Johnston, TJie Brain of Petromyzon. 23 



funiculi, nucleus of the spinal V, cerebellum, and probably also 

 in the acusticum. 



A discussion of the general significance of these facts is 

 reserved for the theoretical part of this paper. 



d. Internal arcuate fibers. 



The course of the internal arcuate fibers from the several 

 cutaneous centers has been described in part. In the caudal 

 part of the medulla the fibers coming from the nucleus funiculi, 

 nucleus trigemini spinalis, and the acusticum course around the 

 periphery of the medulla and cross the ventral raphe near the 

 surface. These fibers are relatively fine and very numerous and 

 form a conspicuous layer in the caudal part of the medulla, but 

 grow relatively less numerous farther forward. In the cephalic 

 part of the medulla the arcuate fibers are coarser and run 

 deeper, crossing a little beneath the floor of the ventricle. 

 Among these are the thick fibers from the spindle cells. The 

 arcuate fibers can not be traced forward as a distinct tract, and 

 in the tectum the tractus tecto-bulbaris and bulbo-tectalis are 

 only imperfectly separated. I believe, however, that the rela- 

 tions of the arcuate fibers are the same here as in Acipenser. 



e. The fasciculus communis center. 



As in Acipenser the fasciculus communis system is repre- 

 sented in the medulla of Petromyzon by a vagus lobe entending 

 from the caudal end of the medulla to the level of the IX nerve, 

 and by a bundle of fibers extending forward from this to the 

 root of VII, the sensory root of which constitutes the bundle. 

 Caudally the vagus lobes are connected above the ventricle by 

 the commissura infima Halleri, and there continue into the 

 cord median tracts of fibers which seem to be related to the 

 vagus lobes alone. 



In the cord in the region of the first and second spinal 

 nerves a few cells situated in that part of the gray matter which 

 represents the dorsal horns send dendrites into both the deeply 

 staining and pale tracts. Other cells, situated nearer the median 

 line, dorsal to the central canal, send dendrites into the pale 

 tracts alone. Farther forward the latter cells become sharply 

 distinguished in form and size (8-32 X 30-80 p) from the cells 



