SPINAL NERVES. 445 



There is much uncertainty in regard to the morphological value of the 

 various cranial nerves, but the following conclusions may be stated : — 



(1) The nerves arise either as outgrowths of the central system or as 

 specialisations of peripheral cells. 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 developing from a neural ridge of the 

 dorsal surface of the brain. In many cases this root divides into 

 "dorsal," "ventral," and other branches. As may be well studied 

 in 9, these typically innervate a gill-arch and slit, and the branches may 

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

 branchial, pra^-branchial, etc. In the course of growth the nerve often 

 shifts from the position whence its root originated. 



(2) Some of the cranial nerves 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- 



V /•/ 



Fig. 190. — Diagrammatic section of spinal cord. 



p.f., Posterior fissure; p.c, posterior column of white 

 matter ; d.p.s., dorsal, posterior, sensory_ or afferent 

 root ; g., ganglion ; v.a.vt., ventral, anterior, motor or 

 efferent root ; c.n., compound spinal nerve with 

 branches; s.g., sympathetic ganglion; a.c, anterior 

 column; the anterior fissure is exaggerated; g.c, 

 ganglion cells ; g.in., grey matter ; it'.m., white matter. 



motor is a ventral loot associated with the ophthalmicus profundus, that 

 the trochlear 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 possible that each segmental nerve was associated with a 

 branchial sense organ (Beard and Froriep). These organs arise above 

 the gills, and grow thence into various parts of the head, and along the 

 trunk as the "lateral line." It is possible 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, and it seems that parts of them arise in development 

 independently of the brain. It may be that nose and ear were originally 

 branchial sense organs. 



Spinal nerves. — Each spinal nerve has two roots — a 



