462 



CRANIAL NERVES. 



cavity, described by me as the preman- 

 dibular cavity. Owing to the early con- 

 version 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 

 unable 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 1 ) that the head and trunk 

 had become differentiated from each other 

 at a stage when mixed motor and sensory 

 posterior roots were the only roots pre- 

 sent, and I supposed the cranial and 

 spinal nerves to have been independently 

 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 cranial nerves. 



The views put forward by me on this 

 subject, though accepted by Schwalbe 



fb 



FIG. 272. TRANSVERSE SECTION 

 THROUGH THE FRONT PART OF THE 

 HEAD OF A YOUNG PRISTIURUS 



EMBRYO. 



The section, owing to the cranial 

 flexure, cuts botli the fore- and the 

 hind-brain. It shews the praeman- 

 dibular and mandibular head-cavities 

 \pp and iff, etc. 



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

 mouth ; pt. upper end of mouth, 

 forming pituitary involution ; i ao. 

 mandibular aortic arch; ipp. and 

 ipp. first and second head-cavities ; 

 ivc. first visceral cleft ; V. fifth 

 nerve ; atm. ganglion of auditory 

 nerve ; VII. seventh nerve ; aa. dor- 

 sal aorta ; acv. anterior cardinal 

 vein ; ch. notochord. 

 (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 spinal nerves. Such simplicity, 

 which is clearly not found, I have never asserted to exist ; I have only 

 stated that the cranial nerves, in acquiring the complicated character they 

 have in the adult, do not develop anterior roots comparable with those 

 of the spinal nerves. Marshall also strongly objects to my views, and has 

 made some observations for the purpose of testing them, leading to some 

 very interesting results, which I proceed to state, and I will then explain my 

 opinion concerning 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 



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

 unable to satisfy myself of their presence. 



