Johnston, flu^ Radix MesenceplHilica Trigemint. 613 



other preparations with perfect clearness around the lateral part of 

 the tectum to the nucleus magnocellularis tecti. The fibers run in a 

 broad bundle immediately outside the ependymal layer and at the 

 anterior border of the tectum are seen in transverse sections bending 

 toward the median line and joining the large cells (Fig. 11). 



In the turtle the whole course of the bundle is remarkably clear. 

 At its exit there is some intermingling with the motor roots, which is 

 not seen in fishes or amphibians and suggests the complex relations 

 seen in adult mammals. 



In mammals the structures in the region of the trigeminal roots 

 are more crowded than in lower vertebrates and the relations of the 

 mesencephalic root have been difficult to make out on that account. 

 So long as the fact that the cells of origin lay in the brain did not 

 seem to be of especial importance for the interpretation of the bundle, 

 authors inclined to believe that it joined the sensory root. After the 

 formulation of the neurone doctrine and the law that sensory fibers 

 arise from peripheral ganglion cells and motor fibers from central 

 cells, most authors saw a connection of the mesencephalic bundle with 

 the motor root of the trigeminus. I believe that this accounts for the 

 change of opinion from Meynert and the older authors to Kolliker, 

 Van Gehuchten, Cajal and recent authors. The mesencephalic root 

 is very difficult to trace to its exit in mammals, the differences of 

 opinion are most natural, and the opportunities for seeing what one 

 wishes to see are admirable. "When I first studied this root in 

 Scyllium I fully expected to find it join the motor root, as I had 

 been convinced so far as mammals and birds were concerned by the 

 work of Cajal and of Wallenberg. I was greatly surprised by what 

 I found in Scyllium, but in all the forms studied since I have never 

 been able to trace the bundle into the motor root or to find good 

 reason for doubting its connection with the sensory root. 



Instead of burdening the paper with an extended description and 

 with numerous figures of the various brains, I will make one or two 

 notes on the course of the bundle in adult brains and depend upon the 

 figures from the human embryonic and icctsil brains to demonstrate 

 the essential relations in mammals. It is necessary to give attention 

 only to the relations of this bundle to the motor and sensory roots, 

 since the course of the upper part of the bundle is well understood. 



