Johnston, llie Brain of Pctrouiyzoii. 1 1 



can not be traced to their endings in haematoxylin sections and 

 consequently Ahlborn has mistaken for them the motor roots 

 which arise from the lateral column of large cells. 



The motor root of IX is not mingled with its sensory root, 

 but lies considerably ventrad and cephalad from it, nearly in the 

 same transverse section as the postauditory lateral line root 

 (/. /. A', Figs. I, 9, lo). It is larger than the motor roots of X. 

 The fibers of the motor VII arise from the cells of the 

 lateral motor column in the caudal part of its cephalic half 

 (Fig. ii). The fibers are fine and collect into two bundles as 

 they leave the motor column ; one bundle pierces the ventral 

 part of the spinal V tract, the other the dorsal part. The bun- 

 dles are not traceable as such through this tract, but are broken 

 into small fasciculi or single fibers. The fineness of the fibers 

 makes it difficult to distinguish them from the fasciculus com- 

 munis fibers of VII, with which indeed they seem to be min- 

 gled as they emerge from the medulla. In the absence of a 

 knee bend in the root of the motor VII, Petromyzon seems 

 to differ from all other vertebrates. 



As Ahlborn states, the motor V receives fibers from two 

 sources (Figs, ii, 12, 13). The main portion of the cephalic 

 half of the lateral motor column is the place ol origin of the 

 greater part of the large root. A small part of the fibers form 

 what Ahlborn called the ascending root of the motor V. He 

 traced these fibers caudally in the medulla but could not find 

 their place of origin. I find that they come from the isolated 

 cephalic portion of the ventral motor column which lies at the 

 level of the VII-VIII root complex. 



I am unable to verify Ahlborn's account of the VI nerve. 

 A root of considerable size arises from the cephalic end of the 

 lateral motor column (Fig. 13), runs independently for some 

 distance, and joins the motor V root as the latter enters the 

 Gasserian ganglion. Thus far Ahlborn's account is clearly 

 correct. Beyond this point I am unable to trace the bundle 

 with certainty, but it seems to become permanently united with 

 the V trunk. I am inclined to the opinion that the root in ques- 

 tion beloncfs to the V and that the VI is to be looked for else- 



