640 Journal of Comparative Neurology and Psychology. 



hypothesis the motor collaterals would be analogous (not homologous) 

 to the motor collaterals in the spinal cord, and serve for direct reflexes 

 between the sensory surfaces about the mouth and the muscles con- 

 trolled by the trigeminus. This is perhaps the most important 

 function of the mesencephalic root bundle and is sufficient to account 

 for the growing predominance of the motor collaterals. 



I have gone thus far with these speculations in order to show that 

 very interesting problems lie here for other workers — to determine 

 more completely the morphology of these neurones in all classes of 

 vertebrates, the disposition of their central processes, their origin 

 and development and especially the history of the several processes, 

 the axones and the motor collaterals. The size of the mesencephalic 

 root and its constancy in all classes of vertebrates are sufficient proof 

 of its importance and of the value of further studies along the lines 

 indicated. 



The peripheral course of the fibers is the one point of crucial 

 importance in the question at issue. In showing that the mesen- 

 cephalic root bundle in selachians, ganoids, urodeles, anura, reptiles, 

 insectivores, rodents, ungulates, carnivores, and man leaves the brain 

 in the sensory root, I believe that I have established the strongest 

 probability that the bundle is sensory in function. In mammals and 

 man the motor root runs over the surface of the trigeminal ganglion 

 without interchange of fibers. In many lower forms the motor root 

 is almost as distinct. I do not know of any case in vertebrates in 

 which motor fibers leave the brain in a sensory root and join the 

 motor rami peripherally. 



Exact and conclusive proof of the sensory character of this bundle 

 would be obtained by one or a combination of the following opera- 

 tions: (a) Destruction of the cells of origin or cutting the mesen- 

 cephalic root at any point central to the Gasserian ganglion without 

 injury to the latter, followed by examination of the peripheral trunks 

 of the trigeminus by the Marchi method to determine the distribution 

 of the degenerated fibers. (&) Violent tearing out of one of the 

 peripheral sensory rami in each of several animals with later study 

 of the cells of origin to determine which ramus it is whose rupture 

 causes destruction of the cells by retrograde degeneration. I have 

 not found time or favorable conditions to attempt these operations. 



