ORGANS OF THE BODY 



645 



more and more, its nervous constituents commence to disappear, while 

 the sustentacular matter gains the preponderance. The optic fibres cease 

 gradually to constitute a separate layer, the ganglion cells appear at wider 

 intervals, the granular layers become shallower, and the rods and cones 

 shorter &c. (Muller, Merkel, Schultze). Finally, the nervous elements 

 disappear entirely from the membrane, leaving behind reticula of con- 

 nective-tissue fibres, which give way in their turn to a nucleated and 

 eventually non-nucleated homogeneous substance. This becomes, then, 

 still thinner, until, about 2 mm. from the ora serrata, the retina terminates 

 by fusing with the membrana hyaloidea. This is the relation of parts as 

 described by Hitter, .while other observers seem to have come to different 

 conclusions on the point. Thus Koelliker states that the membrana linii- 

 tans extends in the form of a system of cylindrical cells decreasing in height, 

 over the ciliary processes (intimately united to them and to the zonula 

 %innii) as far as the external border of the iris. Briicke and H. Muller, 

 on the other hand, suppose them to reach as far as the edge of the pupil. 



Schultze found several forms of these cells, but did not observe any 

 transition into the fibrous matter of the zonula Zinnii ( 313). He 

 regards the cells as corresponding to the radiating sustentacular fibres. 



The blood-vessels of the retina (fig. 584) are derived from the arteria 

 and vena centralis, enclosed within the optic nerve. They thus consti- 

 tute a separate vascular system of the bulb, which forms, however, con- 

 nections in the manner described in 312 with the vessels of the ciliary 

 system (e). From the splitting 

 up of the artery just mentioned 

 "is formed a delicate wide- 

 meshed network of capillaries 

 0-0056-0-0045 mm. in dia- 

 meter (fig. 599, b). This vas- 

 cular plexus lies chiefly in the 

 internal portion of the retina, 

 but extends, however, as far as 

 the internal granular layer. It 

 was formerly supposed by many 

 that at the ora serrata this 

 retinal system of vessels com- 

 municate with that of the 

 choroid ; but this is not really 

 the case (H. Muller, Leber). 

 In the yellow spot numerous 

 capillaries are to be seen, but 

 no vessels of any size. 



The optic nerve itself only re- 

 ceives small twigs from the art. 

 cent., but, on the other hand, 

 numerous branches from the in- 

 ternal sheath (fig. 584, 1), and 

 a few from the external sheath. 



A highly developed network of this kind does not, however, occur 

 in the retina of all mammalia. In the horse, for instance, we only find 

 a narrow belt of delicate radiating vessels around the entrance of the 

 optic nerve. The retina of the hare and rabbit, also, only presents a 

 narrow vascular zone corresponding, as a rule, with that of the medullated 



Fig. 599.- 



Vessels of the human retina, a, arterial; c, 

 venous twig ; 6, capillary network. 



