Johnston, TJie Brain of Petromyzon. 47 



number of these organs in about two millimeters in length of 

 the body at the level of the first gill slit is about sixty. The 

 organs are usually found singly, but often two, three or four are 

 placed near together. In the anterior part of the head they 

 do not appear to be more numerous than in the gill region, 

 b. The cranial nerves. 



I. The vagus group (Figs. 7, 29). 



After emerging from the wall of the medulla the mixed 

 communis and motor roots of X run as four or five bundles for 

 some distance within the cranial cavity, from which they emerge 

 just within the caudal end of the auditory capsule. In trans- 

 verse section the bundles in the cranial cavity appear in a dorso- 

 ventral row, the most ventral being the cephalic root. As they 

 near the foramen of exit these roots come close together and be- 

 yond the foramen they form a single nerve trunk which passes 

 backward as the vagus proper. The IX nerve has a single com- 

 munis root which runs parallel with the X roots in the cranial cav- 

 ity, but outside of and below them. Immediately above it is the 

 post-auditory lateral line root which is much larger than any of 

 the IX or X bundles, and is laterally compressed and covers the 

 outer surface of the X roots. As all these roots approach their 

 foramina the IX becomes closely applied to the lateral line root 

 and emerges through a separate foramen slightly dorsal to that 

 for X. The lateral line root has still another more dorsal fora- 

 men, but the IX joins it again beyond the foramen. The 

 lateral line root has a large V-shaped ganglion of large cells, 

 one limb of which extends dorsally on the outer surface of the 

 cranium while the other extends latero-ventrally. The root of 

 IX accompanies this lateral hmb of the ganglion as a weh de- 

 fined bundle of fibers which are connected with small cells form- 

 ing the ventral part of the ganglion. The whole lateral limb 

 of the ganglion is figured by Ahlborn ('84) as the ganglion of 

 IX (j5r. /.). As it leaves its ganglion the trunk of IX receives 

 a considerable bundle of fibers from the large cells of the dor- 

 sal part of the ganglion. These lateral line fibers are slightly 

 thicker than the IX fibers, but can not be traced to their desti- 

 nations in my preparations. It is probable that they innervate 



