12 Journal of Comparative Neurology. 



Nerve Fibers. 



Although the course of the nervous processes of the various 

 types of cells has already been described individually, yet it 

 might be advisable to consider again here in particular the 

 behavior of the neurites in their totality and mutual relations, 

 especially as these relations are of a peculiar nature in the cere- 

 bellum of the Selachii. Besides this there are still those axis- 

 cylinders to be considered wJiich enter the cerebellwn from other 

 parts of the central nervous system. The presence of such nerve 

 fibers in the cerebellum of selachians, also, is to be assumed a 

 priori both from the standpoint of comparative anatomy and 

 from the necessity that external impulses must be transmitted 

 to the cerebellum. Notwithstanding this, the positive proof of 

 these fibers has presented the greatest difficulty to me. After a 

 painstaking search through my preparations, I have only been 

 able to actually demonstrate one isolated fiber unquestionably 

 of this kind. This one is shown in figure i6 (Plate III). We 

 see this fiber ascend in an irregular course through the granular 

 layer and break up in its terminal arborization in the molecular 

 layer. This very meager demonstration of the existence of 

 '^ascenditig fibers" in the selachian cerebellum must suffice at 

 present. In its caliber and morphological characteristics this 

 axis-cylinder observed by me scarcely differs from that of the 

 Purkinje cell. Nothing was observed of " moss-like ontgroivths " 

 at definite intervals, such as were described by Ramon y Cajal 

 and others on the ascending fibers of Mammalia {fibres mous- 

 se Jises). 



The tangle of nerve fibers in all layers of the cerebellar 

 plate is infinitely complicated and it is, in fact, very difficult to 

 find one's way. As already mentioned above, the neuraxons 

 of the Purkinje cells, before they enter the granular layer, usu- 

 ally proceed a longer or shorter stretch in a horizontal direction 

 beneath and in the layer of the Purkinje cells. In this way they 

 form a dense nervous plexus (Plate III, fig. 17, above). This 

 plexus is especially strongly developed where there is no gran- 

 ular layer under the Purkinje cells, in consequence of which all 



