552 THE NERVOUS SYSTEM. 



of which arrangement they are called rod-granules. These gran- 

 ules also give off processes which pass in an inward direction and 

 terminate in the outer molecular layer. In the outer nuclear layer 

 are also cone-granules ; these are connected with the cones of the 

 ninth layer externally, and internally by a thick process which 

 becomes bulbous, the so-called cone-foot; they terminate in fine 

 fibers in the outer molecular layer. 



8. Membrana Limitans Externa. This is, together with the 

 membrana limitans interna and the fibers of Muller, a part of the 

 supporting structure of the retina. Indeed, some authorities 

 include neither of these membranes in the list of layers which 

 compose the retina, and hence describe it as made up of eight 

 rather than of ten layers. 



9. Layer of Rods and Cones. This is called also Jacob 's mem- 

 brane and bacillary layer. It is characterized by the presence of 

 rods and cones, of which the former are much more numerous 1 , 

 taking the retina as a whole. Relatively the cones are more 

 numerous at the back of the retina, but less so in the anterior 

 part. 



Rods. A rod is a solid body set perpendicularly to the sur- 

 face at whatever part of the retina it may be. It is made up of 

 an outer and an inner portion, the two being cemented together. 

 The outer portion is cylindrical and characterized by transverse 

 striae ; it has during life a purplish-red color. The inner portion 

 has striae longitudinally arranged. This portion is slightly bulged, 

 and becomes stained with carmine or iodin, while the outer portion 

 does not take the stain. 



Cones. Each is conical in form, with its base lying on the 

 membrana limitans externa. The apex is tapering. Like the 

 rods, each cone is made up of an outer and an inner portion ; the 

 outer conical apex presenting transverse striae, the inner portion 

 being striated longitudinally. Both rods and cones present a 

 granular appearance near the membrana limitans externa. 



Schafer describes the outer nuclear layer and the layer of rods 

 and cones as one, under the name sensory or nerve-epithelium 

 of the retina, inasmuch as their elements are continuous through 

 the two layers. He says : " The elements of which this nerve- 

 epithelium consists are elongated, nucleated cells of two kinds. 

 The most numerous, which we may term the rod-elements, consist 

 of peculiar rod-like structures (rods proper) set closely side 

 by side, and each of which is prolonged internally in a fine 

 varicose fiber (rod-fiber) which swells out at one part of its 

 course into a nucleated enlargement, and ultimately ends (in 

 mammals) in a minute knob within the outer molecular layer, 

 where it is embedded in the ramifications of the dendrons of the 

 rod-bipolars. The rod proper consists of two segments, an outer 

 cylindrical and transversely striated segment, which during life 



