THE INTERNAL COAT THE NERVOUS TUNIC G59 



The expanded and flattened bases or inner extremities of these glia 

 cells arc so closely approximated to one another as to form a complete 

 investment for the inner surface of the retina, which is known as the 

 internal limiting membrane (memlrana limitans inierna) and is fre- 

 quently classed as the innermost layer of the retina. Under low magnifi- 

 cation it appears as a continuous membrane, but under higher powers it 

 is readily resolved into the broad, conical, basal expansions of which it 

 consists. From these initial expansions the glia cells may be traced 

 outward through the nerve fiber and ganglion cell layers by means of 

 the numerous coarse processes or glia fibers. 



The glia fibers then pass in a fairly straight course through the 

 inner reticular layer. In this portion numerous short, fine, lateral off- 

 shoots from the main stem support the neurospongium of the reticular 

 layer. Continuing through the inner nuclear layer the glia substance 

 is somewhat thickened; it sends off fewer but coarser lateral processes, 

 and in the mid-region of this layer presents an enlargement which is 

 almost entirely occupied by the large ovoid nucleus. 



The fiber cell, somewhat narrowed, may then be traced through the 

 outer reticular to the outer nuclear layer, where its processes form a 

 dense network about the nucleated segments of the rod and cone visual 

 cells. 



The terminal processes of the fiber cells become again flattened, some- 

 what after the manner in which the internal limiting membrane is 

 formed, and are so closely approximated as to form an external limiting 

 membrane., a distinctly membranous structure which derives a reticular 

 appearance from being pierced by each of the innumerable rod and 

 cone elements. 



From the outer surface of the expanded ends of the Miillerian 

 fiber cells which form the external limiting membrane, minute fibrils 

 are continued between the bases of the non-nucleated portions of the 

 rod and cone cells to form shallow sockets, the rod and cone sockets, into 

 which the bacillary portions of these elements are fixed. The neuroglia 

 supporting tissue includes also the ordinary long- and short-rayed astro- 

 cytes, limited, however, to the cerebral portion of the retina. 



THE MACULA LUTEA 

 (Yellow Spot) 



The macula lutea being apparently the most highly developed portion 

 of the retina, deserves some special consideration. The macula is a cir- 



