CHAPTEK VIII. 



PARIETAL AREA. 



WE saw that the anterior half of the postcentral gyms was covered by a specialised 

 type of cortex to which the name " postcentral " was affixed. We also found that the 

 posterior half speaking roughly of the same gyms presented a structural arrangement of 

 a similar lint less developed and less complex nature, and this was distinguished by the 

 name " intermediate postcentral." Now on passing further back, into the superior parietal 

 lobule, it may be noticed that while certain postcentral characters still remain, the disposition 

 of both cell and fibre constituents is, by comparison, one of another grade ; and since the 

 formation also differs from that in other investing areas, I have decided to give it and 

 the field it covers independent consideration. But I must explain that this step is not 

 being taken without some misgivings, because when the function of the part is discussed, 

 physiology will be found to give meagre support to the separation. 



Reference to Plate I, in which the distribution of this area is represented by short 

 vertical lines, will show that the term " parietal " is being employed, not because the area 

 corresponds to the portion of the brain which the anatomist distinguishes by that name, 

 but for convenience in indication. At the same time while the field does not cover the 

 whole parietal lobe it is distributed over parts of it, and hence in some measure the designa- 

 tion is justifiable. 



Analysing the structure of the enveloping grey matter we will first consider the type 

 of fibre arrangement, pointing out incidentally the manner in which it differs from the 

 formation in surrounding areas. 



TYPE OF FIBRE ARRANGEMENT. (Plate XX, fig. 1.) 

 Zwial Layer. 



The zonal layer must be described as being of weak development. It is made up for 

 the most part of fine varicose fibrils, and contains only a few large varicose elements. 

 Large evenly-niedullated fibres are entirely wanting, and the inferior border of the layer is 

 indefinite. 



Supraradiary Layer. 



Immediately below the zonal layer the fibres are scanty, but elsewhere the display is 

 one of average richness, and this, as usual, increases towards the line of Baillarger. The 

 contained fibres are nearly all of fine calibre and form an irregular network ; only occasionally 

 a vertical fibre of Martinotti, which may be said to be of medium size, is seen. 



