280 Tike Homologies of the Visual Area 



the preliminary statement that I accept Dr Elliot Smith's definition of the same fissure, as seen 

 in the brains of lower animals, and by the "true calcarine" fissure understand the sulcus which limits 

 the area striata anteriorly (sulcus " praestriatus"), and which corresponds to or produces the calcar 

 avis 1 . In the brains of the animals I have examined, the interesting observations have been made 

 that this fissure forms a constant limit to the spread of the visual area, that the downward or 

 caudal extent of the area is coterminous with the lower extremity of the fissure and, further, 

 that one wall only, the posterior, peripheral or outer, is coated by visual cortex. Proceeding 

 forwards, I have next noticed that the boundary on the mesial surface of the hemisphere is 

 completed by the intercalary fissure, in fact, this forms as definite a limit as does the 

 calcarine. 



The truth that the combined calcarine and intercalary sulci form a continuous limit, and that 

 individually considered both constituents of the complex have a similar relation to the visual area, 

 is one of great significance in throwing light on the subsequent history of these fissures. First, 

 as to the "true calcarine" fissure, the collateral histological study of its investing cortex in lower 

 mammals and in Anthropoidea supplies further proof of the contention, first suggested to us by 

 its anatomical relation to the calcar avis, that this portion of the complex becomes the " stem " 

 of the calcarine fissure in Homo. The histological proof is this : in man, as Bolton first showed, 

 and in the man-like ape as I have indicated and as Elliot Smith has since emphasised, the " stem " 

 of the calcarine fissure, like the "true calcarine" fissure of lower animals, is only covered by visual 

 cortex on that wall furthest removed from the splenium ; the inner wall forms part of the 

 limbic area. 



With this point settled, we have next to determine what happens to the intercalary 

 fissure. According to Elliot Smith, as a result of the prolongation of the hemisphere, the calcarine 

 fissure becomes widely separated from the intercalary and the latter eventuates in the calloso- 

 marginal. But this is an interpretation of events to which histology not only gives no support, 

 but is directly antagonistic. Obviously the great stumblingblock to the suggestion is the truth that 

 in Carnivorae and Suidae, animals in which the intercalary sulcus is well represented, this sulcus 

 stands in close relation to visual cortex, whereas in the brains of those members of the simian family 

 which I have examined, as in man, even the tail of the calloso-marginal fissure, is widely removed 

 from the visual area. If then the intercalary sulcus becomes the calloso-marginal, we must 

 necessarily regard the remarkable relation between the former and the visual area, in lower orders, 

 as a purely fortuitous circumstance, but lessons taught us by similar relations between sulci and 

 functional areas in other regions render this impossible. 



A final solution of the difficulty may not be arrived at until the brains of intermediate orders 

 are examined, but, for the present, I prefer to believe that the calloso-marginal fissure is formed 

 at the expense of some other sulcus, probably the genual ; that the vertical clefts resulting from 

 the occipital prolongation of the hemisphere, represented by the parieto-occipital fissure and 

 "AflFenspalte," occur on the frontal side of the visual area, and of all the fissures to which it is 

 related ; and that, in this process, the intercalary sulcus, instead of increasing in length and 

 importance, becomes attenuated, and, finally, in Homo, is swallowed up in the lower extremity of 

 the parieto-occipital fissure and either wholly lost or but feebly preserved as the lower boundary of 

 the submerged gyrus intercuneatus. 



1 In Sus, Felis, Canis, and many other animals, a long and continuous fissure indents the mesial surface of 

 the brain, above and behind the spleuium corporis callosi ; Elliot Smith speaks of this as the "calcarine complex"; 

 in the hinder portion he sees the "true calcarine" fissure, and for the anterior portion he has coined the term 

 "intercalary." In many German monographs the same complex is called the "sulcus splenialis" and the word 

 "calcarine" does not appear. 



