vn] Linibic Lobe <ntd Olfactory Area 181 



is no ensv matter. At first I was doubtful whether or not to extend it to the Hoor of the 

 tissura hippocampi, but as the fissural or subicular portion of the hippocampus shows certain 

 peculiarities of structure I have decided on a subdivision of the area; this differentiation 

 will be found to be in accord with Ramon y Cajal's observations, and the dividing line lies 

 along the lip of the hippocampal fissuiv and corresponds with the periphery of the thick 

 .superficial layer of nerve fibres known to histologists as the lamina medullaris externa, or 

 lamina medullaris involuta. This leads up to an account of the fissural cortex of the 

 hippocampus. 



CORTEX OF THE FISSURA HIPPOCAMPI. 



Arrangement of Nerve Fibres. (Plate XVIII, fig. 1.) 



By far the most important feature of this region is the extraordinary development of 

 the zonal layer, giving rise to the dense and thick lamina medullaris externa just alluded 

 to. Thickest at the floor of the fissure where it is in contact with the alveus, this lamina 

 gradually loses in depth as it creeps upwards, and soon fades away after crossing the lip 

 and reaching the free surface of the gyrus. In the subjacent cortex it is impossible to 

 define the usual layers ; it is all richly filled with fibres interlacing and intertwining in every 

 direction, and it is divided into panels by broad pillars or bands of fibres which extend 

 from the outer lamina to the white substance, bands which might be the equivalents of 

 the projection bundles of Meynert. 



From the general trend of fibres in this part it is evident that it has intimate connections 

 with the area last described and also with the cornu ammonis. 



Cell Lamination. (Plate XVIII, fig. 2.) 



Two features characterise the cell lamination of this part : one is the presence of curiaus 

 independent islets of minute cells in the plexiform layer, and the other is the distinctive 

 arrangement and appearances of the deep layer of large pyramidal cells. 



The islets find a place in the extremely deep plexiform layer and are composed of 

 a hundred or more minute, deeply-stained, triangular cells with a diameter of about 5 /j,. 

 These extraordinary cell-groups are not numerous, only two or three being visible in each 

 section, and there appear to be more of them in the anterior part of the area than in 

 that adjoining the splenium. What lamina they represent it is impossible to say, unless 

 they are the equivalent of the layer of small pyramidal cells. 



The nests of large polymorphous cells noted in the last region examined are not 

 discoverable; at the same time in parts of the area one does see a few cells which, as far 

 as one can judge from Nissl specimens, belong to the large polymorphic variety ; and they 

 are occasionally converted into groups by the broad bands of medullated nerve fibres which 

 unite the lamina medullaris and the white substance. 



As for the rest of the cortex it is occupied by one deep layer of rather large pyramidal 

 cells, pyramids which are elongated in form and have a particularly long apical process ; being 

 arranged in regular parallel rows, the layer they form has accordingly been given the name 

 "stratum raditttum." Traced in the central direction this layer seems continuous with the 

 prominent layer of pyramidal cells which is to be found in the cornu ammonis, and at first 



