THE SENSE OF SIGHT. 



603 



The relation of these nerve-cells one to another and to the supporting 

 neuroglia tissue and the manner in which they unite to form the 

 above-mentioned layers are schematically shown in Fig. 280. 



The pigment layer is composed of hexagonal cells. Though 

 formerly described as forming a part (the inner layer) of the chorioid, 

 these cells belong embryologically to the retina. From their retinal 

 surface delicate pigmented processes extend into and between the 

 rods and cones. On expo- 

 sure to light these processes 

 elongate and push themselves 

 between the rods. In the 

 dark they retract and with- 

 draw into the cell-body. 



The visual cells which 

 form the layer of rods and 

 cones are of two varieties, the 

 rod-shaped and the cone- 

 shaped. 



The rod-shaped visual cells 

 consist of a straight elongated 

 cylinder extending through the 

 entire thickness of Jacobson's 

 membrane and a fine fiber 

 containing a nucleus, which, 

 after piercing the external 

 limiting membrane, passes 

 into the outer molecular layer, 

 where it terminates in a spheric 

 enlargement. The outer por- 

 tion of the rod is clear and 

 homogeneous, though contain- 

 ing a pigment known as visual 

 purple or rhodopsin ; the inner 

 portion of the rod is slightly 

 granular. 



The cone - shaped visual 

 cells also consist of two por- 

 tions, a conic portion situated in Jacobson's membrane between the 

 rods, and a fine fiber, containing a nucleus, which, after piercing the 

 external limiting membrane, passes into the outer molecular layer, 

 where it terminates in a fine tuft. The inner portion of the cone is 

 thicker than the rod and rests on the limiting membrane; the outer 

 portion tapers to a fine point and is known as the cone-style. The 

 cones, as a rule, are shorter than the rods. The proportion of rods 

 to cones varies in different parts of the retina, though there are on 



FIG. 280. CROSS-SECTION OF THE RETINA 

 FROM A MAMMAL. A. Layer of rods 

 and cones. B. Visual cells (outer gran- 

 ules). C. Outer molecular layer. E. 

 Bipolar cells (inner granules). F. In- 

 ner molecular layer. G. Ganglion cells. 

 H. Layer of nerve-fibers, a. Rods. b. 

 Cones, e. Bipolar rod. f. Bipolar 

 cone. r. Lower ramification of a bi- 

 polar rod. f. Lower ramification of a 

 bipolar cone, g, h, i, j, k. Ganglion 

 cells in various stages, branching from 

 F. x, z. Bipolar contact of rods and 

 cones, t. Mii Her 's supporting fibers. 

 S. Centrifugal nerve-fibers. (After Ra- 

 mon y Cajal.) 



