TII] Distribution of the I>r<'<; ntrl TI/JH- of Cortc.i- 29 



Tin- line along which this change of character takes placr varies in different brains, 

 hut striking an average from the lH'inis]>]ioivs which I have examined it may be located 

 as follows : coining over the upper margin ot tile hemisphere, at a point 1'5 to 2 ctm. 

 anterior to the upper extivmitv of the fissure of Rolando, it drops vertically downwards to 

 the sulcus precentrahs superior, which goes to form a boundary; the " precentral " features 

 do not cross tins sulcns, and although in some cases the arrangement does not reach so 

 tar forward, the usual thing is for it to just touch the sulcus and extend down the anterior 

 wall for a short distance. 



In this, its upper third, the area has its greatest sagittal breadth, for after quitting the 

 level of the sulcns pivcentralis superior the field in its further course downwards undergoes 

 a progressive narrowing, and thinning away to a point, ceases opposite the lower extremity 

 of the fissure of Rolando. It seems to be uncommon for the area to be limited by the 

 sulcus pivcentralis inferior, and as a rule, in this situation, the anterior limit runs mid-way 

 down the ascending frontal gyrus. 



The downward extent of the area appears to be dependent on the fissure of Rolando ; 

 thus in one case, where the fissure of Rolando was a short one, the " precentral " type of 

 cortex stopped a considerable distance above the operculum, while in other cases, where the 

 fissure was of normal length and came close to Sylvius, the area followed suit, so that on 

 the whole we may say that the lower limit is coterminous with the lower extremity of 

 the fissure of Rolando. 



DISTRIBUTION OF THE PRECENTRAL TYPE OF CORTEX IN THE HIGHER APES. (Plate II.) 



If the human and anthropoid areas be examined side by side, a most striking resem- 

 blance will be observed ; thus in both, the fissure of Rolando forms a fixed and definite 

 boundary: on the mesial surface the area is insignificant in extent; and, anteriorly, the 

 limits bear a like relation to the precentral sulci. The only obvious point of difference between 

 the two is that the human area, relative to the entire cortical surface, is less extensive than 

 that of the anthropoid, and this only illustrates a fact which others have drawn attention to, 

 viz.. that the higher we progress in the animal scale, the smaller does the motor area become, 

 relative to the whole surface. 



TYPE OF CELL LAMINATION IN THE MOTOR AREA. (Plate III., Fig. 2.) 



As this work deals solely with the topographical distribution of variations in cortical 

 structure, in describing the constituents of the various laminae, I shall disregard minute 

 detail, in order that points of differentiation may be full}' emphasised. 



Plemiform Layer. 



The plexiform layer has an advantage in depth over most other cortical regions. In 

 association with this it is to be remembered that it has to accommodate a heavy zonal layer 

 of nerve fibres, also, that this is a situation in which the apical extensions of the sub- 

 jacent giant cells ramify and terminate (Ramon y Cajal and Bevan Lewis). In the adult 



