898 



A MANUAL OF PHYSIOLOGY 



clean-cut image of the object on the retina, with the excitation of 

 a sharply-bounded area of that membrane, follows as a geometrical 

 consequence from the theory of lenses. 



We have to consider (i) the mechanism by which an image 

 is formed on the retina, and (2) the events that follow the for- 

 mation of such an image and their relations to the stimulus that 

 calls them forth. 



Structure of the Eye. The eye may be described with sufficient 

 accuracy as a spherical shell, transparent in front, but opaque over 

 the posterior five-sixths of its surface, and filled up with a series of 



FIG. 381. DIAGRAMMATIC HORIZONTAL SECTION OF THE LEFT EYE. 



transparent liquids and solids. The shell consists of three layers 

 concentrically arranged, like the coats of an onion : (i) An external 

 tough, fibrous coat, the sclerotic, the anterior portion of which appears 

 as the white of the eye. In front this external layer is completed by 

 the transparent cornea. (2) A vascular layer, the choroid, which, 

 in the restricted sense of the term, ends in front in a series of folds 

 or plaits, the ciliary processes. The choroid contains a greater or 

 smaller quantity of the black pigment melanin. The ciliary processes 

 abut on the outer boundary of the iris, which may be looked upon as 

 an anterior continuation of "the choroidal or middle coat of the eyeball. 

 Between the corneo-sclerotic junction and the anterior portion of 

 the choroid is interposed a ring of unstriped muscular fibres, the 

 ciliary muscle. (3) The inner or sensitive coat, termed the retina 



