464 CRANIAL NERVES. 



ganglion. The gangliated root of the third nerve is purely motor 1 , and its 

 so-called anterior roots again differ from the anterior roots of spinal nerves, 

 in the same manner as those of the fifth nerve. 



With reference to the glossopharyngeal and vagus nerves I would 

 merely remark that no anterior root has even been suggested for the 

 glossopharyngeal nerve and that the posterior roots of both these nerves 

 contain a mixture of sensory and motor fibres. 



In view of these facts, my original hypothesis appears to me to be 

 confirmed by Marshall's observations. 



The fact of all the posterior roots of the above cranial nerves (except 

 the third which may be purely motor) being mixed motor and sensory roots 

 appears to me to demonstrate that the starting-point of their differentiation 

 was a mixed nerve with a single dorsal root ; and that they did not therefore 

 become differentiated from nerves built on the same type as the spinal 

 nerves with dorsal sensory and ventral motor roots. The presence of such 

 non-gangliated roots as those of the third and fifth nerves is not a difficulty 

 to this view. Considering that the cranial nerves are more highly differen- 

 tiated than the spinal nerves, and have more complicated functions to 

 perform, it would be surprising if there had not been developed non- 

 ganglionated roots analogous to, but not of course homologous with, the 

 anterior roots of the spinal nerves 2 . 



As to the sixth nerve further embryological investigations are requisite 

 before its true position in the series can be determined ; but it appears to 

 me very probable that it is a product of the differentiation of the seventh 

 nerve. 



The fourth nerve. No embryological investigations have been 

 made with reference to the fourth nerve. It is possible that it is a segmental 

 nerve comparable with the third nerve, and that the only remnant still left 

 of the segment to which it belongs is the superior oblique muscle of the eye. 

 If this is the case there must have been two praemandibular segments, viz. 

 that belonging to the third nerve, and that belonging to the fourth nerve. 

 Against this view of the fourth nerve is the fact, urged with great force by 

 Marshall, that the superior oblique muscle is in front of the other eye 

 muscles, and that the fourth nerve therefore crosses the third nerve to 

 reach its destination. 



The Olfactory nerve. It was shewn in my monograph on Elas- 

 mobranch Fishes that the olfactory nerve grew out from the brain in the 



1 If Marshall's view about the ramus ophthalmicus profundus (p. 461) is correct, 

 the third must still be, as it no doubt was primitively, a mixed motor and sensory 

 nerve. 



2 In the higher types, as is well known, the fifth nerve has its roots formed on the 

 same type as a spinal nerve. The fact that this is not the case in the lower types, 

 either in the embryo or the adult, is a clear indication, to my mind, that the mam- 

 malian arrangement of the roots of the fifth nerve has been secondarily acquired, a 

 fact which is a most striking confirmation of my views as to the differences between 

 the cranial and spinal nerves. 



