99 



The relationships of the sulcus postcentralis inferior and the sulcus 

 parietalis horizontalis have already been dealt with sufficiently fully 

 above. 



It would be difficult to imagine a sulcus which presented so per- 

 plexing and diverse a form as the postcentralis superior. And it is 

 not until one realises that one is dealing with an extremely composite 

 furrow that the task becomes easier. 



Finally, a word on sulci limitantes in general. It seems to me 

 advisable to draw some distinction in classification between sulci which 

 separate areas of extreme age and importance from sulci which are of 

 secondary nature in that they merely divide specialised portions of 

 the same area from one another. For example, the sulcus centralis 

 separates motor from sensory areas, the postcentralis sensory from 

 parietal areas, the paroccipital visuo-sensory from parietal, and so on. 

 All these furrows separate areas of totally different function from one 

 another and may be called " Limiting sulci of the first degree." On 

 the other hand the superior, inferior and horizonal parietal sulci 

 merely separate specialised portions of the parietal association area 

 from one another, or again the sulci limitantes dorsalis et ventralis area 

 striatae et parastriate simply separate differentiated parts of visual 

 cortex. Such sulci might be called " Limiting sulci of the second 

 degree." For though it is obvious that all such furrows are of the 

 same function, yet those which separate the primitive areas of motion, 

 sensation, and vision are of far greater antiquity than those divide up 

 into specialised parts those areas which are highly developed only in 

 anthropoids. In such a classification it will be observed that though 

 the sulcus postcentralis superior of the anthropoids belongs to the 

 first class, the sulcus praeparietalis must be relagated to the second. 

 For the latter furrow is merely a valley, as it were, between two distinct 

 parts of sensory cortex. It is seen at its best in animals such as pithecus 

 in whom the sensory area is widely spread out. When the common 

 sensory area becomes more compressed, as is the case in man, there 

 is not so much room for the lesser individual furrows which are forced 

 to coalesce and so to lose their individuality even if they are not sup- 

 pressed entirely, becoming lost in the walls of the great limiting sulci 

 of the first degree. 



It is hoped that the above remarks will emphasise the manner 

 of the true formation of sulci — how they depend essentiallj'^ on spe- 

 cialisation of cortical structure as Elliot Smith pointed out some 



7* 



