Chap. XXIII. ] 



OF THE HUMAN BRAIN. 



445 



volutions were examined and originally described by 

 Lockhart Clarke =^^ in 1863. 



He observed the divergence of bandies of fibres in a 

 fan-like manner from the central stem of white substance, 

 and their passage 

 between long ver- 

 tical groups of 

 nerve-cells situ- 

 ated in the deeper 

 grey layers (fig. 

 159). Some of 

 the fibres, he be- 

 lieved, were con- 

 tinuous with the 

 processes of the 

 cells, whilst others 

 turned round and 

 pursued a hori- 

 zontal course 

 (either in a trans- 

 verse or in a lon- 

 gitudinal direc- pj^^ ISS.— Transverse section thrmigh anterior part of 

 tion). The bun- ^®ft Frontal Lobe, showing shape of Convohxtioi.s and 

 -.- e Xil • relative thickness of Grey Matter, a, Third frontal 



ClleS 01 nbreS m convolution, a magnified section of which is shown in 



this manner be- the next figure. 



come reduced in size, and at the same time the com- 

 ponent fibres become finer as they approach the surface — 

 apparently in consequence of the branches which they give 

 off, in their course, to contiguous nerve cells. When they 

 arrive at the third layer from the surface, they '' are 

 reduced to the finest dimensions, and form a close net- 

 work with which the nuclei and cells are in connection." 

 The two layers superficial to this are paler in colour and 



■* Proceed, of Royal Society, vol. xii. p. 716. 

 20 



