310 ELEMENTS OF HISTOLOGY, [chap, xxxix. 



layer. No other elements of the retina are present at 

 the papilla, except a continuation of the limitans 

 interna. At the papilla nervi optici the arteria and 

 vena centralis nervi optici also enter, and spread 

 out with their branches in the inner layers of the 

 retina, A large lymph space is also found there. 



(6) The macula lutea and fovea centralis will be 

 considered after the various layers of the retina have 

 been described. 



(c) At the ora serrata all cellular and nuclear 

 elements of the retina except the pigmented epithe- 

 lium and the nerve-fibres, come to an end ; but the 

 limitans interna, with its peculiar radial or Miiller's 

 fibres, is continued over the ciliary processes in the 

 shape of columnar epitheloid nucleated cells men- 

 tioned above : this is the pars ciliaris retinae. 



436. Structure of the layers of the retina (Figs. 

 158, 159). 



(1) The membraiia limit ans interna is 

 composed of more or less polygonal areas, which are 

 the ends or bases of pyramidal, finely-striated fibres 

 the radial fibres of Muller. Each radial fibre passes 

 from the limitans interna in a vertical direction 

 through all layers to the limitans externa, and on its 

 way gives off numerous lateral branchlets, fibrils, and 

 membranes, which anastomose with one another so as 

 to form a honeycombed strorna or matrix for all 

 cellular and nuclear elements of the retinal layers. 

 In the nerve-fibre layer the radial fibres are thickest, 

 this being, in fact, the pyramidal basis ; in the inner 

 nuclear layer each possesses an oval nucleus. 



437. (2) The layer of nerve -fibres. The 

 optic nerve-fibres at their entrance into the eyeball 

 lose their medullary sheath, and only the transparent 

 axis cylinder is prolonged into the retina. In man, 

 medullated nerve-fibres in the retina are very excep- 

 tional ; in the rabbit there are two bundles, whose 



