426 SPECIAL PHYSIOLOGY. 



membrane, called the membrana limitans, which separates it from the 

 so-called vitreous body. As seen during life, by the aid of a magnify- 

 ing glass, the retina presents a reddish color, due to the bloodvessels 

 contained in it. Branches of the central artery of the retina are seen 

 ramifying over it; and these, as well as the capillary network, are 

 situated upon, or near, its inner surface. When examined immediately 

 after death, the retina is found to be of a pinkish color, soft, and trans- 

 parent ; but it quickly becomes white or semi-opaque. In the centre 

 of the back of the eyeball, i. e., exactly in the antero-posterior axis of 

 the globe, and, as we shall explain, in the line of most perfect vision, 

 the retina presents, soon after birth, an elliptical yellowish spot about 

 y^th of an inch wide, called the macula lute a ; in the middle of this 

 yellow spot-, the margins of which are slightly elevated, is a darker 

 circular depression, named the fovea centralis (fovea, a pit). The 

 retina being exceedingly thin in this situation, the pigment of the 

 choroid is seen through it, and this gives rise to the deeper color of the 

 fovea centralis ; the yellowish margin of the macula lutea, is owing 

 to the presence of some special but delicate pigmentary matter. A 

 little below the yellow spot, and about y^th of an inch internal to it, 

 is the colliculus, or point of entrance of the optic nerve ; the central 

 artery of the retina, also passes into the eyeball at the same place. 

 The thickness of the retina gradually diminishes from behind forwards ; 

 its thickness varies from yj^th to 2i<j tn f an inch. 



Examined microscopically, the retina is found to consist essentially 

 of three layers, which are held together by a very delicate connective 

 tissue ; it also contains bloodvessels. The external layer, formerly 

 called Jacob's membrane, and now the bacillary layer, is very thin, and 

 consists of a stratum of evenly-disposed, transparent, colorless rods, 

 called bacillce, intermixed with other larger bodies, named cones. These 

 rods are solid and highly-refractive bodies, packed closely side by side, 

 and, except those near the anterior part of the retina, which, at least 

 in the frog, are disposed obliquely, are arranged more or less nearly 

 perpendicularly to the centre of the eyeball ; under the action of 

 water, these rods swell, undergo distortion, and show a division into an 

 outer and inner segment; the outer segment is the more highly re- 

 fractive ; the inner one, which becomes curved on the application of 

 water, is connected with a fine fibre, which passes vertically inwards, 

 into the next or middle layer of the retina. The cones, or bulbous 

 particles, disposed at regular intervals between the rods, present a 

 similar structure, and the same connection with the middle retinal 

 layer. The diameter of the rods is, on an average, T 3f ?<j tn ' tnat f 

 the cones about ^ggth of an inch. The middle layer of the retina, 

 also named t\\s granular layer, consists of two strata of granular or 

 nucleiform bodies, which are connected, on the one hand, with the fine 

 fibres proceeding from the rods and cones, and on the other, by fine 

 processes, with the nervous elements belonging to the third or so-called 

 nervous layer of the retina. Amongst these nuclear fibres, are some, 

 forming the radiating fibres of II. Miiller, which pass through the 

 whole thickness of the retina, from between the rods and cones of the 

 outer layer, to the membrana limitans, on its extreme inner surface, 



