168 ANATOilY OF THE CENTRAL NERVOUS SYSTEM. 



there are numeroiis cells, which, for the most part, cannot be recognized as 

 ganglion-cells, bitt rather retain the character of neuroblasts throughout life. 

 Between them, however, lie true ganglion-cells, with profusely branching 

 dendrites and slim neuraxons. The majority of these neuraxons may be 

 traced upward toward the brain-surface; but a small minority take their 

 course between the ganglion-cells and the epithelial cells, and mark the be- 

 ginning of the siibcoiiical medullary la^er. Whither they go in amphibia is 

 not known, but it is probableThey go mostly in the commissures of the 

 mantles. From single fibers which pass outward from this subcortical 

 medullary layer and from those which pass direct from the cells to the sur- 

 face of the brain there is formed just under the surface of the brain a fine 

 net-work: the tangential reticulum. Besides the two sources named, the 

 neuraxons of cells which lie in the tangential reticulum itself participate in 

 the formation of the net-work. 



This quite irregularly disposed apparatus must be looked upon as the 

 fundament of a brain-cortex, because in reptiles one finds just the same 

 elements, only in much greater number and thickness, arranged, further- 

 more, in more regular and unmistakable strata. In these animals no fur- 

 ther doubt can exist that one has in this structure to deal with a cortex, 

 from which, as we shall see later, the highly specialized and well known 

 cortex of higher vertebrates may be derived. 



It is, indeed, one of the greatest services which S. Eamon y Cajal has 

 bestowed in the field of brain-anatomy that he has demonstrated the type 

 which recurs in the structure of the brain-cortex in all classes of vertebrates 

 and that he designated the features that characterize a brain-cortex. The 

 author's own investigations on amphibia and reptilia coincide throughout 

 with the important discovery of the Spanish savant. 



A most essential featitre of the cortical structure, and one always recog- 

 nizable, is the fact that these fibers originate and end, and that there exist 

 innumerable possibilities for the association of incoming and outgoing fibers. 



In the cortex of the reptile one may from without inward differentiate 

 (1) the tangential layer of fibers, (2) a molecular cell-layer, (3) a layer of 

 pyramidal cells, (4) the layer of the Ple.xus suhcoiiicctlis, (5) the medullary 

 center, and (6) the ventricular epithelium (compare Fig. 117). 



This relatively simple apparatus is, however, so constructed, even in 

 vertebrates of so low rank as the reptiles, that it affords an almost infinite 

 possibility for combinations of single cells and tracts (study, again. Fig. 117). 



But the cortex is not, by any means, uniform over the whole mantle. 

 Even in reptiles one can, differentiate particular cortical areas one from 

 another. The author would differentiate at least three separate areas in 

 the cortex of the reptile (see Fig. 103), to which might be added as a fourth 

 the cortex on the Conu.s frontalis pallii, which belongs possibly to the olfac- 



