THE ANATOMICAL BASIS 9 



since the colouring matter of the blood is contained in highly refractile 

 corpuscles the blood column is opaque. 



The part of the retina to which most attention must be directed 

 for the present purpose is that comprising the rods and cones and the 

 pigment epithelium. It is shown conclusively by Purkinje's experi- 

 ment, which depends upon the position of shadows thrown by the retinal 

 vessels upon the percipient layer of the retina, that the primary seat 

 of the visual impulses is in the layer of rods and cones. Here the most 

 sharply defined image is formed by the optical system. As already 

 stated it is not perfectly defined physically, but as will be seen later 

 physiological and psychological compensation tends to counteract the 

 physical defects. 



The rods and cones, as their names imply, are minute cylindrical 

 and conical structures. They project vertically — or more accurately 

 radially — from the surface of the outer limiting membrane of the retina. 

 Almost exactly at the posterior pole of the eye is situated a small area 

 in which vision is most distinct. This area is impregnated with a yellow 

 pigment and hence is called the macula lutea or yellow spot. In the 

 centre of the yellow spot there is a conical pit, the fovea centralis, caused 

 by thinning out of the retina. In this minute area the structures are 

 reduced to little more than neuroepithelium and ganglion cells. 

 Moreover the neuroepithelium in this region consists entirely of cones, 

 though the cones are slender and elongated and are more rod-like here 

 than elsewhere^. 



This change in structure of the cones may be evidence of some 

 physiological combination of the functions of both rods and cones in 

 this situation. 



Passing peripherally in every direction from the central fovea it is 

 found that rods gradually make their appearance between the cones, 

 and soon the number of rods in a given area becomes greater than that 

 of the cones, so that at the extreme periphery of the retina only a few 

 scattered cones are to be found. 



The following are some measurements of the diameter of the foveal 

 cones : Max Schultze, 2-8 /m ; H. Miiller, not more than 3 ^ ; Merkel, 3/x ; 

 Welcker, S'l— 3-5;Lt; Wadsworth, 2-5 /x; Kuhnt. 2—2-5^; Kolliker, 

 4"5 — 5'4ju,; Koster, 4-4: /x; Greeff, 2-5 /x; Dimmer, 3 — 3'5/x. 



The foveal region is an elliptical area with the long axis horizontal. 

 The long axis measures about 0'3 mm., the vertical 0*2 mm., and the 

 total area is 0"5 — 0*6 sq. mm. Taking the diameter of the inner hmbs 



^ Greeff, in Graefe-Saemisch Handb. d. ges. Augenheilkunde, Theil i. Bd. i. Cap. v. 1900. 



