658 THE BRAIN. 



spinal cord, processes carrying bloodvessels and bearing lymph-spaces pass 

 inward to supply the gray matter with blood. But while, as we shall see 

 later on, the supply of bloodvessels to the gray matter is considerable, the 

 truly connective-tissue elements of the pia mater processes are soon merged 

 into neuroglia. Immediately beneath the pia mater forming the immediate 

 surface of the cortex is a thin layer consisting of neuroglia only. 



563. The nerve-cells of the above several kinds are arranged more or 

 less distinctly in layers parallel to the surface, so that the whole thickness of 

 the cortex may by means of them be, more or less successfully, divided into 

 a series of zones, one above the other ; and we may, as we have said, recog- 

 nize on the one hand a general arrangement common to the whole surface, 

 and on the other hand modifications existing in the several regions. The 

 general arrangement may be said to be one of five layers or zones, usually 

 counted from the surface inward. 



The fifth layer, lying next to the central white matter, fairly uniform in 

 characters and thickness (about 1 mm.) over the greater part of the brain, 

 is characterized by the presence of somewhat sparsely scattered " fusiform " 

 cells, though other branched cells are present. It is broken up into vertical 

 columns by the bundles of vertical fibres, and its demarcation from the white 

 matter below is somewhat indistinct, owing to the fact that in the brain the 

 white matter, especially that lying beneath the cortex, contains cells and 

 small groups of cells lying between the bundles of fibres to a much greater 

 extent than does the white matter of the spinal cord. 



The fourth layer, lying above the preceding, varies much more both in 

 thickness (0.35 mm. to 0.15 mm.) and in its characters. The constituent 

 cells are on the one hand large pyramidal cells, and on the other hand 

 " granule " or "nuclear " cells. In some regions it may be subdivided into 

 two layers, the small " nuclear " cells being so abundant as to form in the 

 upper part of the layer a separate layer called the "granule" or "nuclear" 

 layer. This fourth layer, like the preceding fifth layer beneath it, is split 

 up into vertical columns by the bundles of vertical fibres, but to a less degree. 

 It is marked in its lower part by a horizontal streak due to numerous, mostly 

 fine, medullated fibres running horizontally. In the cortex of the island of 

 Reil this horizontal layer is developed into a conspicuous sheet of medullated 

 fibres, separating the fourth and fifth layers by a distinct interval of obvious 

 white matter. This fifth layer of fusiform cells, thus detached from the rest 

 of the cortex, is what is called the claustrum (Figs. 138, 139, c/.). 



In the third layer, the constituent cells are the characteristic pyramidal 

 cells. These are for the most part large, though diminishing in size from 

 below upward, and the layer has been called the " layer of large pyramidal 

 cells," though in certain regions the largest pyramidal cells, and notably the 

 giant-cells, are found in the preceding, fourth, layer. The cells are, on the 

 whole, scattered somewhat sparsely, though frequently gathered into small 

 groups, and among them occur small " nuclear " and other cells. The 

 bundles of vertical fibres spread out rapidly in this layer, so that the 

 columnar arrangement becomes lost, and many of the fibres undoubtedly 

 become axis-cylinder processes of the pyramidal cells. Though the layer 

 varies in thickness (1 mm. to 0.4 mm.), and in some of its features in different 

 regions, the characteristic pyramidal cells are present over the whole surface 

 of the hemisphere. In the lower part of the layer a second horizontal streak 

 of closely interwoven horizontal fibres frequently makes its appearance. 



The second layer, generally a thin one, though varying from 0.25 mm. to 

 0.75 mm. in thickness, is also formed by pyramidal cells, but is distinguished 

 from the layer below by the absence of large and medium-sized cells and by 

 the presence of numerous small cells closely packed together ; it has been 



