526 THE NERVOUS SYSTEM. 



these images are not seen by the individual, and we should not even 

 know of their existence except for the results of experiment and obser- 

 vation. But it shows that all the light coming from each part of the 

 object is made to fall upon a single point of the retina ; and it thus 

 becomes possible to perceive the figure of an object, as well as its 

 luminosity. 



Retina. The retina is the most essential part of the organ of vision, 

 since it is the only one directly sensitive to light. It forms a nearly 

 transparent membrane, composed of nervous elements, situated between 

 the inner surface of the choroid and the outer surface of the hyaloid 

 membrane, and extending from the entrance of the optic nerve to the 

 commencement of the ciliary body. Here it terminates by an indented 

 border, the ora serrata, nearly at the plane of the posterior surface of 

 the crystalline lens. In front of this region it is replaced by an attenu- 

 ated layer, in contact with the surface of the ciliary body, which con- 

 tains no nervous elements. It has, accordingly, the form of a mem- 

 brane moulded upon a nearly hemispherical surface, with its concavity 

 directed forward, and receiving the rays admitted through the pupil. 

 Its greatest thickness is in the immediate vicinity of the optic nerve, 

 where it measures, according to Kolliker, 0.40 millimetre. At a short 

 distance from this point it is reduced to 0.20, and thence becomes gradu- 

 ally thinner in its middle and anterior portions. At the ora serrata, 

 it is only 0.09 millimetre in thickness. 



The retina consists of superimposed layers, containing many different 

 microscopic elements. In regard to its physiological properties, so far 

 as they have been determined, four of these layers may be distinguished 

 as representing its essential constituents. These layers, counting from 

 the inner to the outer surface of the retina, are as follows : 1. The layer 

 of nerve fibres ; 2. The ganglionic layer of nerve cells ; 3. The layer 

 of nuclei ; 4. The layer of rods and cones. 



1. Layer of Nerve Fibres. The optic nerve joins the posterior part 

 of the eyeball about 2 millimetres inside its longitudinal axis, and at a 

 slightly lower horizontal plane. Its neurilemma becomes continuous 

 with the sclerotic coat, while its nerve fibres penetrate the cavity of 

 the eyeball. Up to this point the optic nerve consists of dark-bordered 

 medullated fibres, having, according to Kolliker, a diameter of from 1 

 to 4.5 mmm. But at their entrance into the eyeball they become much 

 smaller, being reduced, on the average, to less than 2 mmm., and many 

 of them to less than 1 mmm. in diameter. Owing to these changes, 

 the nerve appears suddenly diminished in size at its passage, through 

 the sclerotic. Internally it forms a slight prominence at the fundus of 

 the eye, the so-called papilla ; and the central artery and vein of the 

 retina emerge at this point. From the papilla as a centre the optic 

 nerve fibres, which have thus reached the inner surface of the retina, 

 radiate laterally in every direction under the form of a closely set layer. 

 This layer diminishes gradually in thickness from the centre outward, 

 owing to the fact that its fibres terminate successively in the deeper 



