142 EDWARD PHELPS ALLIS, JUN. 



way in its length " ? Or does one of the muscles^ either at 

 its origin or elsewhere^ ever traverse a nerve " midway in its 

 length. " ? I assumed, in my discussion of the subject, that 

 neither of these things occur, and I know of no single fact 

 that proves the contrary. I also assumed, in my discussion 

 of these nerves and muscles, and it was definitely implied 

 though not definitely stated, that an eye-muscle nerve once 

 laid down on one side of any of the eye-muscles, or on one 

 side of any of the nerves that innervate those muscles, or 

 that traverse the orbit, would always be laid down in the 

 same relation to that structure. I then further assumed 

 that the nerve once laid down never changed this embryonic 

 relation to another nerve, but that it might and did change 

 its relation to a muscle in the manner set forth in the 

 principle above referred to as invoked by me. In this 

 assumption that the nerves are always laid down in the same 

 relations to the eye-muscles I was quite unquestionably in 

 error, as my present work on Mustelus shows. For it is 

 evident that since the nerves that innervate the eye-muscles 

 or that traverse the orbit are relatively well developed before 

 the eye-muscles have acquired their attachments on the skull, 

 they might be so placed, because of correlation to other 

 parts, as to obstruct a muscle as it sought its cranial attach- 

 ment, and hence deflect it, or even split it, as the obstructing 

 nerves split the rectus internus in Mustelus. This is, how- 

 ever, in reality simply an application, in embryonic stages, 

 of the principle I invoked for adults ; that is, the muscle 

 here, in principle, traverses, at the end that is to become its 

 end of origin on the skull, a nerve that it encounters. This, 

 it will be readily seen, is a totally different thing from the 

 assumption that the nerve cuts through the muscle fibres; 

 and also totally different from another principle frequently in- 

 voked in this connection, and which is very definitely expressed 

 by Herrick (32, p. 237) in the following sentence : — "If this 

 be true (that the eye-muscle nerves grow directly out from 

 the brain), I see no reason why a given motor nerve should 

 not grow out either above or below some other structure 



