560 THE CEREBRUM. 



tudinal striation. The smaller angular corpuscles are also nucleated 

 and provided with processes which run in various directions, and pro- 

 bably unite into a fine network. Bounded cells, tolerably uniform in 

 size, (about --s^yoth of an inch in diameter) also occur. They have no 

 visible processes. Both these and the angular cells often appear to lie 

 within clear spaces in the matrix. 



The fibres radiate from the white centre of each convolution in all 

 directions into the grey cortex, having a course for the most part per- 

 pendicular to the free surface. In passing through the grey substance 

 they are arranged in bundles about xs^oo^h of an inch in diameter, and 

 thus separate some of the nerve cells, giving them a columnar arrange- 

 ment. The direction of the fibres varies, according to the part of the 

 convolution in which they occur, whether near the summit or the base, 

 and the radiating fibres are wanting in the sulcus between two convo- 

 lutions, where the fibres have an arciform course, corresponding to the 

 surface of the sulcus, and seem to connect the adjacent convolutions. 

 Other fibres pass in all directions tlirough the grey substance, connecting 

 its several layers. Gerlach has called attention to the presence of 

 bundles of medullated fibres at right angles to the radiating bundles, 

 and forming with them a large-meshed network, in the interstices of 

 which is a still finer network, composed of the finest non-medullated 

 nerve fibres, and formed, he believes, by the interlacement and anasto- 

 mosis of the ramifying processes from the nerve cells. The coarser 

 fibres are said to arise from the cell-processes, either directly, or indi- 

 rectly through this fine network. 



Layers of the cortex. — The form and arrangement of the cells 

 differ at various depths from the surface of the convolution, and as 

 these variations possess considei-able uniformity, several layers are con- 

 stituted, having more or less definite histological characters. Their 

 correspondence to the stratification distinguisliable by the naked eye is, 

 however, somewhat doubtful. Differences exist between tl)e arrangement 

 of the structural elements in different parts of the brain. The most 

 common type is that which is best seen in the convolutions of the parietal 

 lobe. In this most observers agree in recognising five layers * (fig. 391). 



1. The most external layer is narrow, about yV^h of the whole thickness 

 of the grey cortex. It is pale, and contains few cells, and those are small, 

 oval, pyramidal, stellate, with fine processes, and are embedded in a 

 granular material. A few nerve fibres occur in it, and have been said to 

 be connected with the nerves of the pia mater. Some observers have 

 described a network of fine fibres throughout the layer connected with 

 the processes of its cells. On account of the small number of nervous 

 elements the connective tissue elements of the cortex can be seen in this 

 layer with more distinctness than elsewhere. 



2. The next layer, of nearly the same width, is composed of small 

 thickly-set nerve cells, oval, angular, or pyramidal, with branching 

 processes. 



3. The third layer is of paler tint and much greater width. It con- 

 tains pyramidal branching cells, large and small, arranged as above 

 described, with the pointed extremities towards the surface of the con- 

 volution, and separated into groups by bundles of radiating fibres. 

 The inner portion of the layer, in which the cells are larger and the 



* The division into five layers, described by Jlevnert, is substantially that of Dr. 

 Lockart Clarke (Proc. Royal Soc, 1863). 



