380 



CRANIAL NERVES. 



to the liyoid cavity; and, as Marshall has recently shewn, the main branch 

 of the third nerve adjoins the posterior border of the front cavity, described 



by me as the premandibular cavity. 

 Owing to the early conversion of the 

 walls of the posterior head-cavities into 

 muscles, their relations to the nerves are 

 not quite so clear as in the case of the 

 anterior cavities, though, as far as is 

 known, they are precisely the same. 



Anterior nerve-roots in the brain. 



During my investigations on the de- 

 velopment of the cranial nerves I was 

 iniable to find any roots comparable with 

 the anterior roots of the spinal nerves, 

 and propounded an hypothesis (suggested 

 by the absence of anterior spinal roots 

 in Amphioxus') that the head and trunk 

 iiad become differentiated from each other 

 at a stage when mixed motor aiid sensory 

 posterior roots were the only roots pre- 

 sent, and I supposed the crai^ial and 

 spinal nerves to have Leen independeutly 

 evolved from a common ground form, 

 the resulting types of nerves being so 

 different that no roots strictly comparable 

 with the anterior roots of spinal nerves 

 were to be found in the cra?iial nerves. 



The views put forward by me on this 

 subject, though accepted by Schwalbe 

 (No. 357), have in other quarters not 

 met with much favour. Wiedersheim 

 holds that it is impossible to believe that 

 the cranial nerves are simpler than the 



Fig. 272. Transverse section 

 through the front part of the 



HEAD OF A YOUNG PrISTIURUS EMBRYO. 



The section, owing to the cranial 

 flexure, cuts both the fore- and the 

 hind-brain. It shews the preman- 

 dibular and mandibular head cavities 

 \pp and 2pp, etc. 



fb. fore-brain ; I. lens of eye ; m. 

 mouth; pt. upper end of mouth, 

 forming pituitary involution ; lao. 

 mandibular aortic arch ; Ipp- and 

 2^2^. first and second head cavities; 

 Ivc. first visceral cleft ; V. fifth 

 nerve; aun. ganglion of auditory 

 nerve ; VII. seventh nerve ; aa. dor- 

 sal aorta ; acv. anterior cardinal vein ; 

 ch. notochord. 



spinal nerves. Such simplicity, which is 

 clearly not found, I have never asserted to exist ; I Jiave only stated that the 

 cranial nerves, in acquiring the complicated character they have in the adult, 

 do not develop antex-ior roots comparable with those of the spinal nerves. 

 Marshall also strongly objects to my views, and has made some observa- 

 tions for the purpose of testing them, leading to some very interesting 

 results, which I proceed to state, and I will then explain my opinion con- 

 cerning them. 



The most important observation of Marshall on this subject concerns 

 the sixth nerve. In both the Chick and Scyllium he has detected a nerve 

 (the first development of which has unfortunately not been made out) 

 arising by a series of roots from the base of the hind-brain. By tracing 

 this nerve to the external rectus muscle of the eye he has satisfactorily 

 identified it as the sixth nerve. " Neither in tlie nerve nor in its roots are 

 Oiere any ganglion cells.'' This nerve he finds to be placed vertically below 



* Schneider holds that anterior roots are present in Amphioxus, but I have been 

 unable to satisfy myself of their presence. 



