in] Tj/pc of Amnif/( iiifitf of Ncri'c /< T /7>ms- 27 



impulses, anil, secondly, that the richness (if the iuterradiary plexus is dependent upon the 

 large size of the nerve cells met with in this situation. In support of the latter state- 

 ment, we have the knowledge that the axis cylinder processes of these giant cells are not. 

 only of large calibre, but that their collateral offshoots are numerous, and furthermore, we 

 have a definite statement amounting to a law, from S. Ramdn y Cajal, to the effect that 

 the density of the intercellular network is directly proportionate to the diameter of the 

 bodies of the nerve cells present. 



Variations m Type <>f Arninijeinent in Different Parts of the Area. 



It must not be supposed that the type of arrangement just described remains absolutely 

 constant over the whole of the area; on the contrary, certain variations occur of which it 

 is important to give an acnmnt. 



Disregarding details and speaking in general terms, it must be noted that the point 

 which strikes one most in an examination of seriatim sections is that, proceeding from above 

 downwards, there occurs a gradually progressive diminution in the fibre wealth of all the 

 systems of fibres, and possibly also a reduction in the calibre of the large fibres. Thus, 

 the fibre wealth is seen at its greatest in the vicinity of the upper margin of the hemi- 

 sphere, while towards the lower end of the fissure of Rolando the cortex assumes an 

 intermediate type of arrangement. 



From the foregoing it may be gathered that it is impossible to subdivide the field 

 according to structural variations in such a way as to correspond with the physiological 

 subdivisions: we can only sav that in the area, which is credited with the control of move- 

 ments of the leg and body, the nerve fibre plexus is denser than it is in the arm area ; 

 while in the arm area it is denser than in the face and neck area ; but there is nothing 

 approaching an abrupt change in fibre richness, or anything in the shape of an alteration 

 in general arrangement, at the lines of contact of the various physiological areas, which will 

 admit of a corresponding anatomical subdivision. 



Arrangement of Nerve Fibres in the Precentral Area of tlie Anthropoid Ape. 



I have made an exhaustive examination of this area in the brains of three apes, and 

 compared the sections most carefully with those from the human brain, but have discovered 

 no fundamental difference in fibre constitution, so that the description already given of the 

 human " precentral " area may be applied to the anthropoid brain, with this addition only, 

 that, in the ape, the fibres in general are of distinctly smaller calibre. 



DISTRIBUTION OF THE PRECENTRAL TYPE OF CORTEX. (Plate I.) 



On the lateral surface of the hemisphere, the area embraces practically the whole of 

 the wall of the fissure of Rolando, and from the lip of the fissure extends forwards over 

 rather more than half of the exposed surface of the precentral or ascending frontal convo- 

 lution, the Sylvian opercular portion being excepted, and, on the mesial surface, it is 

 confined to a small coterminous portion of the paracentral lobule, lying anterior to the 

 fissure of Rolando. 



42 



