THE USURPATION OF NERVES. 259 



III. — The Usurpation of Nerves. 



From the preceding account it appears that the modifi- 

 cations to which the primitive segmental nerves of the 

 head have been subjected are of two main t)''pes : in the 

 first place by the loss of some one or more of the pri- 

 mary components, as the loss of the general cutaneous 

 fibres of the IX and VII, or by the addition of components 

 not primarily present, as in the case of the addition of 

 special cutaneous components to the VII; and in the 

 second place by the prolongation of nerves of one segment 

 so that they encroach upon the area of another. This 

 encroachment may take place either by the terminal organ 

 of the nerve migrating secondarily into the adjacent seg- 

 ment and carrying its nerve with it, or the nerve may 

 effect secondary connection with the terminal organ which 

 belongs primarily in the adjacent segment. The occur- 

 rence of the latter case, it is true, is somewhat doubtful 

 and is denied absolutely by some authors. Yet it would 

 seem in the present state of our knowledge to be at least 

 probable in some cases, as, for example, the r. intestinalis 

 vagi, and the innervation of the skin of the head between 

 the vagus and trigeminus by general cutaneous fibres 

 from these nerves. 



The way in which a nerve can be carried to the most 

 distant parts of the body by a vagrant terminal organ is 

 best illustrated by the development of the lateral lines and 

 their nerves, as described by Wilson ('91 and '97), Ayers 

 ('92), Mitrophanow ('93), Piatt ('96) and others. The 

 development of the so-called hypoglossus musculature 

 from the post-otic myotomes is another illustration and 

 Ruge ('97) suggests the same for the development of 

 the facial musculature of the mammals, viz., that these 



