Johnston, The Radix Mescenphalica Trigemini. 637 



part of the root remains wholly distinct from the motor root and 

 nucleus ; all the fibers of this root mingle intimately in the sensory 

 root of the trigeminus with the fibers which end in the chief sensory 

 nucleus and with those which go to form the spinal trigeminal tract. 



Discussion and Conclusions. 



The important considerations regarding this mesencephalic root 

 are (1) the character and significance of its cells of origin; (2) the 

 course, position and connections of the bundle in the brain and 

 (3) the peripheral distribution of its fibers. 



That the fibers arise from cells in the brain in all classes of verte- 

 brates there is now, I believe, no reasonable doubt. I have shown in 

 this paper that in fishes and amphibians the great majority of these 

 cells lie in or near the mid-dorsal line in the tectum mesencephali. 

 Since the same root bundle of the trigeminus arises from these cells 

 and from those in the locus coeruleus in mammals, I know no reason 

 to doubt, and I know of no author who doubts, that the cells in the 

 two situations belong to the same category. We are dealing with the 

 same set of cells throughout all vertebrates and those in mammals 

 must be regarded as having migrated farther from the place of origin, 

 at the mid-dorsal line. The facts are all in favor of the supposition 

 that these cells have been derived from the neural crest just as the 

 giant ganglion cells in the spinal cord have been. The cells located 

 in the locus coeruleus in mammals w^ill require some additional ex- 

 planation such as the supposition that they have migrated probably 

 from the mesencephalic segment along the course of the nerve bundle. 

 I see no possibility of denying that these cells arise in the extreme 

 dorsal region of the brain in all vertebrates and I know of no other 

 structures with which they can be compared than the giant cells in 

 the spinal cord. The reason for comparing these with one another 

 is that both send their processes out in sensory nerves. 



The similarity of the mesencephalic cells in size, form and struc- 

 ture to the spinal ganglion cells and to the dorsal cells of the spinal 

 cord is another strong argument. It is important to notice that in 

 both classes of cells bipolar and unipolar examples are found which 

 give rise to coarse peripheral processes (dendrites) and slender cen- 



