286 OUTPOSTS OF THE INTELLIGENCE SERVICE 



form synaptic connection exclusively with one bipolar neurone 

 each, i.e. the stimulation of one cone is not associated with the 

 stimulation of any other cone till the disturbance has reached the 

 cortex. They are thus fitted for point vision and are capable of 

 transmitting fine detail if the light is not too intense. The fovea 

 contains cones closely packed together ; the surrounding area, cones 

 and some rods. Then comes an area peripheral to this, with rods 

 and some cones, and, finally, at the very edge of the effective 

 sensitive surface very few cones are found. They are, therefore, 

 the elements responsible for direct vision in good light, are capable 

 of transmitting detail and are sensitive to all parts of the visible 

 spectrum. They are not fitted for twilight vision and are not 

 associated with the visual purple. 



Fovea Centralis. As this tiny depression in the retina is the 

 screen on which inverted images of external objects are clearly 

 focused under ordinary conditions, ^e must devote a little more 

 attention to it. In this area the rays of light transmitted through 

 cornea, aqueous humour, lens and vitreous body and " stopped 

 down " by the iris, come otherwise undistorted into direct contact 

 with the bases of the cones, which here are larger, more highly 

 developed, and more rod-like than elsewhere. In order to get this 

 freedom from the dissemination of light by the colloidal structures 

 interposed in other parts of the retina, the nervous connections of 

 the cones are led away from the fovea into the surrounding 

 macula lutea. There are no blood vessels in the fovea, and this, 

 also, contributes to exactness of vision. The cones, too, are so 

 closely packed together that, in section, they become hexagonal, 

 each flat side being in contact with the flat side of a neighbour. 

 That is, in the fovea, there are essentially only two sets of elements, 

 viz. cones and the cells of the stratum pigmenti. 



Pigments. Several coloured substances have been described as 

 being present in the retina. (1) In the outer layer, i.e. the layer 

 lying between the layer of rods and cones and the chorioid coat, 

 there is a pigment fucsin, which acts as a " damper " for the 

 ethereal waves, i.e. the needles, prisms or plates of this pigment 

 found in the processes of the cells of the stratum pigmenti prevent 

 light from spreading by reflection from the outer limbs of rods and 

 cones. (2) Visual purple, or rhodopsin, is found associated with the 

 rods. In fact some investigators, like Edridge-Green, are inclined 

 to consider that the main if not the sole function of the rods is to 

 elaborate and secrete this pigment. Rhodopsin has been extracted 

 from retinae and its photo-sensitiveness tested. Strong light 

 bleaches it very rapidly, forming probably two substances, a 

 principal one and an accessory one. These products of photolysis 



