1 62 ANIMALS 



the connected nervous elements are developed from the brain. 

 The vertebrate eye is exceedingly complex, and only the more 

 essential features will be called to mind: i. The sclera is a hol- 

 low shell of approximately spherical form, composed of a thick 

 and dense layer of connective tissue. It is the protective and 

 supporting framework of the eye. The cornea is the trans- 

 parent, more convex portion of the sclera on the side where 

 the light enters the eye. 2. The choroid layer is a layer of 

 blood vessels and capillaries, which lines the inner surface of 

 the sclera. In front it forms the iris, and an opening in the 

 latter is the pupil. 3. The retinal layer is double. Against 

 the surface of the choroid layer there is a layer of pigment cells, 

 which extends from the point where the optic nerve enters the 

 eye to the pupil. The retina proper is the innermost layer and 

 extends from the optic nerve to within about 60° of the centre 

 of the pupil, where it thins out into an endothelium, and as 

 such, continues on to the edge of the pupil, where it merges into 

 the pigment layer. 



380. The nervous elements of the retina are arranged in 

 three layers: i. The sensory layer, proper, is composed of two 

 types of cells, rods and cones, as they are called. The rods are 

 much more numerous than the cones, except at the point of 

 most distinct vision — the fovea centralis — where the rods are 

 entirely wanting. The "rods" consist of a slender cylinder, 

 which tapers at one end into a short fibre. The latter is more 

 or less beaded and ends in a small knob. The cones are shorter 

 and thicker than the rods and, as the name signifies, are conical 

 in form. From the base of the cone a rather stout fibre pro- 

 ceeds, but ends shortly in a broad disc. The nuclei of the 

 "cones" are rather large and located at the base of the cone. 

 The nuclei of the rods are smaller and lie somewhere along the 

 course of the fibre. The cylindrical and conical portions of 

 the rods and cones, respectively, project into the pigment layer 

 in such a way that their ends are completely surrounded by 



