variation in the make-up of the separate nerve bun- 

 dles, and while undoubtedly the best criterion we possess, it 

 cannot be employed in all cases. It is the most reliable in its 

 application to the axial muscles, where there has been the least 

 amount of differentiation, and where the primitive segmenta- 

 tion is still evident or but slightly disguised ; it has also proven 

 of value in the muscles of the visceral system, especially in the 

 case of fishes and amphibians, but in such cases as the highly 

 differentiated limb muscles, this method is difficult of appli- 

 cation, and cannot be followed beyond the homologizing of 

 the larger groups. Here the character most to be depended 

 on is the insertion, since these are in most cases by narrow 

 tendons and hence very definite, while the origins spread over 

 a greater or less area in proportion to the size of the muscular 

 " belly," or fleshy mass, and are thus somewhat inconstant in 

 individuals of the same species, or probably in the same indi- 

 vidual at different periods of its life. 



Although, in the study of the morphology of the muscles, 

 through the incompleteness of the embryological record and 

 the technical difficulties in the way of examining it, reliance 

 has hitherto been placed mainly on the comparison of adult 

 forms, much may undoubtedly be learned concerning the his- 

 tory of individual muscles and muscle groups from their on- 

 togeny, especially in those cases in which the animal passes 



