CHAPTER V. 



VISUO-SENSORY AND VISUO-PSYCHIC AREAS. 



VlSUO-SENSORY OR CALCAR1NE AREA. 



IN the cortex of those convolutions which bound and form the walls of the calcarine 

 fissure there exists a prominent lamina with which all anatomists are familiar; it is known 

 by the name " line of Gennari." This lamina can be easily recognised with the naked eye 

 in slices of a fresh brain, and appears as a distinct white line situated midway between 

 the surface and the white substance ; its white colour in the fresh condition suggests that 

 nerve fibres enter largely into its constitution, and that this is so, treatment with a stain 

 which will display nerve fibres unquestionably proves. 



Recent researches afford indisputable grounds for believing that the area of cortex, 

 characterised by the possession of this lamina, is the chief end-station of the optic radiations, 

 and therefore constitutes the cortical centre for the primary reception of visual sensations ; 

 hence particular interest attaches to a detailed examination of its constituents. 



TYPE OF ARRANGEMENT OF NERVE FIBRES. (Plate X, fig. 1.) 



Concerning medullated nerve fibres, the following description of the type of arrangement 

 has been facilitated by reference to the writings of other workers, notably Kaes, Botazzi, 

 and Kolliker, who have given excellent accounts of these fibres, and whose observations 

 I have been able to .check and to confirm in almost every particular, because they happen 

 to have displayed the fibres by methods resembling that which I myself have employed. 

 I have also found it a great advantage to be able to consult Ramdn y Cajal's extremely 

 thorough description of the appearances presented by his chrome silver preparations of the 

 visual cortex. 



Passing to a consideration of the individual layers : 



Zonal Layer. 



Compared with most other parts of the brain this layer is relatively well-developed; 

 at the same time it is not nearly so prominent as it is, for instance, in the precentral 

 cortex, also its lower border is lacking in sharpness of definition. While it is chiefly com- 

 posed of varicose fibrils, it is important to notice that it contains a few evenly medullated 

 fibres of moderately large size and some coarse varicose fibres. Kaes mentions the existence 

 of these large fibres, and an examination of chrome silver preparations seems to prove that 

 they represent the radial prolongations of the subjacent giant cells or solitary cells of Meynert 

 (Ramdn y Cajal). 



