EYE 



445 



453). The rods are elongated cylinders (60 n long and 2 ju thick) consisting 

 of a homogeneous outer segment, in which the visual purple is found 

 exclusively, and a finely granular inner segment. In the outer third of 

 the inner segment there is said to be an ellipsoidal, vertically striated 

 structure (which in some lower vertebrates is very distinct). The por- 

 tion of the rod cells below the limiting membrane is a slender thread, 

 expanding to surround the nucleus which is characterized by from one 

 to three transverse bands. Beneath the nucleus the protoplasm again 

 becomes thread-like, and this basal prolongation of the cell terminates 

 in a small club-shaped enlargement, without processes (Fig. 454). 



Cone cell 

 Rod cell. 



Stellate ganglion cell. 

 Bipolar cells. 



Amakrine cells. 

 Centrifugal nerve fiber. 



Multipolar ganglion cell. - 



11 Layer of rods and cones. 



Membrana limitans 

 ' externa. 



Outer nuclear layer. 



Henle's fiber layer. 

 1 Outer reticular layer. 



Dinner nuclear layer. 



Inner reticular layer. 



Ganglion cell layer. 

 ==^-_ = Nerve fiber layer. 



Collateral. Pyramidal bases 

 of radial fibers. 

 FIG. 454. DIAGRAM OF HUMAN RETINA. SUPPORTING SUBSTANCE RED. 



The cones likewise consist of an outer and an inner segment. The 

 conical outer segments are shorter than those of the rods. The inner 

 segments are thick and somewhat dilated so that the entire cone is flask- 

 shaped. Moreover, the inner segment contains a vertically striated 

 "fiber apparatus." The nuclei of the cone cells are situated just beneath 

 the limiting membrane; below the nuclei the protoplasm forms a fiber, end- 

 ing in an expanded pyramidal base. 



The entire visual cells therefore form three layers of the retina, namely, 

 (i) the layer of rods and cones; (2) the outer nuclear layer, containing the 

 nuclei of the rod and cone cells; and (3) Henle's fiber layer, composed 

 of the basal processes of these cells. The three layers next beneath are 

 formed essentially of superposed parts of the radially arranged bipolar 

 nerve cells, which constitute the ganglion retina. Immediately beneath 

 Henle's fiber layer, dendritic processes of these cells form an outer reticular 

 layer, whereas their nuclei are situated in an inner nuclear layer, and their 

 centripetal processes, or neuraxons, enter an inner reticular layer. There 



