SUSTENTATIVE AND INTEGRATIVE CELLS 49 



as the connective-tissue of the central nervous system, their place is taken 

 in the retina by the Miiller fibers, which do the same job even better and 

 other jobs in addition. We may fairly consider the glial cells of the 

 retina to be meaningless, and present only because of their inheritance 

 from the brain wall of which the retina is, after all, a part. Occasionally 

 they seem to resent their idleness and become altogether too busy, gener- 

 ating a 'glioma' — a particularly disastrous type of tumor whose presence 

 calls for the immediate removal of the eye to prevent a fatal involve- 

 ment of the brain by way of the optic nerve. 



Horizontal and Amacrine Cells — Although the Miiller fibers and 

 neuroglial cells are certainly not impulse-conducting elements, the 'hori- 

 zontal cells' are under suspicion of performing some sort of integration 

 of the retina. If we think of the visual^bipolar->ganglion-cell chain as 

 running vertically through the retina, then the amacrines and horizontal 

 cells do their work in a horizontal direction. The horizontals have their 

 cell-bodies among those at the outer surface of the inner nuclear layer 

 (Fig. 19, h). In lower vertebrates the horizontal cells are chunky and 

 epithelioid, or ropy and anucleate, and seem only to have a supporting 

 function like the Miiller fibers. In higher vertebrates, however, they more 

 often have many spider-leg processes running in the outer plexiform 

 layer. Thus they may give the appearance of nerve cells and very prob- 

 ably do conduct laterally, tying up one area of the retina with another 

 just as regions of the cerebral cortex are interconnected by association 

 fibers. Those of mammals (Fig. 19, h) are certainly conductive, and in 

 man have their stubby dendrites connected with cones and their long 

 axons connected with distant rods and cones. 



The 'amacrine' cells ordinarily have this same horizontally integrative 

 function. Their exact action and its effects upon subjective visual phe- 

 nomena are about the biggest remaining mystery in the physiology of 

 the retina. Their nuclei tend to lie in the inner half of the inner nuclear 

 layer and each gives off a single process which passes vitread and then 

 branches more or less, the branches being short or very long (Fig. 19, a). 

 The amacrines seem to associate the bipolar^ganglion-cell synapses, per- 

 forming for the inner plexiform layer the same function that the con- 

 ductive types of horizontal cells do for the outer. 



The action of these two types of cells would appear to be detrimental 

 to the preservation of the pattern of the retinal image during its 'wire- 

 photo' transmission to the brain in the form of nerve-impulses. If all the 



