Descriptive Biology 115 



ber of nerve and muscle rudiments appear in the embryo, 

 which disappear in the adult. 



The modifications which nerves and muscles undergo in 

 evolution are perhaps nowhere more beautifully shown than 

 in the evolution of the muscles and nerves of the human face. 

 These muscles, known as the "mimetic" or mimicking muscles 

 of higher apes and man, produce the wonderful play of expres- 

 sion of which the human face is capable, and through control 

 of the lips aid very largely in speech. They are controlled 

 by the seventh or facial nerve, which in lower animals sup- 

 plies the upper neck and lower jaw. In both racial and indi- 

 vidual development the association of muscle and nerve is 

 very constant. Each motor nerve is, as it were, assigned the 

 duty of controlling a certain muscle, and regardless of the 

 wanderings of its muscle, it remains faithful to its charge, 

 so that the best criterion for the comparison of muscles in 

 two animals and for determining the segments to which they 

 belong, is the nerves which supply them. So it comes to pass 

 that when the muscles of the neck wander out over the face, 

 or when those of the shoulder region spread themselves over 

 the back, their nerves must needs go along with them, and we 

 are in this way enabled to tell the original source of most 

 of the complicated muscles of the higher vertebrates. And 

 so when we speak or smile or frown we are using muscles, 

 which, in some ancestor of the long forgotten past, controlled 

 the throat and jaws and probably served it in chewing and 

 swallowing its food. 



Thus in a few words we have outlined the field of the 

 morphologist. Perhaps some of my readers may deem the 

 sketch one worthy of cubist art, or modern poetry. If per- 

 chance however we have gained even a glimpse of the mani- 

 fold problems here involved and of some of the larger facts 

 thus far discovered, the attempt will not have been in vain. 



