6 14 'Journal of Comparative Neurology and Psychology. 



The first mammal in which the bundle was followed with certainty 

 was the mole, whose brain is comparatively simple and primitive. 

 In it the bundle runs caudad over the motor nucleus without any 

 complex relation to it and then bends downward and forward to 

 enter the spinal trigeminal tract and leave the brain in the sensoiy 

 root. The large cells in the locus coeruleus are widely separated from 



C 



Fig. 12. Sketches to show the course of the meseucephalic root in the 

 commou rat. A, B, C, camera outlines from transverse sections ; D, diagram 

 of the roots projected on the sagittal plane. In A, the mesencephalic bundle 

 bends down through the substantia gelatinosa to enter the spinal trigeminal 

 tract. Its fibers then run forward to leave the brain in the sensory root (C). 



the motor nucleus and closely related to the granular layer of the 

 cerebellum. I hope to study and describe this relation later, but it 

 is to be noted here that there is nothing to suggest any relation of 

 the locus coeruleus to the motor nucleus of the trigeminus or to any 

 other motor area. 



The essential features seen in the mole gave the key to the study 

 of other mammals. In the rat (Fig. 12) and cat it was found 



