PHYSIOLOGICAL ANATOMY OF THE EYEBALL. 



777 



FIG. 14.-Rods of the retina. 



(Schultze.) 



From the monkey. A. Kods, after ma- 

 ceration in iodized serum, the outer 

 segment (li) truncated, the inner 

 segment (a) coagulated, granular, 

 and somewhat swollen ; c, filament 

 of the rods ; rf, nucleus. B. Kods 

 from the frog: 1. Fresh, magnified 

 500 diameters; , inner segment; 

 &, outer segment ; c, lentiform body ; 

 d, nucleus. 2. Treated with di- 

 lute acetic acid and broken up into- 

 plates. 



senformiger Korper). The entire inner segment is somewhat granular, and it often pre- 

 sents a granular nucleus at its inner extremity. The outer segment apparently differs 

 in its constitution from the inner segment and is not similarly affected by reagents. 

 Treated with dilute acetic acid, the outer segment becomes 

 broken up transversely into thin disks. These points in 

 the anatomy of the rods are referred to particularly, for 

 the reason that they have lately been used as an anatomi- 

 cal basis for a theory of the perception of colors. They 

 can be readily understood by reference to Fig. 244. 



The cones are probably of the same constitution as the 

 rods, but that portion called the inner segment is pyri- 

 form. The straight portion above (the outer segment) is 

 sometimes called the cone-rod. The entire cones are 

 about half the length of the rods and occupy the inner 

 portion of the layer. The outer segment is, in its consti- 

 tution, precisely like the outer segment of the rods. The 

 inner segment is slightly granular and contains a nucleus. 

 The cones are connected below with filaments passing 

 into the deeper layers of the retina. The arrangement of 

 the rods and cones is seen in Fig. 245, which shows the 

 different layers of the retina. 



At the fovea centralis, the external layer is composed 

 entirely of immensely elongated cones, with no rods. 

 These are slightly increased in thickness at the macula 

 lutea, but are diminished again in thickness, by about 

 one-half, at the fovea centralis. At the fovea, the optic 

 nerve-fibres are wanting; and the ganglion-cells, which exist in a single layer over 

 other portions of the retina, here present from six to eight layers, except at the very 

 centre, where there are but three layers. Of the layers between the cones and the 

 ganglion -cells, the external granule-layer and the inter-granule layer (cone-fibre plexus) 

 remain, in the fovea, while the internal granule-layer and the granular (molecular) layer 

 are wanting. At the fovea, indeed, those elements of the retina which may be regarded 

 as purely accessory seem to disappear, leaving only the structures that are concerned 

 directly in the reception of visual impressions. 



The external granule-layer is composed of large granules, looking like cells, which are 

 each nearly filled with a single nucleus. These are connected with the filaments from the 

 rods and cones. They are rounded or ovoid and measure from T^^-g- to ^^Vo of an ^ ncn 

 in diameter. The inter-granule layer (cone-fibre plexus) is composed apparently of mi- 

 nute fibrillse and a few nuclei. The internal granule-layer is composed of cells nearly 

 like those of the external granule-layer, but a little larger, and probably connected with 

 the filaments of the rods and cones. The granular (molecular) layer is situated next the 

 layer of ganglion -cells. 



The layer of ganglion-cells is composed of multipolar cells, like those in the brain, 

 measuring from ^Vjr to T ^-g- of an inch in diameter. In the centre of the retina, at the 

 macula lutea, the cells present eight layers, and they diminish to a single layer near the 

 periphery. The smaller cells are situated near the centre, and the larger, near the periph- 

 ery. Each cell sends off several filaments (from 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, from 5o ^ 60 to g-gooo f an i n h i n diameter. These do not call for special 

 description. 



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



