Johnston, The Brain of Petroniyzon. 25 



the spinal V tract, and others pass through the lateral nucleus 

 of the acusticum and the space separating this nucleus from the 

 spinal V tract. A considerable bundle of fine fibers collects 

 above the inner border of the spinal V and a little farther cau- 

 dally this appears as a larger bundle above the internal arcuate 

 fibers from the acusticum. This bundle runs caudally mesial 

 to the acusticum until it joins the front end of the vagus lobe. 

 The course of this root has not been described by previous 

 authors. Its manner of entrance, through the spinal V nucleus, 

 corresponds to that of the sensory IX and X roots. 



The sensory VII, IX, and X roots are not large and the 

 whole fasciculus communis system is poorly developed in com- 

 parison with the same system in Acipenser. It is a noticeable 

 peculiarity that the vagus lobe is an important body of grey 

 matter at the level of the commissura infima Halleri and con- 

 tinues for some distance caudally from that. It is caudal to 

 the commissure that the largest bundle of secondary tract fibers 

 is to be found. The calamus scriptorius and the commissura 

 infima Halleki are very far forward with respect to the IX and 

 X roots and the body of the vagus lobe. At the very caudal end 

 of the medulla, at the point at which the canal shows the 

 slightest enlargement, I have noticed in one series a bundle of 

 fine fibers passing from the region of the fasciculus communis 

 (cervical bundle) close over the lateral surface of the cavity and 

 spreading out in the latero-ventral tracts. These are possibly 

 collaterals of the fibers of the median dorsal tracts. It is pos- 

 sible that there is a mingling of the dorsal tracts proper with the 

 cervical bundle in the cord and that these collaterals come from 

 the cutaneous fibers. Such a mingling would explain also the 

 fact that certain cells send their dendrites into both the dorsal 

 tracts and the tracts which belong to the fasciculus communis 

 system. 



(f ) The lower olive (Fig. 6). 



This body presents about the same appearance in Petro- 

 myzon as in Acipenser, but is smaller. It consists of a group 

 of spindle shaped cells disposed in the transverse plane parallel 



