638 Journal of Comparative Neurology and Psychology. 



tral processes (axones). The ground for this comparison is as 

 perfect as can be desired so far as the animals thus far studied are 

 concerned. 



I have already pointed out that the bundle has essentially the same 

 course and position in the brains of all classes and I need only repeat 

 that it is always situated in the dorsal zone of the brain. There is 

 not known in any part of the brain or spinal cord of any vertebrate 

 a root bundle of any motor nerve which runs longitudinally in the 

 dorsal half of the brain wall. The trochlearis is the only motor 

 nerve which enters the dorsal zone of the brain in any part of its 

 course, and that only to decussate. On the other hand, all the sensory 

 nerves, their roots and longitudinal bundles in all vertebrates are 

 strictly confined to the dorsal half of the neural tube. This is not a 

 demonstration that this bundle is sensory in function, but it shows 

 that the presupposition is that it should be sensory. We expect all 

 primary bundles in the dorsal half of the brain wall to be sensory ; the 

 burden of proof rests with those who would consider this bundle to 

 be motor. 



The connections of the bundle in the brain require further study. 

 Since all the evidence goes to show that the large processes of the 

 cells are peripheral sensory fibers comparable to the peripheral pro- 

 cesses of the spinal ganglion cells or the dorsal cells of the spinal 

 cord, we should look for connections in the brain similar to those set 

 up by the central processes of the spinal ganglion cells. I have 

 shown (1900) that the giant cells in the cord of fishes are bipolar 

 and that their axones run in the dorsal tracts, so that these neurones 

 resemble embryonic spinal ganglion cells in everything but the posi- 

 tion of their cell bodies. The cells in the mesencephalon of the toad 

 are unipolar, bipolar or multipolar and possess true axones which 

 enter the substance of the tectum. The same is probably true of the 

 bipolar cells described by Merkel and Krause and of those which I 

 have seen in the rabbit. I consider it probable also that one of the 

 several processes which KoUiker saw on these cells may have been a 

 true axone. The same may be said of the two cells figured by Van 

 Gehuchten. Each has a slender ascending process which is probably 

 the true axone. My line of reasoning here is that the existence of 



