116 



ANATOMY OF THE CENTEAL NEEVOUS SYSTEM. 



able terminal ramifications. There are similar nerve-terminations in several 

 of the deeper layers. The terminal ramifications come into manifold rela- 

 tions with the dendrites of cells which lie at various levels. A small number 

 of such cells appear to send fibers down into the optic nerve itself, but the 

 majority — especially a long layer of very large cells — send their neuraxons 

 ventrally, where they form a definite layer of the deep medullary layer: 

 Stratum medullare profundum. But into this layer, as into the optic layer, 

 numerous fibers enter from other terminal ganglia. Through this structure 

 there arises an extraordinarily great opportunity for the transmission of 

 light-impressions to the general sensory tract, since the deep medullary layer, 

 as far as now known, is in connection with ends of other sensory nerves. 



' Eefer to Figs. 64 and 71, and see how, at the posterior end of the mid- 

 brain, the complex of Tractus tecto-spinales et teeto-bulbares just anterior 



Fig. 67. 



to the cerebellum rises abruptly into the midbrain and there enters the deep 

 medullated layer. There we have again found connection with a bundle 

 already familiar, and may turn our attention to others. 



Faturalh', it is not very easy in all the fiber-systems which fill the 

 ventral portion of the midbrain to recognize the separate relations. To 

 solve this problem in adult animals seems quite impossible. As most wel- 

 come simple objects, the larvse of amphibians offer themselves. Here the 

 system of the deep medullary stratum develops itself before all other fiber- 

 systems of the midbrain, even earlier than the optic nerve. This layer has 

 medullary sheaths at a time when no other system in this region is medul- 

 lated, except, probably, the nuclei of the cranial nerves. If one, in observing 

 a frontal section, passes from the ventricular epithelium outward, one 

 comes first upon a laj^er of loose tissue with few cells, — the Ependym, — then 

 a simple tissue with large ganglion-cells, and beyond it into the only medul- 



