48 THE VERTEBRATE RETINA 



fibers. This phenomenon of summation is of the utmost importance in 

 the physiology of the retina, and will be discussed again when certain 

 other concepts have been introduced. 



The three kinds of retinal elements so far mentioned — visual, bipolar, 

 and ganglion cells — are those concerned in the simple, straightforward, 

 projective pathway of the visual impulse to the brain. There are four 

 other types of cells which remain to be described: Miiller fibers, neu- 

 roglial cells, horizontal cells, and amacrine cells. 



Miiller Fibers — Miiller fibers may be likened to rivets which run 

 through the whole thickness of the retina proper and bind its layers 

 together (Fig. 19, m). Their outer ends form the external limitans and 

 their inner ends are expanded into trumpets or pyramids whose bases, 

 against the vitreous, are six-sided and are fitted together as an unbroken 

 mosaic, the internal limiting membrane of the retina. This is not a true, 

 isolable membrane but simply the inner surface of the retina. The vitre- 

 ous which touches the internal limitans may be a little tougher than the 

 rest, like the skin on a cornstarch pudding; but it is still part of the 

 vitreous — there is no distinct layer at the retino-vitreal interface which be- 

 longs to neither structure. The retina and vitreous are simply in contact. 

 The nucleus of a Miiller fiber lies about half-way through the thick- 

 ness of the inner nuclear layer and is very easily identified by its elon- 

 gated oval form. The boundary surface of a Miiller fiber, however, 

 cannot be made out at all unless the cell is isolated by the procedure 

 of macerating the retina; for the Miiller fibers have irregular expansions 

 and cavities in them, and occupy a surprising amount of the total volume 

 of the retina. If we imagine building a model of the retina by using wires 

 for the nerve fibers in it, and large glass beads for their nuclei, we could 

 then represent the whole population of Miiller fibers best by filling in all 

 the empty space with wax or some such substance. All of the nuclei in 

 the retina sit in pockets in the Miiller fibers, which at the levels of the 

 nuclear layers form a sort of sponge-work. Every nerve-fiber is likewise 

 insulated from every other by a film of intervening Miiller-fiber sub- 

 stance; and only at the synaptic handclasps between nerve-fiber ends is 

 there opportunity for separate nerve cells actually to come in direct con- 

 tact. 



Neuroglia — The neuroglial cells of the retina are small and not num- 

 erous. They are like one of the chief types of glial elements in the brain 

 and spinal cord. While glial cells are abundant and im.portant in serving 



