442 STRUCTURE OF VERTEBRATA. 



into "dorsal," "ventral," and other branches. As these typically 

 innervate a gill arch and slit, as may be well studied in 9, the branches 

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

 branchial, prae-branchial, &c. 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- 

 motor is a ventral root associated with the third or ciliary nerve, 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 likely 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 



P, C Pi 



FIG. 141. 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.m., 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., a 

 ganglion cell; g.m.) grey matter ; iv.m., white matter. 



lay over each primitive gill cleft, and had an associated ganglion. The 

 ganglia known as ciliary, gasserian, &c., 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 dorsal, posterior, or 

 sensory, and a ventral, anterior, or motor. These arise 

 separately and independently, but combine in the vicinity 

 of the cord to form a single nerve. The dorsal root exhibits 

 at an early period a large ganglionic swelling the spinal 



