28 Prcceittral or Motor Area [CHAP. 



In discussing the boundaries of the area in detail, we will begin with the posterior 

 one. This is perfectly sharp and definite and is soon described, as it is formed by the 

 fissure of Rolando in its whole length, but it is important to note that the cortex under 

 consideration does not extend quite to the floor of the fissure, in any part. 



Passing next to the exact limits of the small area on the mesial surface of the hemi- 

 sphere, we find that the posterior boundary of the field is almost as definite as it is on 

 the lateral surface, it is marked, namely, by the upper extremity of the fissure of Rolando; 

 and in those cases in which the upper extremity of this fissure stops short of the upper 

 margin of the hemisphere and fails to gain the mesial surface, a peninsula of " precentral " 

 cortex is still carried over, and is then bounded by a line dropped down in continuation 

 with the fissure of Rolando and parallel with the upturned tail of the calloso-marginal fissure. 

 From which it may be gathered, that that part of the paracentral lobule which lies behind 

 the line of the fissure of Rolando is devoid of motor characters, and as a matter of fact 

 it is covered by a totally different type of cortex, which I have designated " post-central." 

 Just as the fissure of Rolando does not, as a rule, descend the mesial surface for more 

 than 1 to 1'5 ctm., so also the " precentral " cortex soon stops short and does not seem 

 ever to extend more than half-way down the paracentral lobule, remaining separated from 

 the calloso-marginal fissure by a fairly broad zone of cortex, in which " intermediate pre- 

 central " and " post-central " characters are commingled. The anterior border of the area 

 curves upwards and forwards to reach the margin of the hemisphere at a point 1'5 to 2 ctm. 

 anterior to the fissure of Rolando. (For further particulars vide appendix.) 



Turning next to the anterior boundary on the lateral surface, it may at once be 

 mentioned that this is tortuous and does not appear to be influenced by fissures 1 , and, 

 furthermore, that the transition from what is taken to be the " motor " to the next type, 

 is much less abrupt than it is posteriorly ; indeed, I think it more correct to state that 

 the area showing a " motor " type of arrangement of fibres is fringed by a definite zone 

 in which the formation is intermediate in character. 



The points which guide one in determining this line of demarcation are as follows : 

 (1) in the examination of a transverse section of the anterior central convolution from any 

 part, it matters not whether it be from the upper or lower end, if the radiary zone be 

 carefully traced round, it will be observed to suffer a diminution in fibre wealth. This 

 is neither a very distinct nor abrupt change, and a coincident reduction in the depth of 

 the cortex is barely noticeable, but visible and obvious proof of the transition in type is 

 afforded by the fact that the radiate fasciculi begin to stand out with greater boldness, 

 and the general coloration of the zone becomes fainter, appearances which are to be accounted 

 for by the loss of fibi-es in the interradiary plexus, and also the disappearance of fibres 

 of large calibre, (2) The fibres of the zonal layer thin out and the formation loses its band- 

 like character. (3) The plexus of fibres in the supraradiary field becomes less intricate, and, 

 lastly, if the white substance immediately subjacent to the cortex be inspected, the onset 

 of a less intense purple staining, pointing to an inferiority in fibre strength, will be observed. 



1 To explain the fact that the position of the anterior line of demarcation of the motor area is not influenced or 

 determined by the presence of fissures, while the posterior boundary is, the surmise seems tenable that the elements, 

 from which the motor nerve cells and fibres spring, are deposited, either synchronously with, or after the formation 

 of the " primary fissures," but prior to the development of " the secondary sulci." In accordance with this view the 

 fissure of Eolando, being, as we know, a " primary fissure, " forms a fixed boundary, while the precentral sulci, being 

 "secondary" ones, bear an inconstant relation to the area. 



