618 



THE EYE. 



Fig. 428. 



inner end may be rounded, pointed, square, or even branched ; the 

 sides of the cells, too, are sometimes uneven. 



These cells are considered by KoUiker to coiTespond with the Miillerian fibres 

 of the retina, but according to Schwalbe these fibres, or rather their united inner 

 ends, are more probably represented by a delicate memlirane, which covers the 

 inner ends of the columnar cells and sends fine offsets around and between 

 them, and which he believes to be continuous with the membrana limitans 

 interna. 



Vessels of the Bietina. — A single artery (arfcria centralis retina) 

 passes between the bundles of fibres of the optic nerve to the inner 

 surface of the retina at the middle of the papilla optici (fig. 418, a. 



It is accompanied by the correspond- 

 ing vein and soon divides into branches 

 (fig. 417), usually two, one above, 

 the other below, each of these again 

 dividing into two branches which 

 arch out towards the sides ; the outer 

 ones are somewhat the larger, and as 

 they bend round the macula lutea they 

 send numerous fine branches into it 

 which end at the margin of the fovea 

 centralis in capillary loops. The main 

 branches of the vessels pass forwards 

 in the nerve-fibre layer, dividing dichoto- 

 mously as they proceed, and giving otf 

 fine offsets to the substance of the 

 retina, where they form a close capil- 

 lary network, but tliis does not extend 

 beyond the inner nuclear layer. So that 

 the outer retinal layers are entirely desti- 

 tute of blood-vessels, and moreover the 

 vascular system of the retina is nowhere 

 in direct communication with the choroidal vessels. Near the 

 entrance of the optic nerve, however, it comes into communication 

 with some offsets from the sclerotic coat, and the choroidal vessels 

 also send in branches, which join the long-meshed network in the 

 optic nerve furnished by the central arter}'. The arteries of the 

 retina have the usual coats, but the veins resemble capillaries in 

 structure, their walls consisting of a single layer of epithelioid cells. 

 Outside this layer is a sjjace (perivascular lymphatic, His) both m 

 the veins and capillaries, bounded externally by a second epithelioid 

 layer (forming the wall of the 13'mphatic). Outside this again is found, in 

 the case of the veins, a layer composed of a peculiar retiform tissue ; 

 they appear to have no plain muscular tissue in their wall. These 

 perivascular lymphatics are in communication with the lymjDhatics 

 of the optic nerve, and may be filled by injecting coloured fluid 

 underneath the sheath which that nerve derives from the pia mater. 

 Other lymph-spaces also become injected by the same process, viz., 

 the interstices between the nerve bundles which radiate from the 

 papilla optici, the capillary space between the limitans interna and 

 the hyaloid membrane of the vitreous humour, and finally even the 

 irregular interstice between the pigmentary layer and the layer of rods 

 and cones (Schwalbe). 



Fig. -128. — A Sjiall Portion of 

 THE Cii.iARV Part of tiiI'^ Re- 

 tina (KolHker). o50 Diameters. 



A, human ; B, from the ox ; 1, 

 pigment-cells ; 2, cells forming the 

 pars ciliaris. 



