278 The Homoloyies of the Motor Area 



We have next to enquire whether the fissuret, just described, is the homologue of the whole, 

 or of a part only of the fissure of Rolando. I believe the latter to be the case. We know 

 that, on developmental, and I may say, also on physiological grounds, the Rolandic sulcus is 

 divisible into two portions, an upper and a lower, that the division is marked by the superior 

 annectant gyrus or buttress, and that the gyrus sometimes remains on the surface and makes the 

 separation morphologically complete in extrauterine life. And as what I take to be typical motor 

 cortex ceases on reaching the floor of the coronal fissure, and as also the results of experiment 

 favour the assumption, I would join forces with those who look upon the coronal fissure as the ante- 

 cedent of the lower portion of the fissure of Rolando. 



Supposing then that we are construing matters aright, we get a reproduction in phylogeny, 

 of an ontogenetic condition in the immature primate brain, namely, a separation of the great 

 central fissure into two discrete portions ; and, summarising the homology, the " compensatory 

 ansate fissure " of Carnivorae ' becomes the upper Rolandic segment of man, and the coronal fissure 

 the lower segment, while the intervening substance is moulded into the superior annectant gyrus 

 or buttress. 



It still remains for me to state reasons for refusing to believe that the sulcus cruciatus 

 is the forerunner of the fissure of Rolando. My strongest objection is again founded on histology. 

 We have gathered from our study of the human brain that the fields governing those functions 

 we have hitherto succeeded in locating, are all deposited in direct relation to some important 

 sulcus, which in one direction, at any rate, forms a sharp and perfect boundary ; thus, the 

 olfactory area is restricted in its spread by the fissura rhinica ; the "stem," which we may 

 consider the fundamental component of the calcarine fissure, prevents the expansion of visual 

 cortex on to the postlimbic gyrus ; the auditory area is closely related to a portion of the Sylvian 

 fissure ; and, lastly, there are strong grounds for assuming that the fissure of Rolando acts as 

 a dividing line between motor and sensory cortex. Obviously, in the case of all these limits, 

 two totally different types of cortex are found on the apposed walls of the fissure. And so 

 far as my study of the lower animal brain has gone, I see no cause for doubting the occurrence 

 of a repetition of this association between external morphology and functional distribution. But, 

 when we proceed to apply our experience to the sulcus now holding our attention, the sulcus 

 cruciatus, we soon find in regard to its covering of grey matter, that it differs entirely from 

 the fissure of Rolando, inasmuch as both walls, instead of only one, are coated by motor cortex ; in 

 brief, it has none of the characters of a dividing line. 



Discarding then the Rolandic fissure as the homologue of the cruciate, we have now to examine 

 the human brain and differentiate the remains of the latter fissure. For my own part I believe 

 that a remnant exists : indeed, considering the prominence of the sulcus in the lower animal brain 

 it would be remarkable if it did not. The fissure I have in view is to be sought on the oval 

 or paracentral lobule, it is placed immediately below and on the frontal side of the upper 

 extremity of the fissure of Rolando, or of a line drawn on in continuation with that extremity; 

 it is so small that it is perhaps better described as a fissuret, and, it is not to be confounded 

 with the anterior boundary of the paracentral lobule, namely, the preoval or paracentral fissure, 

 nor with the inconstant sulcus precentralis marginalis. From an inspection of the large collection 

 of brains and casts in our museum at Rainhill, I am satisfied of its stability, but, as is the 

 case with every other fissural element, its appearance is rarely the same in any two specimens ; in 

 its commonest form it is an isolated, vertical, or oblique furrow, but occasionally it lies hori- 



1 I have not been afforded an opportunity of examining a series of brains to determine the constancy of this 



flssuret in other animals, but in the very instructive figures reproduced in the Catalogue of the Koyal College of 



Surgeons Museum, a corresponding indentation is illustrated in the brains of Ursidae, Otariidae, Mustelidae, and 

 other Carnivores. 



