v] Vi&uo-Sensory T 7 ///"' of Cell LtnufiHif/oi/ 117 



Layer of Giant Pyramidal Cells (Solitary Cells of Meynert). 



These solitary cells of Meynert constitute another very important and distinctive feature 

 of the calcarine cortex. 



Thi'ir number is not great, in a single low power microscopic field one can seldom 

 see more than half-a-dozen, and they are best described as lying in a single row from 

 "2 to '3 mm. below the stellate layer, although dislocated members often lie at a higher 

 in- lower level. 



In shape they are triangular or stellate, and they possess large bodies from 25 to 

 in diameter, in which chromophilic particles are distinctly evident. According to Ramon y 

 Cajal, the radiary or apical process of these cells, although relatively speaking more delicate 

 than the same process in giant pyramidal cells of other regions, is still of great length and 

 reaches up to the plexiform layer : further, the basilar processes are very numerous and thick, 

 and so peculiar that they serve to distinguish these cells from homonymous cells in any 

 other portion of the brain ; thus in addition to being thick and numerous they run almost 

 exclusively in a horizontal direction, giving off branches as they go, while their length often 

 exceeds that of the apical process. 



Ramon y Cajal has seen similar cells in the visual cortex of the cat and dog, and 

 I will now add that such cells are also recognisable in the Orang and Chimpanzee. 



Layer of Medium-sized Pyramidal Cells. 



This layer is best seen in those parts of the calcarine cortex which lie on the free 

 surface ; they form a fairly definite lamina, and the component cells, which are triangular 

 or fusiform, are separated into columns by the intervening radiations of Meynert. 



Layer of Fusiform and Triangular Cells. 



The cells of this lamina are triangular, ovoid or fusiform, and few in number. Along 

 the walls of the calcarine fissure they, together with the overlying medium-sized pyramids, are 

 compressed within a very narrow compass and are flattened so that their main processes 

 point in a horizontal direction, in consequence of which that part of the cortex lying between 

 the solitary giant cells and the white substance has a very curious packed appearance. 



Summary. 



Briefly put, the characters which distinguish the calcarine type of cell lamination are, 

 first, the almost unique external layer of large stellate cells usurping the position occupied 

 by the external layer of large pyramidal cells in other regions; secondly, the existence 

 of pale-stained zones above and below the uncommonly well-marked layer of stellate cells, 

 the upper of which marks the position of the line of Gennari; thirdly, the presence in 

 the depths of the cortex of the layer of solitary cells of Meynert, cells which differ from 

 homonymous cells in any other part of the brain. 



