THE ORGANS OF THE SPECIAL SENSES. 275 



with ciliated epithelium continuous with that of the nasal cavity into 

 which its other extremity opens. 



5. Sight. — The receptive nervous organ of vision is the retina. 

 This has an extremely complicated structure, which may be divided 

 into the following nine layers : 



1. The layer of pigmented epithelium, which lies next to the 

 choroid coat of the eye, and is, therefore, the most deeply situated 

 coat of the retina ; 2, the layer of rods and cones ; 3, the external 

 limiting membrane ; 4, the outer granular layer ; 5, the outer molec- 

 ular layer; 6, the inner granular layer; 7, the inner molecular 

 layer ; 8, the ganglionic layer ; 9, the layer of nerve-fibres. 



Internal to the ninth layer is the internal limiting membrane, 

 which separates the retinal structures from the vitreous humor 

 occupying the cavity of the eyeball. The general character and 

 associations of these layers are shown in Fig. 258. 



1. The layer of pigmented epithelium is made up of hexagonal 

 cells, which are separated from each other by a homogeneous 

 cement and form a single continuous layer upon the external sur- 

 face of the retina. They are in contact with the rods and cones 

 of the next layer, and send filamentous prolongations between those 

 structures. The pigment lies within these filamentous processes 

 and the portion of cytoplasm continuous with them, but its position 

 varies with the functional activities of the organ. When the eye 

 has been exposed to light the pigment is found lying deeply between 

 the rods. When the eye has been at rest for some time the pigment 

 is retracted in greater or less degree within the body of the cell. 



2. The rods and cones are the terminal structures of cells which 

 extend from the fifth layer to the first. The nuclei of these cells 

 lie within the fourth layer, to which they give a granular appear- 

 ance (Fig. 258). 



3. The external limiting membrane is formed by the cuticularized 

 outer ends of certain sustentacular epithelial cells, the " cells of 

 Muller" (Fig. 258, x), which extend from this layer to the in- 

 ternal limiting membrane and serve to support the various elements 

 of the retina. The nuclei of these cells lie in the seventh layer, to 

 the granular character of which they contribute. The portion of 

 the cell which lies in the fourth layer of the retina is indented 

 with numerous oval depressions receiving the nuclei of the cells 

 carrying the rods and cones, which they both support and isolate 

 from each other. The filamentous cell-bodies of those elements 



