382 STRUCTURE OF VERTEBRATES. 



There is much uncertainty in regard to the morphological 

 value of the various cranial nerves, but the following con- 

 clusions are important : — 



(1) The nerves arise as outgrowths of the central system. Each 

 spinal nerve has two roots — a dorsal and a ventral, but in most cases at 

 least a cranial nerve has primitively a single dorsal root arising from 

 a neural ridge of the dorsal surface of the brain. In many cases this 

 root divides into "dorsal," "ventral," etc., branches. As these typi- 

 cally innervate a gill-slit, as may be well studied in g, the branches may 

 be called (as Beard proposes) supra-branchial (dorsal), post-branchial, 

 prse-branchial, etc. In the course of growth the nerve often shifts from 

 the position whence its root originated. 



(2) Some of them mark distinct segments of the head, while others 

 are secondary derivatives. It is likely that I, 3, 5,7, 8, 9, and several 

 parts of 10 mark segments. It is possible that the oculo-motor is a 

 ventral root associated with the third or ciliary nerve, that the troch- 

 lear is a ventral root of the trigeminal, that the abducens is a ventral 

 root of the facial. , 



(3) It is possible that each truly segmental nerve supplied a primitive 

 gill-slit, as 7 supplies the spiracle, 9 the first branchial, 10 the second, 

 third, fourth, and fifth branchials. 



(4) It is likely that each segmental nerve was associated with a 

 branchial sense-organ (Beard and Froriep). These organs occur about 

 the gills, and are continued in the lateral line of the trunk. It is likely 

 that a branchial sense-organ lay over each primitive gill-cleft, and had 

 an associated ganglion. The ganglia known as ciliary, gasserian, etc. , 

 may be the ganglia of branchial sense-organs. It may be that nose and 

 ear were originally branchial sense-organs. 



The Spinal Cord. — The spinal cord arises like the brain 

 from the medullary groove, whose sides fold over and form 

 a medullary canal. 



This canal is for a time continuous posteriorly with the 

 food-canal beneath, so that a :=) -shaped tube results. The 

 connection between them is called the neurenteric canal, 

 and though it is only temporary its constant occurrence 

 is of much interest. 



As the medullary canal sinks in and becomes differ- 

 entiated into the spinal cord, its cavity becomes a rela- 

 tively minute ciliated canal, while the walls become very 

 thick. 



In some cases at least the spinal cord has from the first a 

 distinct bilateral symmetry ; and this is always emphasised 

 by longitudinal fissures, which run along it dorsally and 

 ventrally. 



