447 



carine" sulcus of the human braiu are really homologous to the caudal 

 part of the superior occipital sulcus of the Apes. If we apply the name 

 intrastriate to the whole series it will obviate such a confusion. The 

 fact that in many human brains both the mesial intrastriate (retro- 

 calcarine) and the lateral intrastriate (superior occipital) furrows assume 

 the same Y-shapes as in the Apes, although they must (for the reasons 

 just mentioued) occupy ditierent parts of the area striata, shows that 

 the Y-shape is chiefly determined by the mechanical conditions oper- 

 ating near the caudal edge of the hemisphere. It occasionally happens, 

 however, in the human brain (I have seen it several times) that the 

 bifid ends of the retrocalcarine and superior occipital sulci occupy the 

 same position in relation to the area striata (but not to the edge of 

 the hemisphere) as they do in the Anthropoid Apes: in other words 

 the bifid end of the superior occipital is swept on to the mesial sur- 

 face and the bifid end of the retrocalcarine is placed relatively far for- 

 ward. In order to adequately compare the conditions which exist in 

 a series of Apes' brains with human specimens it is desireable to have 

 one name for the whole retrocalcarine-superior-occipital series of fur- 

 rows. I have therefore introduced the term ""intrastriatus". Cun- 

 ningham is the only writer so far as I am aware who has used a 

 terminology which might serve this purpose: he calls the sulcus intra- 

 striatus mesialis "posterior calcarine" and the s. intrastriatus lateralis 

 "external calcarine". The use of the term calcarine in this sense 

 seems to me to be undesireable : moreover I wish to emphasize the 

 causal relationship which this series of furrows presents to the cortical 

 area containing the stria Gennari. 



The gradual shifting of the area striata on to the mesial surface 

 naturally produces profound disturbance in the so-called superior occi- 

 pital sulcus in the human brain. Thus it is rare to find this sulcus 

 exhibiting such a typical Y-shaped form as is represented in Figure 1 

 (s. i. I. and s. i. I. [r. d.]). 



The slipping-over of the area striata on to the mesial surface also 

 exercises (indirectly) a disintegrative etfect on the sulcus infrastriatus 

 (occipitalis inferior). This furrow, called into being as an inferior 

 limiting furrow of the area striata lateralis, ceases to present this 

 typical relation when the stria- bearing cortex retracts (or, rather, is 

 pushed backward) in the Anthropoid Apes and in Man. Hence in 

 these highest Primates (see Fig. 1 and 2) the inferior occipital sulcus 

 loses the constancy of form which distinguishes it in the Cebidae and 

 most Cercopithecidae. In many human brains an additional true infra- 

 striate sulcus developes on the mesial surface of the hemisphere. 



