NERVOUS SYSTEM OF THE VERTEBRATA. 463 



it as the sixth nerve. " Neither in the nerve nor in its roots are there any 

 ganglion cells" This nerve he finds to be placed vertically below the roots 

 of the seventh nerve ; and it is not visible till much later than the cranial 

 nerves above described. 



In addition to this nerve Marshall has found, both in the third nerve 

 and in the fifth nerve, a series of non-gangliated roots, which arise in a 

 manner not yet satisfactorily elucidated, considerably later than, and in front 

 of, the main roots. These roots join the gangliated roots on the proximal 

 side of the ganglion or in the ganglion 1 ; and Marshall believes them to be 

 homologous with the anterior roots of spinal nerves, while he holds the 

 sixth nerve to be an anterior root of the seventh nerve. 



In addition to these nerves Marshall holds certain ventral roots, which 

 occur in Elasmobranchs close to the boundary of the spinal cord and 

 medulla, and which probably form the hypoglossal nerve of higher types, to 

 be anterior roots of the vagus. It is very difficult to prove anything 

 definitely about these nerves, but, for reasons stated in my work on 

 Elasmobranch Fishes, I am inclined to regard them as anterior roots of one 

 or more spinal nerves. 



Before attempting to decide how far Marshall's views about the so-called 

 anterior roots of the seventh, the fifth and the third nerves are well founded 

 it will conduce to clearness to state the characters and relations of the two 

 roots of spinal nerves. 



The posterior root is (i) always purely sensory ; (2) it always develops a 

 ganglion. The anterior root is (i) always purely motor ; (2) it always joins 

 the posterior root below the ganglion, except in Petromyzon (though not in 

 Myxine) where the two roots are stated to be independent. 



How far do Marshall's anterior and posterior roots of the cranial nerves 

 exhibit these respective peculiarities ? 



With reference to the sixth and seventh nerves he states " we must 

 regard the sixth nerve as having the same relation to the seventh that the 

 anterior root of a spinal nerve has to the posterior root." On this I would 

 remark (i) that the posterior root of this nerve is a mixed sensory and 

 motor nerve and therefore differs in a very fundamental point from that of 

 a spinal nerve ; (2) the sixth nerve though resembling the anterior root of 

 a spinal nerve in being motor and without a ganglion, differs from the 

 nearly universal arrangement of spinal nerves in not uniting with the 

 seventh. 



With reference to the fifth nerve it is to be observed that it is by no 

 means certain that the whole of the motor fibres are supplied by the so- 

 called anterior roots, and that these roots differ again in the most marked 

 manner from the anterior roots of spinal nerves in joining the main root of 

 the nerve above (nearer the brain), and not as in a spinal nerve below the 



1 These non-gangliated roots of the fifth nerve are not to be confounded with the 

 motor root of the fifth nerve in higher types. They appear to form the anterior root 

 of the adult which gives origin to the ramus ophthalmicus. 



