423 



has its main origin from the dorsal rami of the seventh and tenth 

 nerves. It leaves the vagus at the junction of its ganglion and dorsal 

 ramus, where the vagus receives its loop from the seventh nerve: and 

 some cells from the dorsal half of the vagus ganglion pass into the 

 lateralis here. If we look then upon the lateralis as forming posteriorly 

 a commissural system connecting dorsal rami of posterior spinal roots, 

 we may say that this commissural system is continued forward, cra- 

 nially at least as far as the seventh, and possibly even the fifth nerve. 

 Although the Lamprey presents a well-marked lateralis nerve, it 

 has not also a regular lateral line, for the sense-organs of the skin are 

 scattered and without segmental arrangement. 



The sense-organs do not therefore appear to be in direct relation 

 with the spinal ganglia, and the view of the close connection between 

 them (Spencer, Beard, Fro rie p) does not receive support. 



The recent work of On odi, according to whom, in the Fish and 

 Lizard, the spinal ganglia are purely proliferations of the dorsal part 

 of the medullary tube, casts further doubt upon this view. 



It seems more natural to consider the lateralis as a relic of the 

 extensive and irregular commissural system connecting the posterior 

 roots of Amphioxus. 



The scattered hair-cells of Amphioxus are irregularly grouped in 

 Petromyzon, and it is only in higher Chordata that a definite segmental 

 arrangement obtains and a corresponding relation to the spinal ganglia. 

 The close relation of ganglion to sense-organ, which is asserted in 

 some developmental histories, is probably therefore secondary. And if 

 we remember how in Selachii the lateralis nerve, still lying deeply 

 seated and close to the spinal nerve-roots, sends long branches 

 through the intermuscular septa to the sense-organs of the 

 skin, we perhaps get an idea of a condition contributing to the seg- 

 mental arrangement of the latter. 



Bdellostoma. The anterior and posterior roots are distinguished 

 as in the Lamprey by the size of their fibres, but they unite to form 

 mixed nerves. 



In some regions of the body, if not throughout, there are two 

 anterior roots for each posterior root: — a fact of interest as tending 

 towards the condition found in Amphioxus. The two anterior roots 

 send up each a dorsal ramus, and these unite into a single branch for 

 the back. The posterior root presents a distinct spinal ganglion, and 

 divides into a dorsal and ventral ramus. The ventral ramus unites 

 with the corresponding rami of two anterior roots. We have not yet 

 traced a junction of the dorsal ramus with that of the anterior roots. 

 The dorsal rami of posterior roots have no commissural connections. 



