94 



I am unable to say to what area the sulcus paracinguli is a true limiting 

 furrow. None of the brain maps hitherto published are very clear on 

 this particular region, which is by no means simple. 



We may conclude this portion of the paper by stating that the 

 form of the superior post-central sulcus is very largely dependent 

 upon the state of development of the sulcus and arcus cinguli, and 

 upon the sulcus paracinguli, which last is called into existence by the 

 arcus afore-mentioned. Its shape also depends to an unknown but 

 certainly lesser extent on other factors which will be discussed later. 



The Morphology of the Sulcus postceutralis superior. 



This sulcus has, in man, a separate developmental history. It 

 is the last of the various portions of the sulcus interparietalis (B.N.A.) 

 to appear, and it may or may not subsequently become confluent with 

 the rest of this furrow system. Out of 80 hemispheres examined in the 

 course of this research it was found to be separate both from the 

 sulcus postcentrals inferior and also from the sulcus parietalis hori- 

 zontalis mihi in 61.3% of brains. These figures are in marked 

 contrast with those of most other observers. Thus Eetzius (6) finds 

 the sulcus postceutralis superior separate in only 24 % of cases, and 

 Cunningham (6) in 25.4 %. 



The percentages obtained by these authorities refer, however, to 

 superficial continuity of sulci only, whilst I have been at some pains 

 to open up the depths of all fissural connections. I have considered 

 deep annectant gyri (B.N.A. gyri transitivi) as being as important as 

 superficial ones. Such deep convolutions never grow up in the course 

 of a singly-developed, and thus originally uninterrupted, sulcus. 

 They occur at the junction of two furrow elements of separate develop- 

 ment and origin. In another paper (1) I have given reasons for be- 

 lieving the sulcus parietalis horizontalis (mihi) (the ramus horizontal! 

 of the intraparietal sulcus of Turner of the Old Terminology, the inter- 

 parietalis propius of Ecker) to be a new fissure in the anthropoid 

 series, and that it has no homologue in the pithecidae and new-world 

 apes. I have further shown that the so-called sulcus interparietalis 

 of such animals is, in fact, not interparietal at all but really post- 

 central, in that it limits the sensory from the true parietal area, as 

 Brodmann's maps so clearly show (T). The great development in the 

 anthropoids of the parietal association area necessitates the formation 

 of new furrow^s or the exaggeration of old ones. 



