CHAP, ii.] THE BRAIN. 1029 



whole surface of the cortex, dipping down into 'the sulci ; and 

 from it, as in the case of the spinal cord, processes carrying blood 

 vessels and bearing lymph spaces pass inwards to supply the grey 

 matter with blood. But while, as we shall see later on, the supply 

 of blood vessels to the grey 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. 



650. 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 recognize 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 inwards. 



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 some- 

 what 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 ('35 mm. to '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 horizon- 

 tally. In the cortex of the Island of Heil, 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. 115, 116, 

 &c., cl). 



In the third layer, the constituent cells are the characteristic 

 pyramidal cells. These are for the most part large, though 



