6 Journal of Comparative Neurology. 



the cervical bundles of Cajal which continue the fasciculus 

 communis system into the cord. 



The great increase of the grey matter dorsally results in 

 an apparent or relative crowding ventrad of the lateral and ven- 

 tral tracts, and the medulla of Petromyzon in front of the cala- 

 mus scriptorius assumes the appearance of an entirely charac- 

 teristic fish medulla. The must dorsal and median portion of 

 each lateral wall is occupied by the fasciculus communis center 

 (lobus vagi). Latero-ventral to this is the nucleus funiculi, or, 

 farther forward, the tuberculum acusticum with the spinal V 

 tract bounding it ventrally. Ventral to this are the lateral and 

 ventral tracts with the central grey matter adjoining the cavity. 

 In this there are two distinct columns of motor cells, one lateral 

 and one ventral in position. In the region of the calamus the 

 large hypoglossus nerve takes its origin from the ventral group 

 of motor cells by several rootlets which are surrounded by the 

 cells of the lower olive. The sensory and motor roots of the 

 IX and X nerves take their apparent or external origin high up 

 on the lateral surface of the medulla, at about the level of the 

 spinal V tract, as will be described more fully below. 



Passing forward, the acusticum grows larger and by the 

 time the level of the VII and VIII nerves is reached the fasci- 

 culus communis has lost its dorsal position and has become a 

 small bundle of fine fibers recognizable with difficulty between 

 the acusticum and the central cavity (Figs. lo, ii). At the 

 same time the lateral group of motor cells has grown extremely 

 large and projects into the cavity as a great rounded ridge 

 which extends from just behind the root of VII to the point of 

 exit of the sensory V. The fasciculus communis disappears 

 from sections in front of the VII root, the spinal V tract rises 

 to a more and more dorsal position, and finally emerges from 

 the dorso-lateral surface of the medulla at its extreme cephalic 

 end as the sensory root of V (Figs, i, 12, 13). Somewhat 

 further caudally the large lateral column of motor cells gives 

 rise to the very large motor root of V, and immediately in 

 front of this to Ahlborn's VI, and then quickly disappears from 

 sections. 



