62 



Prof. G. Elliot Smith. The Morphology of [Dec. 1, 



If a section be made through any part of the retrocalcarine sulcus (or 

 sulci) the stria Gennari will be found to occupy both lips of the furrow 

 (fig. 1 B). Moreover, it extends upward for a short distance into the 

 cuneus and downward for a similar extent into the gyrus lingualis. 

 It sometimes happens (as in the specimen which I have represented in 

 fig. 1) that superior (x) and inferior (y) limiting furrows of this area 

 striata make their appearance. When the posterior end of the 

 retrocalcarine sulcus is bifid, or when it is represented by a separate 

 vertical furrow (fig. 1, r 3 ), the area striata becomes extended so 

 as to completely surround the depression. In some cases * the 

 retrocalcarine sulcus may be placed close to the superior margin of 

 the hemisphere; its anterior part may be less than 1 cm. distant 

 from the fossa parieto-occipitalis, in fact, this sulcus or its separate 

 constituents may occupy in different human hemispheres any position 

 within the triangle bounded below by the tentorial margin, above 

 by the dorsal edge, and in front by the fossa parieto-occipitalis. But 

 wherever this retrocalcarine sulcus or any of its sub-divisions may 

 happen to be placed, it will be found to be invariably within the area 

 striata. The obvious inference is that the sulcus retrocalcarinus is 

 produced b}^ the folding of the visual cortex itself. The variability of 

 its form and constitution points to the conclusion that it is the result 

 of the mechanical conditions to which a limited cortical area must be 

 subjected when it begins to expand. 



In his great monograph on the "Surface Anatomy of the Cerebral 

 Hemispheres,"* Cunningham has expressed the opinion that there is 

 no " posterior calcarine fissure " (sulcus retrocalcarinus mihi) in the 

 Apes, the so-called "calcarine fissure" being the representative of the 

 "stem" only (i.e., the sulcus calcarinus mihi) of the Human brain 

 (p. 42). I have already strongly opposed this interpretation on general 

 morphological grounds, because the retrocalcarine sulcus of the Human 

 brain is often apparently formed by the backward prolongation of 

 the true calcarine sulcus, as Gustaf Retzins has demonstrated.! 

 The distribution of the area striata in the Apes enables us to 

 settle this matter decisively. I have examined sections of more 

 than 50 hemispheres of the genera Cebus, Cercopithecus, Macacus, Papio, 

 Cynopithecus, Semmpithecus, Hylobates and Simia to determine the 

 distribution of the stria Gennari, and have found that it occupies both 

 Hps of the greater portion of the sulcus commonly called calcarine. 

 This shows that the greater part of this furrow represents the sulcus 

 retrocalcarinus of the Human brain. 



The examination of foetal brains of Semnopithecus seems to indicate 

 that the retrocalcarine sulcus is very precocious in those brains in 

 which the area striata is of great extent. It may develop con- 



* ' Royal Irish Academy Memoirs,' 1892. 

 f ' Das Menschenhriii,' 1896. 



