196 HISTORY OF THE HUMAN BODY 



muscle and the nerve which supplies it, since the nerve which 

 originally supplies a given primitive element, such as a rnyo- 

 tome or a limb-element, never forsakes it, but follows it 

 through all its vicissitudes and continues to supply its deriva- 

 tives, of whatever complexity or form, through all the changes 

 of relation, which are often very great. A conspicuous ex- 

 ample of this is the facial nerve (Vllth cranial), which, in 

 spite of its name, is not, so far as it is a motor nerve, origi- 

 nally associated with the face but with the hyoid region of the 

 neck. In the lower mammals this nerve supplies an integu- 

 mental muscle covering the side of the neck, of which the 

 human platysma is a remnant, and as it happens that in higher 

 forms this sheet becomes extended over the face and differen- 

 tiates into the various slips that form the mimetic musculature, 

 its nerve follows it, multiplying its branches in strict accord- 

 ance with the growth and differentiation of the muscle, until 

 it covers the entire face with its ramification and earns the 

 name of facialis. Another branch of the same nerve supplies 

 the digastric muscle of the mandible in the lower vertebrates 

 (the equivalent of the posterior belly of the like-named mus- 

 cle of mammals), and when in reptiles a small slip detaches 

 itself from this muscle and wanders into the middle ear to 

 become the stapedius, a minute branch of the nerve in question 

 follows it to its ultimate location and furnishes it with its 

 nerve supply. 



Were it possible to follow each motor nerve fiber from its 

 origin to its connection with its muscle, it would probably 

 serve as an absolute criterion for muscular homology, but 

 there is much chance of error in the fact that an anatomical 

 nerve is not a single fiber, but a bundle of them, and while 

 each fiber is presumably constant in its supply, there is some 

 variation in the way in which they ere put into bundles, so 

 that no one can be sure that a given nerve is always quite 

 homologous with one in a like location in another animal. 

 This is especially true of the innervation of the limbs, where 

 the nerve supply, after proceeding from a certain fairly definite 

 number of nerve roots, passes into a plexus, in which the 



