NERVOUS CONSTITUENTS OF THE RETINA. 'I'll 



and optbalmoscopically. Two varieties have been noticed, in one of 

 vliich the medullated fibre tract is continuous with the optic disk,* 

 \\hilst in the other and less common case isolated white spots appear 

 upon the retina at some distance from the disk, so that here the 

 medulla, after disappearing at the point of entrance of the optic nerve, 

 reappears after the fibres have run for a certain distance.! 



Amongst Mammals it has been known since the time of Bowman 

 that medullated nerve fibres were prolonged into the retina in the 

 Rabbit and Hare.J In these animals two white fasciculi run diverg- 

 ingly outwards in opposite directions, which render the retina mode- 

 rately opaque, but perhaps are not altogether incapable of perceiving 

 light, since, as I have satisfied myself, the bacillar layer is well de- 

 veloped behind them. 



A small quantity of medullary substance, which however scarcely 

 interferes with the transparency of the optic-fibre layer, is found, as 

 Leydig has pointed out, around the nerve fibres of the retina in many 

 Fishes, and he remarks that the primitive nerve fibres of Sharks and 

 Rays are " sharply contoured and varicose. H. Miiller also men- 

 tions the fact that some of the fibres within the globe of the eye in 

 Fish are composed of axis-cylinder and medullary sheath. || A some- 

 what similar appearance may also be seen in Birds. 



A very remarkable deviation from the normal condition is exhibited 

 in the thickenings of the nerve fibres of the retina, which in cases of 

 Bright's disease were regarded as the cause of certain white spots 

 occurring in this affection, and were considered to be bipolar ganglion 

 cells. These varicosities resembling bipolar ganglion cells were first 

 described by Zenker and Virchow, and their true nature was recog- 

 nized by H. Miiller.^" They are formed of fusiform thickenings and 



* Donitz (Reichert and Du-Bois Reymond's Archiv, p. 741, 1864), by 

 whom this persistence of medulla was first demonstrated ophthalmoscopi- 

 cally, established the fact that in his own eye the part in question was, 

 like the disk itself, blind; i.e., is either quite incapable of transmitting 

 light, or that there is no layer of rods and cones behind it. 



t See for example the cases given by v. Recklinghausen in Virchow's 

 Archiv, Band xxx. , p. 375. 



J See H. Miiller, Zcitschrift far ivissenschaftliche Zoologie, Band viii., 

 p. 64. 



Beit-rage zur Mikroskop. Anat. und Entwicklungsgeschichte tier Rochen 

 n,td Haie, p. 24, 1852. 



j| Zdtschrift fiir ivissenschaftliche Zoologie, Band viii., p. 22. 



IT v. Grafe's Archiv fiir Ophthalmologie, Band iv., 2, p. 41. 



Q2 



