nerve comes out of the brain stem as two distinct parts be- 

 low the root of the seventh. The ninth and tenth have a 

 common root but soon branch. 



The spinal nerves each have dorsal and ventral roots in 

 contrast to the lamprey where a dorsal root nerve is sepa- 

 rate from a ventral root nerve. The decision as to which of 

 these situations is the more primitive is based on the condi- 

 tion in Amphioxus, where there are separate dorsal and 

 ventral root nerves which alternate with each other. The 

 spinal nerves of the lamprey have visceral motor fibers in 

 the dorsal root nerve. Presumably this is also the case in the 

 myxinid and in most fishes. 



General observations 



Since the head shows a certain amount of segmentation in 

 having muscular somites, it has been assumed that the 

 cranial nerves are in fact only highly modified nerves similar 

 to the spinal nerves. The separation of motor and sensory 

 divisions is not unlike the primitive spinal condition. 



Various attempts have been made to count the number of 

 segments in the head and to associate with these parts of 

 the cranial nervous system. These attempts are summarized 

 in Table 31-1. Opinions vary as to the number of preotic 

 segments: ranging from three segments in the case of Young 



pharyngeal branches 



facial nerve (VII) root temporal branch IX 



1 of VII 

 nerve 

 lateral line X 



visceral X 

 hyomandibulor branch VII 



boculor branch of 



St spinal nerve 



superficial ophthalmic VII and V 

 buccal branch VII 



trochlear nerve (IV) 

 abducens nerve (VI) / 1st spinal nerve 



II "I 



maxillary branch, V; 

 mandibular branch, V3 artery 



hyomandibular branch VII 



Figure 13-16. Cranial nerve distribution in the lamprey. A, nerves; B, nerves in relation to head 

 skeleton. (A after Lindstrom, 1949; B After Marinelli and Strenger, 1954) (For skeletal elements see 

 Figure 5-24 B, p. 129) 



THE CONDUCTING AND INTEGRATING SYSTEM • 399 



