PHYSIOLOGICAL ANATOMY OF THE EYEBALL. 779 



membrane. The vessels lie so superficially that, in a cross-section examined with the 

 microscope, they are seen to project 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 appearance 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. 

 246 than by any detailed description. The figure represents the ophthalmoscopic appear- 

 ance 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 



Fia. m.-Blood-vetael* of the retina; magnified 7* diameters. (E. Q. Loring.) 



vascular twigs which, coming from above and below, extend toward the spot in the cen- 

 tre of the oval which marks the position of the fovea centralis. In opposition, then, to 

 the general opinion, which is that the macula lutea has no blood-vessels, it is the spot of 

 all others in the retina which is most abundantly supplied with minute vascular branches. 

 These vessels can be distinctly seen even with the ophthalmoscope ; and microscopical 

 examination shows that the capillary plexus in the macula lutea is closer and richer than 

 in any other part of the retina. According to Nettleship, the area surrounding the fmva 

 centralis is the part which is most abundantly supplied with arteries, capillaries, and 

 veins. The fovea centralis itself, however, has no blood-vessels." 



The arteries of the retina send branches to the periphery, where they supply a 

 plexus of very small capillaries in the ora serrata. These capillaries empty fa to an mco 

 plete venous circle, branches from which pass back by the sides of the arteries 

 vena centralis. 



