A(t(k'ii<ftim '21 \ 



VISUAL AHEA. 



The visual cortex presents such ;i remarkable arrangement of cell and fibre elements that 

 it can be identified, and its distribution defined with perfect exactitude by simple histological 

 methods. 



Turning to the figures, I would first draw attention to the relation of the area to the com- 

 bined intercalary and calcarine fissures. Visual cortex again coats only the outer walls of these sulci ; 

 and it is interesting to observe that, in front, the area begins at the junction of the cruciate 

 and intercalary fissures, a junction which seems to be associated with the presence of a concealed 

 annectant gyrus ; and that below the area ends exactly along with the calcarine fissure ; and, 

 further, that this lower limit lies several millimetres above the level of the posterior offshoot of 

 the fissura rhinica. Sweeping over the hinder part of the gyrus marginalis and the occipital pole 

 the area finds external limits, first, in the hinder part of the lateral sulcus, and then in the shallow 

 postlateral fissure, and it must be mentioned that visual cortex clothes the inner walls only of the 

 last-named fissures. 



One is struck by the great extent of the area relative to the rest of the surface. I have 

 made no accurate estimate, but in rough terms it occupies quite one sixth of the dorsal and 

 mesial surfaces of the hemisphere ; a very different state of affairs from that obtaining in primates. 



Strongly reminiscent of what one has seen in the visuo-sensory cortex of man and the ape, the 

 microscopic characters are unmistakeable. In fibre-stained sections a line of Gennari is visible 

 to the naked eye (N.B. the line is not similarly visible in unstained sections), and microscopically 

 is composed of the usual plexus of fine fibres. In the radiary zone there is an abundance of 

 large fibres, most of them cut obliquely and deeply placed. 



The cell lamination is worth putting down in detail : it is made up of, (1) An uninteresting plexi- 

 form layer. (2) A somewhat closely-packed layer of small polymorphous cells. (3) A layer of well- 

 stained, erect, medium-sized pyramidal cells, among which lie some specially important larger elements, 

 endowed with Nissl bodies. Such cells occur at all parts of the layer, but are not numerous, and it 

 is curious that they are seen at their best along the lateral outskirts of the area. The suggestion has 

 occurred to me that they are representative of the large external pyramidal cells of the visuo- 

 pyschic cortex of Homo. (4) There follows a pallid, rather broad stratum, containing elements 

 reminding one of stellate cells, but they are indifferently stained. (5) Then more medium-sized 

 cells, and mixed with them some solitary cells of Meynert, the latter being of pyramid form, 

 erect, and intensely stained. (6) Some small fusiform cells complete the lamination. 



ECTOSYLV1AN REGION. 



The ectosylvian sulcal arc is much more perfect in Canis than Felis, and forms a clear 

 boundary to a singular type of cortex, occupying what I am calling Ectosylvian Area A (cf. Felis). 



Roughly speaking, it covers the first arcuate gyrus of Leuret, and it has the following nerve 

 fibre characters ; the general wealth is not at all good ; for instance, compared with the convolutions 

 above, the radiary zone is relatively pallid. In the depths, streaming out from the medullary 

 projection are numbers of fibres almost gross in calibre, confined in their distribution to the 

 neighbourhood of the white substance, and running for the most part obliquely or parallel to the 

 surface. I believe that the majority of these fibres strike the cortex towards the hinder end of the 

 area, because, in the frontal direction they gradually lose in number. 



The nerve cell arrangement is equally peculiar. A plexiform layer, and layers of small and 

 medium-sized pyramidal cells, can be identified ; but there are no large external cells. A paler 

 stratum, containing rounded and ill-stained elements, follows, and then come the cells which give character 



