7 8o A MANUAL OF PHYSIOLOGY 



organs more generally diffused in the skin, attain to some such dim 

 consciousness of light and shadow as will enable them to avoid an 

 obstacle or an enemy, to seek the sunny side of a boulder or the 

 obscurity of an overhanging ledge of rock. But the indispensable 

 condition of distinct vision is that an image of each part of an object 

 should be formed upon a separate portion of the receiving or sensitive 

 surface. This condition is, to a certain extent, fulfilled by the com- 

 pound eyes of some of the higher invertebrates (insects, e.g.). Here 

 rays from one point of the object pass through one of the funnel- 

 shaped elements of the compound eye, and rays from another point 

 through another. Rays striking obliquely on the facets are stopped 

 by the opaque partitions between them. In the Cephalopods we 

 find that this compound type of eye has already been abandoned ; 

 the single system of curved refracting surfaces so characteristic of the 

 vertebrate eye has made its appearance ; and the formation of a 

 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 formation 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 trans- 

 parent liquids and solids. The shell consists of three layers concen- 

 trically 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 and pigmented layer, the choroid^ 

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

 folds or plaits, the ciliary processes. These abut on the outer 

 boundary of the iris, which may be looked upon as an anterior con- 

 tinuation 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 (Figs. 287, 288). 

 This covers the choroid as a delicate membrane, extending to the 

 ciliary processes, where it ends in a toothed margin, the ora serrata. 

 The optic nerve forms a kind of stalk to which the eyeball is attached. 

 Its point of entrance at the optic disc is a little nearer the median line 

 than the antero-posterior axis, which nearly passes through the centre 

 of a small depression, the fovea centralis, situated in the middle of 

 the macula lutea, or yellow spot. From the optic disc (sometimes 

 called the optic papilla) the optic nerve spreads over the retina as a 

 layer of non-medullated fibres, separated from the interior of the 

 eyeball only by the internal limiting membrane. This so-called 



