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nient of nerve fibres, represents the exact extent of a field functionally analogous as well as 

 structurally homologous to the primate " precentral " or "motor" area. 



On the finer differentiation of this area into seguiental fields, corresponding with the various 

 groups of muscles, I am not prepared to write, because I feel that the intraterritorial structural 

 variations are insufficiently pronounced to make a statement from the student of normal histology 

 of any value. 1 am sure, however, that just as an examination of the Betz cells for "reaction a, 

 distance," in cases of amputation, proved of great service to me in subdividing the area in the 

 human brain, so experimental study on similar lines, should anybody care to undertake it, will 

 tell us more about the area in the lower animal. Yet, even as it is, we have a sufficiency 

 of experimental evidence to justify the belief that, from within outwards, the sequence of 

 representation follows the same order as in primates, namely, lower extremity, trunk, upper 

 extremity, etc. 



Since the Betz cells suddenly diminish in number on the inner wall of the sulcus coronalis, and 

 almost stop at the floor, one cannot help looking upon this fissure as a boundary to the upper 

 extremity and neck areas ; but, as movements of the tongue, face, etc., have been so constantly 

 obtained from sub- and post-coronal cortex, it is imaginable that the isolated large cells here- 

 abouts may govern such movements. And yet this is a question of some difficulty, for in the 

 ape and man, the structural peculiarities of the area said to dominate the same movements are 

 not nearly so distinctive as those of the fields governing movements of the extremities. 



We can now pass to the homologies of this region, and since in primates, the great central 

 fissure of Rolando bears such important relations to the motor area, we will first attack the 

 vexed problem of the representation of this fissure in lower mammals. Now from a very intimate 

 acquaintance with the. microscopic appearances of the Rolandic cortex, I am satisfied that the 

 fullest reliance can be placed on histology as an aid to the determination of the homology in 

 question, and although the pig, with its bizarre arrangement of sulci, gyri, and cortical constituents, 

 presents difficulties, I ain convinced that, in the dog and cat, I have found the homologue of, 

 at anj' rate, the upper half of the Rolandic fissure; and this is not, as is generally maintained, the 

 sulcus cruciatus, but an isolated, shallow, insignificant-looking tissuret, placed between the cruciate 

 and ansate sulci, indenting the posterior limb of the gyrus sigmoideus not far from its middle, 

 and known as the "compensatory ansate" (in the diagrams it is marked "homologue of Rolando"). 

 In the dog, the fissuret is obliquely placed, running from behind forwards and inwards, and is 

 better developed than in the cat. In the latter animal, the indentation runs more nearly parallel to 

 the sulcus cruciatus, and from a special examination of a number of brains, I have ascertained 

 that it may be represented in one hemisphere and not in the other ; I have also noticed, in 

 those cases in which the formation does not appear as a fissure, that a slight dimple invariably 

 indicates the point where it should be. As to reasons for regarding this sulcus as the representative 

 of the fissure of Rolando, or a part thereof ; in describing the histology of the human and 

 anthropoid cortex, I pointed out that the structure of the precentral cortex was totally dis- 

 similar from that of the postcentral, so unlike, that in itself it suggested a difference in function 

 between the two gyri, and I also showed that the floor of the fissure of Rolando formed the 

 dividing line between these two types. Brodmann's independent study of the cell lamination of 

 the same gyri in man has produced a like declaration. And now, in the dog and cat, to see 

 the lietz cells ceasing, and the fibre arrangement undergoing a change, the moment the fissuret 

 under consideration is reached it is beautifully demonstrated in sections cutting the fissure at 

 right angles is not only a most interesting and instructive histological revelation, but so 

 strongly reminiscent of what one has seen in man and the ape, that one is forced to offer it 

 as proof, and as convincing proof, of the correctness of the thesis here submitted. 



