684: SPECIAL SENSES. 



tre, and the larger, near the periphery. Each cell sends off several fila- 

 ments (two to twenty-five), probably going to the layer of rods and cones, and 

 a single filament which becomes continuous with one of the filaments of the 

 optic nerve. 



The layer formed by the expansion of the optic nerve is composed of 

 pale, transparent nerve-fibres, ?6 ^ 00 to ^TOTJT ^ an i ncn (0'5 to 1 /n) in 

 diameter. These do not require special description. 



The limitary membrane is a delicate structure, with fine striae and nuclei, 

 composed of connective-tissue elements. It is about 2g ft 6() of an inch (1 p.) 

 in thickness. From this membrane, connective-tissue elements are sent into 

 the various layers of the retina, where they form a framework for the sup- 

 port of the other structures. 



The retina becomes progressively thinner from the centre to the periphery. 

 The granular layers and the nervous layers rapidly disappear in the anterior 

 half of the membrane. 



The following is the probable mode of connection between the rods and 

 cones and the ganglion-cells : The filaments from the bases of the rods and 

 cones pass inward, presenting in their course the corpuscles which have 

 been described in the granule-layers, and finally become, as is thought, 

 directly continuous with the poles of the ganglion-cells. The cells send 

 filaments to the layer formed by the expansion of the optic nerve, which 

 are continuous with the nerve-fibres. This arrangement is shown in Fig. 

 248. 



The following description of the blood-vessels of the retina, with Fig. 

 249, was furnished by Loring : 



" The arteries and veins of the retina are subdivisions of the arteria and 

 vena centralis. The larger branches run in the nerve-fibre layer and are 

 immediately beneath the limitary membrane. The vessels lie so superficially 

 that in a cross-section examined with the microscope, they are seen to pro- 

 ject above the general level of the retina, toward the vitreous humor. While 

 the large vessels are in the plane of the inner surface of the retina, the 

 smaller branches penetrate the substance of the retina, to the inter-granule 

 layer. They do not extend, however, as far as the external granule-layer 

 and the layer of rods and cones. These two layers, therefore, have no blood- 

 vessels. 



" The ramifications of the vessels present a beautifully arborescent appear- 

 ance when seen with the ophthalmoscope. The manner in which the vessels 

 are distributed and the way in which the circulation is carried on can be 

 better understood by a study of Fig. 249 than by any detailed description. 

 The figure represents the ophthalmoscopic appearance of a normal eye in 

 young, adult life. The darker vessels are the veins, and the lighter vessels, 

 the arteries. The dotted oval line is diagrammatic and marks the position 

 and extent of the macula lutea. It is seen that this oval space contains a 

 number of fine vascular twigs which, coming from above and below, extend 

 toward the spot in the centre of the oval which marks the position of the 

 fovea centralis. In opposition, then, to the general opinion, which is that 



