56 



THE EYE. 



nerve furnished by the central artery. The arteries of the retina have the usual 

 coats, but the veins resemble capillaries in structure, their walls consisting- of a 

 single layer of endothelial cells without any muscular tissue. Outside the endothelial 

 layer is a space (perivascular lymphatic space, His) both in the veins and capillaries, 

 bounded externally by a second endothelial layer (forming the wall of the lymphatic 

 space). Outside this again is found, in the case of the veins, a layer composed of a 

 peculiar retiform tissue. These perivascular lymphatic spaces are in communication 



nasal 



Fig. 64. SKCTJON THROUGH THE PLACE OP ENTRANCE OF THE OPTIC NERVE (B), TOGETHER WITH THK 



OPHTHALMOSCOPIC VIEW OP THE DISK (A), TO SHOW THE CORRESPONDING PARTS OP THE TWO. 



(Fuchs, after Jaeger.) 



c, d, lines of correspondence ; b, depression in centre of disk ; r, retina ; ch, choroid ; si, so, inner 

 and outer parts of the sclerotic coat, s ; ci, a ciliary artery cut longitudinally ; a, v, central artery and 

 vein ; s, d, subdural space ; sa, subarachnoid space ; du, dural sheath ; ar, arachnoidal sheath of nerve ; 

 p, pial sheath ; n, nerve bundles ; se, septa between them. 



with the lymphatic spaces of the optic nerve, and may be filled by injecting coloured 

 fluid under 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). With one or two exceptions (Chelonia, eel) no 

 vertebrates below mammals have blood-vessels in the retina : even in some 

 mammals the distribution is restricted to the posterior part of the eye and to the 

 nerve-fibre layer. 



Interconnection of the retinal elements. It is only comparatively 

 recently, by the aid of the method of Ehrlich (staining intra vitam with methyl-blue) 

 and that of Golgi (chromate of silver impregnation), that histologists have been able 



