Johnston, The Radix Mesencephalica Trigemini. 605 



In mammals, as already frequently described, the greater collec- 

 tion of these cells is found in the locus cceruleus and they extend 

 forward sparsely even in the superior coUiculus. 



With respect to the ventricle and the layers of the brain wall these 

 cells vary in position within wide limits. I see nothing of funda- 

 mental importance in this connection, but the following facts are of 

 interest. The usual position of the cells is perhaps outside the 

 ependymal layer and among the deepest layers of nerve cells. In a 

 few cases in larval Amblystoma and Desmognathus I have noticed 

 cells in a superficial position similar to that of the giant cells of the 

 cord in the trout (Harrison, 1901). In the frog and especially in 

 the toad many cells are deeply imbedded in the brain wall among 

 the fibers of decussatio tecti. JSTear the median line in selachians 

 and Necturus the cells are often so crowded that the ependymal cells 

 are pushed aside and these cells seem to form the wall of the ventricle. 

 This is especially noticeable in Scyllium, where a large collection of 

 cells is found on the deep face of the decussatio veli and the trocli- 

 learis (Fig. 9). Here in sagittal sections the ependyma seems to 

 be displaced or destroyed and the group of large cells projects 

 directly into the ventricle. Similarly in several instances in 

 Scyllium and Necturus I have found isolated pear-shaped cells 

 hanging down into the ventricle, suspended by their large processes 

 which pass up between the ependymal cells into the tectum. The 

 sections were not broken and all the elements were in perfect order. 



As to the characters of these cells, I have made no extended or 

 minute study of their morphology, but in the course of examination 

 of many brains several important features have been noticed which 

 bear upon their interpretation. The typical form is usually said to 

 be vesicular, pear-shaped or balloon-shaped with a single large process. 

 This statement may, I think, stand for the "typical" form, whatever 

 that may mean. It is undoubtedly true, however, as described by 

 Merkel, Held, Kolliker and Van Gehuehten, that at least some of the 

 cells have more than one process. Sargent (1901) figures these as 

 multipolar cells in several fishes, and in my preparations of Sela- 

 chians, Acipenser and the toad many cells may be seen which have 

 one or more processes in addition to the large process. This larere 



