616 THE SENSES. 



tion, and perforated at its centre by a small opening through which the 

 observer looks. By this means the retina and its vessels, as well as 

 the images delineated upon it, may be distinctly seen. According to 

 the observations of Helmholtz, objects at a certain distance, which are 

 perceived with distinctness, present to the eye of the observer, if suffi- 

 ciently illuminated, perfectly well-defined inverted images upon the ret- 

 ina, like those which would be thrown upon a screen by a system of glass 

 lenses properly arranged. If the eyeball furthermore be taken out from 

 a recently killed animal, and a circular portion of the sclerotic and ch.oroid 

 removed from its posterior part, similar inverted images of illuminated 

 objects in front of the cornea may be seen by transparency upon the 

 exposed portion of the retina. 



It is accordingly certain that luminous rays in passing through the 

 eyeball are brought to a focus at the retina, principally by means of the 

 crystalline lens. The formation of a visible image at this- spot does 

 not by itself explain all the phenomena of vision, since these images 

 are not seen by the individual, and we should not even know of their 

 existence except for the results of physiological experiment and obser- 

 vation. But the formation of such an image shows that all the light 

 coming from each different part of the object is made to fall upon a 

 separate and distinct point of the retina ; and it thus becomes possible 

 to perceive the figure and extension 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 of its tissues directly sensitive to light. It 

 forms a delicate, colorless, 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 outward and forward to the commencement 

 of the ciliary body. Here it terminates by an indented border, termed 

 the or a serrata, which is situated nearly at the plane of the posterior 

 surface of the crystalline lens. In front of this region it is replaced by 

 an attenuated layer, which remains in contact with the surface of the 

 ciliary body, but which contains no nervous elements. The retina 

 proper has, accordingly, the form of a thin membrane moulded upon a 

 nearly hemispherical surface, the concavity of which is directed for- 

 ward, and which receives the luminous rays admitted through the pupil, 

 and traversing the transparent and refracting media of the ej^eball. Its 

 greatest thickness is in the immediate vicinity of the entrance 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 gradually thinner in its middle and anterior portions. At its 

 terminal border, at the ora serrata, it is only 0.09 millimetre in thickness. 



The retina consists of a variety of superimposed Ia3'ers, in which 

 many different microscopic elements alternate with each other. In re- 

 gard to its physiological properties, so far as these have been deter- 

 mined with a sufficient degree of certainty, four of these layers may be 

 distinguished as representing the essential constituent parts of the 



