li_'s Visuo-SenMiri/ <ni<1 Vixno-Pnujchic Areas [CHAP. 



White Projection. 



When the white projection of the " visuo-psychic " cortex is compared svith that of the 

 coterminous parietal and temporal regions, it is noticed at once that the former presents 

 an intensity of staining, which is a certain indication of an advantage in fibre wealth, and 

 especially in fibres of large size ; and, in thin specimens and in specimens in which the 

 process of differentiation has been carried to excess, such fibres can be seen distinctly, spreading 

 out in all directions and apparently running in connection with the optic radiations. From 

 which it may be gathered that an inspection of serial sections of the substance of the occipital 

 lobe offers strong reason for assuming that those fibres of the optic radiations, not specially 

 destined for the calcarine region, come into relation with cells in the visuo-psychic cortex. 



TYPE OF CELL LAMINATION. (Plate XI, fig. 2.) 



As this field of cortex also possesses a distinct type of cell lamination, the examination 

 of sections stained for nerve cells has proved a valuable check to the correctness of the 

 topographic details gathered from an inspection of the nerve fibres, but before proceeding 

 with the analysis of the individual layers, I will describe the behaviour of the visuo-sensory 

 lamination at the point of transition into the visuo-psychic type, for it presents features 

 as interesting as they are peculiar. 



The change is almost entirely confined to the supra- and infra-stellate laminae and their 

 contents. Taking first the upper lamina, it has been mentioned already that in great part 

 this corresponds with the line of Gennari ; also we have seen, in our sections stained for 

 nerve fibres, an abrupt cessation of this line at the point of junction between the two types 

 of cortex. Now coincident with the interruption of the line of Gennari there occurs a sudden 

 disappearance of the external layer of large and small stellate cells, and the pallid lamina 

 gives place to one richly-stocked with giant and other pyramidal cells, particulars of which 

 will be presently given. In like manner the substellate pallid zone along with the layer of 

 solitary cells of Meynert vanish, and medium-sized pyramidal and fusiform cells like those 

 common to many other cortical regions are introduced. The closely-packed layer of small 

 stellate cells interposed between these pallid laminae remains much as it was before. 



We will now analyse the contents of the various laminae beyond the point of transition, 

 i.e. in the visuo-psychip. area, in detail. 



The plexiform and small and medium-sized pyramidal cell layers are all appreciably deeper 

 than they are in the calcarine region, and the cells in the two last-mentioned layers show 

 a uniform increase in size, but being less numerous they stand wider apart from one another. 



The most important layer in this area is unquestionably the external layer of large pyra- 

 midal cells. These cells do not occupy a stratum to themselves, because mixed with them are 

 numerous medium-sized pyramids and some small stellate cells, nor is their number great, 

 but at the same time ten or more may be seen in every low-power field of the microscope. 

 In regard to size they must be included in the giant cell category, for they measure 

 25 by 30 /a, and are obviously larger than the solitary cells of Meynert in the calcarine 

 area. They stand erect, have a long apical dendrite and a pyramidal or pyriform body, from 

 the base of which two or three delicate processes project. The chromophilic particles recognisable 

 are of large size, but are scattered about and do not fill the cell. 



