OEIGIN OF THE COEPUS CALLOSUM. 53 



alveus are tending towards the fimbria, but in the section under consideration many 

 fibres of the alveus, as well as fibres coming from the " pouch " of the fascia dentata, 

 proceed directly into the commissura dorsaUs along with many fibres of the fimbria 

 which have come from more caudally situated regions of the hippocampus. It will be 

 noticed, however, in the figure that as yet only the upper part of tlie fimbria is 

 contributing fibres to the commissure. The tip of the fimbria, which contains fibres 

 coming from the most distant parts (temporal pole) of the hippocampus, is yet undisturbed 

 and has given no contriluition to the dorsal commissure. Between the alveus and the 

 curved lateral part of the hippocanipal fissure there is a broad, more or less definitely 

 stratified region. Slightly nearer to the alveus there is a very uniform and clear layer 

 (in specimens stained by Weigert's method). This is the layer of " ^lyramidal cells," and 

 consists of a closely-packed column of large cells, which stain moderately with ordinary 

 stains. Between the layer of pyramids {i^yr.) and the alveus {ctlo.) there is a stratum 

 containing scattered polymorphous cells and very abmidant medullated nerve-fibres — the 

 layer of poli/morphous cells. The space between the hippocamjjal fissure and the layer of 

 pyramids is usually divided by writers into three zones — a broad clear layer next to the 

 layer of pyramids [stratum radiatum, s.r.), a thin superficial layer [stratum zonale), and 

 an intermediate layer very rich in medullated nerve-fibres [stratu^ii lacunosum, s.l.). 



A large number of fine medullated fibres (.r) extend from the region of the layer 

 of pyramids through the stratum radiatum into the stratum lacunosum. These fibres 

 are very distinct in the region bordering upon the fascia dentata [clde fig.). In the 

 stratum lacvmosum many of these fibres end by means of free branching in relation to 

 the processes of the pyramidal cells, which extend through the stratum radiatum into 

 the more superficial regions. But a considerable number of medullated fibres assume a 

 longitudinal direction (/. e. in the long axis of the hippocampus) as soon as they reach 

 the stratum lacunosum [s.l.). These fibres, which may 1)e called " longitudinal association 

 fibres of the hipjwcamj^us," appear in transverse section [s.l.) as rounded bundles 

 immediately surrounding the lateral or deeper part of the hippocampal fissure. 



In the general cortex [j).) the cells are scattered throughout the whole thickness of the 

 cortex, whereas in the hippocampus (leaving out of account the fascia dentata) the great 

 majority of the nerve-cells are crowded together into the regular row of pyramids. The 

 transition-region from typical cortex to typical hippocampus is generally known as the 

 sub/culuv/, cornu animonis [sub.). AVhether the " subiculum" ought to be grouped with 

 the general cortex or with the hippocampus is a question to which Ave shall have to refer 

 su.bsequently. The deej)est layer of the general cortex consists of a mass of medullated 

 fibres — the corona, radiata. It will be noticed at a glance that while the fibres derived 

 from the hippocampus and " subiculum " proceed into the alveus, those derived from the 

 general cortex proceed away from the alveus, i. e. towards tlie external capsule. At the 

 point of meeting of alveus and corona radiata there is obviously a commingling of fibres, 

 but it is equally clear that any passage of fibres from the alveus into the " corona '' 

 or vice versci is either non-existent or extremely slight and practically negligible. In 

 the adjoining regions there are groups of longitudinal fibres belonging respectively to the 

 two distinct formations, and it is quite possible that an interchange between these two 



