382 CRANIAL NERVES. 



In view of these facts, my ongiiial hypothesis appears to me to ho 

 confirmed by Marsliall'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 difi'erentiated 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. Coni-idering 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-gauglionated 

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

 of the spinal nerves '. 



As to the sixth nerve further embryological investigations are requisite 

 befoi'e its true position in the series can be determined; but it appears to 

 me veiy probable that it is a product of the differentiation of the seven tli 

 nerve. 



The fourth, nerve, ^o 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 supeiior oblique muscle of the eye. 

 If this is the case there must have been two prsemandibular 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- 

 mobrauch fishes that the olfactory nerve grew out from the brain in the 

 same manner as other nerves; and Marshall (No. 355), to whom we are 

 indebted for the gi'eater part of our knowledge on the development of this 

 nerve, has proved that it arises prior to the differentiation of the olfactory 

 lobes. 



The earliest stages in the development of the nerve have not been 

 made out. Marshall, as already stated, finds that in the Chick the neural 

 crest is continued in front of the optic ve.sicles, and holds that this fact is 

 strong a priori evidence in favour of the nerve growing out from it. 

 As mentioned above, note on p. 375, I cannot without further evidence 

 accept Maishall's statements on this point. In any case Marshall has not 

 yet been able again to find an olfactory nerve till long after the disappear- 

 ance of the neural crest. The olfactory nei've at the next stage observed 

 forms an outgrowth of fusiform cells springing oh either side from neai" 

 the summit of the fore- brain; and at fifty hours it ends close to a slight 

 thickening of the epiblast forming the first rudiment of the olfactory pit, 

 with the walls of which it soon becomes united. 



^ 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 mammalian 

 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. 



