94 Journal of Comparative Neurology 



of neuroglia, but penetrated everywhere by fibers, that run gen- 

 erally parallel to the walls of the mesoccele. Near the ectal limit 

 of layer C some cells are found that have their longer axes 

 turned parallel to the ectal surface. Some authors have called 

 this a separate layer for this reason, but as the fibers are 

 slightly more numerous in this layer we doubt if there is any 

 difference between these cells and those lying adjacent, excepting 

 that of position, and that difference is probably the result of the 

 pressure of the fibers. Layer D has numerous cells while layer 

 E is mainly fibrous. 



Layer F is the most clearly defined of the cellular layers 

 and is probably the most important functionally. It consists of 

 large pyramidal cells arranged in compact rows. These cells lie 

 with their longer axes perpendicular to the walls of the mes- 

 occele and their apexes directed peripherad. From the apex of 

 each cell a process is sent off, these processes group themselves 

 together, as shown in the figure, as they penetrate the fibrous 

 layer E. They then extend in a straight line to the retinal 

 fiber layer. According to Van Gehuchten all these processes 

 end in the retinal layer, but with the method used I was not able 

 to demonstrate this though the majority of them could be easily 

 traced to these fibers. From the base of these cells other pro- 

 cesses are given off, which form the greater part of the layer G. 

 These are the axis cylinders. Layer H is composed of endymal 

 cells. 



In Figure 21 near the mesal line are shown some very large 

 cells. These cells are found on either side of the meson 

 throughout almost the entire length of the tectum. They con- 

 stitute the so-called mesencephalic nidus of the trigemini. Some 

 authors, among whom are Deiters and Golgi, refer them to the 

 trochlearis. Golgi, in his paper on the origin of the trochlearis (34) 

 maintains this view. He also figures these cells from the brain of 

 the rabbit as being unipolar and says that they are absolutely so. 

 His figures are taken from cells isolated by the maceration pro- 

 cess, and he is forced to admit that neither by this nor by the 

 silver method has he been able to trace the axis cylinder pro- 

 cesses to their termination. Figure 26 shows two of these cells 



