CHAP. XXIII.] 



OF THE HUMAN BRAIN. 



445 



volutions were examined and originally described by 

 Lockhart Clarke* in 1863. 



He observed the divergence of bundles 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- FlG i5 8 ._ Transverse section through anterior part of 

 left Frontal Lobe, showing shape of Convolutions and 

 relative thickness of Grey Matter, a, Third frontal 

 Convolution, a magnified section of which is shown in 

 the next figure. 



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 Eoyal Society, vol. xii. p. 716. 



tion). The bun- 

 dles of fibres in 

 this manner be- 

 come reduced in size, 



