622 THE SENSES. 



tinuous bright line. As distinct points are actually perceived as such 

 by the retina, although the luminous ray emanating from each one has 

 passed through the whole layer of nerve fibres on its internal surface, it 

 follows that the sensibility of these fibres is not affected by the direct 

 action of light. 



The sensitive elements of the retina are in its posterior or external 

 layers. This fact is deduced partly from the phenomena manifested 

 when the retinal bloodvessels are made visible in the interior of the eye. 

 These bloodvessels and their branches radiate from the central trunk 

 which enters with the optic nerve. Their ramifications, down to a cer- 

 tain size, are all situated in the nerve fibre layer of the retina, and it is 

 only the finest subdivisions which pass into the next layer of ganglionic 

 nerve cells. The two outer layers, namely, the layer of nuclei, and that 

 of the rods and cones, are completely destitute of bloodvessels. Owing 

 to this anatomical arrangement, the posterior or external layers of the 

 retina, situated behind the main branches of the retinal bloodvessels, 

 must lie m the shadow of these branches, the light coming directly from 

 the front through the pupil. The shadows thus thrown are not habitu- 

 ally perceived by any diminution of the light, because the portions of 

 the retina covered by them are always in shadow at the same points, 

 and its sensibility to light is greater, in proportion as the quantity of 

 light reaching it is less. But the shadows may be rendered perceptible 

 by a lateral or oblique illumination, thus causing them to be thrown 

 upon points of the retina unaccustomed to their presence. 



Let a lighted candle be held, in a dark room, about three inches 

 distant from the external angle of either eye, and about 45 degrees in 

 advance of the plane of the iris. On moving the candle alternately 

 upward and downward, the field of vision becomes filled with an abun- 

 dant and elegant tracery of aborescent bloodvessels, the exact counter- 

 part of those of the retina. The form of the vessels is distinctly marked 

 in purpl.e-black, upon a finely granular grayish-red ground. The point 

 of entrance of the vascular trunks may even be seen, witli their division 

 into two principal branches passing respectively upward and downward, 

 and then breaking up into ramifications of various curvilinear form. Jf 

 the candle be held immovable, the appearances rapidly fade, since the 

 shadows in reality are quite faint, and are only made visible from the 

 sudden contrast produced by throwing them successively upon different 

 parts of the retina. 



As the bloodvessels which throw these shadows are at or near the 

 anterior surface of the retina, the extent of their apparent movement 

 on varying the position of the light, gives a means of ascertaining how 

 far behind the anterior surface of the retina its sensitive elements arc 

 situated. According to the measurements of Miiller, 1 this distance must 

 be, in various cases, from 0.17 to 0.36 millimetre; and the same ob- 

 server finds the posterior layers of the retina to be separated from its 



1 Cited in Helmholtz, Optique Physiologique. Paris, 1867, p. 289. 



