THE CENTRAL NERVOUS SYSTEM. 



method. In the deeper parts of the broad third layer a similar 

 well-marked net-work occurs, the interlacing fibres of which sur- 

 round the nerve-cells of the layer. Beyond this plane nervous 

 reticulations occupy the third and second layers and extend into 

 the outer zone. 



A limited number of the nerve-fibres terminate within the outer 

 layer as axis-cylinders which run parallel with the free surface, 

 as do also the terminal ramifications of the axis-cylinder processes 

 of some of the ganglion-cells. The recent investigations of Golgi 

 and others have shown that many fibres end without demonstrable 

 direct anatomical continuity with the nerve-cells, although a 

 close relation between the cells and the fibres undoubtedly exists. 



While the arrangement just described may be regarded as typical 

 for the greater part of the cortex, a few localities are distinguished 

 by modifications which materially affect the histological details. 

 These changes depend upon either an arrested development of the 

 cortex, as in the septum lucidum, or an increased complexity of 

 the cortical arrangement, as in the hippocampal convolution. Less 

 conspicuous variations, affecting one or more layers, are frequently 

 encountered ; thus, the paracentral convolution contains the 

 largest pyramidal cells, the "giant pyramids" (Betz), the entire 

 third layer participating in the increase of size. The occipital 

 cortex is especially differentiated by subdivisions of the third and 

 fourth layers into eight layers (Meynert), while the gyrus cinguli 

 has the third layer separated into an outer group of small and an 

 inner zone of larger cells, the intervening space appearing radially 

 striated on account of the apical processes which cross it ; within the 

 parietal lobes an additional stratum of small pyramidal cells exists 

 between the third and fourth cortical layers. 



The involuted cortex of the hippocampal region, including the 

 cornu Ammonis, or hippocampus major, and the fascia dentata, 

 presents considerable complexity. On observing a section of this 

 region with low amplification, it will be seen that the cornu Ammo- 

 nis consists of a central gray zone bounded both internally and 

 externally by a stratum of white substance ; the gray zone 

 corresponds to the cortex of other parts, and is continuous with 

 the thickened gray mass constituting the fascia dentata above, and 

 with the cortex of the hippocampal convolution below. The 

 medullary substance of the latter becomes greatly reduced in its 

 passage over the cornu Ammonis, the attenuated stratum of fibres 

 being known as the alveus which is prolonged into the thicker 

 fimbria. The white layer enclosing the gray zone on the mesial 

 surface is a conspicuous thickening of the peripheral zone of the 

 hippocampal convolution. 



