236 ANATOMY OF THE CENTHAL NEHTOUS SYSTEM. 



appearance on section. It has therefore been designated as the stratum 

 radiatum. The cells, themselves, in the hardened preparations, appear to 

 lie in large cavities. Thus, their long curved course appears as a clear layer, 

 and has received the name of stratum lucidum. Besides their dendrites, they 

 send a part of their neuraxons also out to the layer of tangential fibers, ex- 

 actly as in the rest of the cortex. The greater part of the neuraxons, how- 

 ever, pass ventrally, and these, with other fibers, then form a true medul- 

 lary layer, the alveus, which lies directly underneath the ventricular epi- 

 thelium. The small space between the stratum lucidum and the alveus 

 is filled by numerous fibers passing into the cornu Ammonis and by fibers 

 leaving it. It contains innumerable fiber-branchings and a number of very 

 remarkable association-cells, which we have only recently become acquainted 

 with through Ramon y Cajal. They are adapted in consequence of their 

 much-branched neiiraxon, which penetrates into the cell-layer of the stratum 

 lucidum, to connect well together the pyramidal cells of the gyrus hippo- 

 campi (see Fig. 9). The entire layer is designated as the stratum oriens. 



All investigations on the cortex of the cornu Ammonis show that there 

 exists here an abundance of cells and a multiplicity of fiber-relations which, 

 so far as is known, has no counterpart in all the remaining cortex. 



If what is typical in the structure of the cerebral cortex be once com- 

 prehended, it is not difficult to recognize the type in regions where it is less 

 distinct. The bulbus olfactorius, for example, formerly was not understood 

 at all. If Fig. 142 is inverted and compared with Fig. 152, the similarity 

 strikes the eye at once. We are here concerned with a cortex in the mo- 

 lecular layer of which the olfactory-nerve fibers enter, and terminate by 

 splitting up into an arborization. But the entire cortex is more condensed. 

 The entrance, also, of the olfactory-nerve fibers into the layer of tangential 

 fibers of the surface, and the different manner in which the dendrites of the 

 cortical pyramids divide and terminate, occasioned by the former, give to 

 the whole an appearance that has hitherto rendered difficult of perception 

 the fact that we are here dealing with nothing other than a common cortical 

 formation. 



The surface of the cerebral cortex is covered in man (Weigert) by a 

 dense net-work of neuroglia, from which numerous, but somewhat widely 

 separated, prolongations radiate down into the region of the smaller pyra- 

 mids. Then the net-work of neuroglia constantly becomes thinner, and in 

 the deepest layers of the cortex it is almost entirely wanting. Within the 

 radii separate fibrils only are perceptible. In the medullary layer there 

 again lies a relatively-dense accumulation of neuroglia, which everywhere 

 surrounds the medullated fibers. 



If the nervous elements of the cerebrum are destroyed — e.g., in paralyses — there 

 appears in their place an hypertrophy of neuroglia which is characterized not only 



