Addendum 279 



zontally, and not infrequently, instead of being isolated, it is joined to the calloso-marginal fissure ; 

 at times it is triradiate, or it may be represented by a dimple only, but, in any case, the truth 

 remains that it is never absent. In the anthropoid specimens I have examined (chimpanzee and 

 orang), a corresponding furrow has also been recognised. 



But while the morphological relations of the fissure just described are in harmony with the 

 suggested homology, it is the microscopic structure of the investing cortex, which, to my mind, 

 almost places the matter beyond doubt. For just as in the lower animals, the deposit of giant 

 cells clings to the sulcus cruciatus, so it is with this fissuret ; in mapping out the distribution 

 of the motor area, I have, in all, examined the paracentral lobule in serial sections quite a dozen 

 times in man, and three times in the anthropoid ape, and I have invariably detected clusters of 

 giant cells in the walls of the fissuret, not all along it perhaps, but always in its upper part ; 

 indeed, so constant is the relation, that this incisure, insignificant though it may appear, can always 

 be taken as a guide to the distribution of the giant cells on the mesial surface of the hemi- 

 sphere ; when the lower margin of the giant cell area is depressed, the fissure falls with it, and 

 reversely. 



VISUAL AREA. 



There is no cerebral area possessing a greater interest for the student of homologies than the 

 occipital lobe. Indeed, so numerous are the publications contributing to our knowledge of the 

 calcarine and parieto-occipital fissures, the " Affenspalte " and the sulcus splenialis, in short, the 

 system of fissures traversing or bounding the occipital lobe, in both primate and lower vertebrate, 

 that not without strong feelings of diffidence I approach this undoubtedly thorny field. And 

 yet, the very warmth of the discussions which have taken place, and are even now proceeding, 

 makes it evident that much still remains to be placed outside the pale of controversy, and since 

 I am breaking new ground, in advancing a thorough histological examination of the grey covering 

 of the brain, over and above naked-eye inspection, as a basis of judgment, my compunction in 

 publishing the ideas set forth in the following paragraphs is relieved. 



It is fortunate that the visual cortex 1 , throughout the whole series of brains coming under my 

 inspection, from the quadruped upwards, offers so many histological points of resemblance and a 

 general structure so distinctive from that of surrounding parts, that its area of location can be 

 defined with absolute exactitude, for this allows me to open with the asset that my map of the 

 distribution of this cortex is correct. Touching this asset, in my opinion, it is one the value, of 

 which cannot be overestimated, it has already proved of signal service in the motor region, it 

 will be found equally useful in others : in fact, I am convinced that if on our naked-eye series 

 of cerebral plans, we could only superimpose another, giving the results of histological examination, 

 this as a preliminary to the final localisation of functional attributes by the physiologist and 

 workers in other departments, all existing doubt concerning various homologies would be summarily 

 removed. 



In discussing homologies associated with the visual area, interest focuses on the calcarine fissure. 

 In Homo, this fissure is usually described as being made up of three constituents, (1) an anterior, deep, 

 fundamental portion, also known as the "stem" or "true calcarine" fissure; (2) a shallower and 

 shorter posterior limb, the tissura calcarina posterior ; and (3) a terminal cross-piece placed on the 

 occipital pole, the fissura extrema of Seitz. We will consider each of these in turn, and begin 

 with the " true calcarine " fissure, one which the comparative anatomist informs us is the most 

 primitive and widely-spread neopallial sulcus in the mammalian brain. And here let me make 



1 Since the particular cortex here referred to corresponds in point of structure with that which I have called 

 " visuo-sensory" in Homo, it might be given that designation; or, since it harbours the line of Gemiari, there is 

 no objection to naming it, as others do, the " area striata." 



