472 CHORDATA 



The relations of these nerves are the most primitive in the Ichthyopsida 

 (Hi,'. 527). Of these, three (I, II, and VIII) are purely sensory, going to the 

 olfactory organ, eye and ear respectively. The third, fourth and sixth are as 

 purely motor, supplying the eye muscles. The remainder are mixed in char- 

 acter. The glossopharyngeal nerve is typical. This leaves the medulla and, 

 at the top of the first gill cleft, divides into two branches, one of which (pre- 

 trr.mnlic) goes in front of the opening; the other (post-trematic) passing behind 

 it, both innervating the muscles of the region and supplying a sense organ above 

 the gill opening. The vagus, in the same wav, has pre- and post-trematic 

 branches for the posterior gill clefts, a condition which leads to the view- 

 that this is a compound nerve. In addition the vagus sends a branch to the 

 Fttitnach and heart. In the Amniotes, with the development of lungs, this 

 branch also supplies the lungs, hence the name pneumogastric. The vagus 

 als:> gives off a nerve which supplies the sense organs of the lateral line of 

 the trunk. 



The facial nerve divides over the spiracle in the same way as the ninth over 

 the first gill cleft. It also supplies the lateral line organs of the head. The fifth 

 nerve also divides at the angle of the mouth, a branch going into each of the 

 jaws, a third going towards the tip of the head. With the loss of gills and 

 lateral line organs in the terrestrial vertebrates the seventh and tenth nerves lose 

 the corresponding branches. The facial nerve, which in the Ichthyopsida is 

 largely sensory, becomes almost exclusively motor in the mammals, as a result 

 of the great development of the muscles of the face, which are practically lacking 

 in the lower forms. 



Since the head undoubtedly consists of several coalesced body segments (at 

 least as many as there are visceral arches, and apparently more), the question 

 arises whether the cranial nerves are as evidently segmental as are those of the 

 trunk. To this is allied the further question whether Bell's Law that a mixed 

 nerve consists of dorsal sensory and ventral motor components is applicable 

 here. In answering this question the olfactory and optic nerves, arising from 

 the prosencephalon, must be excepted. The latter, especially, is not a peripheral 

 nerve, buHs a tract connecting two parts of the brain, since the retina, as we 

 shall see, is but an evagination of a part of the brain towards the periphery. 

 With this cerebral character is connected the peculiarity of the crossing or 

 cliiasma of the optic nerve. In the teleosts the whole right nerve goes to the left 

 eye, ^the whole of the left to the right eye, the nerves either simply crossing or 

 passing through networks in each other. In most vertebrates only a part of 

 the fibres cross outside of the brain. If all of the fibres cross inside the brain 

 (as is the rule for true cranial nerves) the result is the appearance as in thecyclos- 

 tomes, that each eye is supplied by the nerve from its own side. 



)f > the remaining nerves which spring from the hind brain and, with the 



exception of the trochlearis, from its side and base, the oculomotor and abducens 



(possibly also the trochlearis) develop like ventral roots. The eye muscles 



vhich they supply are the remains of the somatic musculature arising from the 



myotomes, which has largely degenerated. All of the other cranial nerves (the 



hypoglossus excepted) arise like dorsal roots, are sensory, and are provided with 



the equivalents of spinal ganglia (genirulate ganglion of the facial, Gasserian 



anghon of the trigeminal, vagus and glossopharyngeal ganglia), but they also 



ontam numerous motor fibres. It is important to note that the muscles in- 



rvated by these motor fibres do not arise from the myotomes and hence do not 



belong to the somatic musculature, but are a part of the 'visceral muscles.' 



since the dorsal roots in Amphioxus contain motor fibres which supply 



:eral muscles, and visceral motor nerves occur in the dorsal roots of the 



es, it is necessary to modify Bell's Law and to say that the ventral roots 



