ORGAN OF VISION. 



217 



a small shining space, destitute of pigment, through which the colours 

 of the membrana Ruyschiana appear. This is termed tapetum. It is 

 met with only in quadrupeds. 



The retina is the last coat, if we except a highly delicate serous 

 membrane discovered by 



Dr. Jacob, 1 of Dublin, and Fi s- 89 - 



called after him Tunica Ja- 

 cobi, which is interposed be- 

 tween the retina and the cho- 

 roid coat. 2 It appears to be 

 composed of cylindrical, trans- 

 parent, and highly refractive 

 bodies, which are arranged 

 perpendicularly to the surface 

 of the retina, their outer ex- 

 tremities imbedded, to a great- 

 er or less depth, in a layer 

 of the pigmentum nigrum. 

 The only plausible suggestion, 

 which, according to Messrs. 

 Kirkes and Paget 3 , has been 

 offered, concerning the use of 

 these bodies, is that of Brticke, 

 who thinks it not unlikely, 



Fig. A. An Enlarged Plan of the Retina, in section. 



rg 

 -mrrmcmf nm rn side of the figure, where the objects are disturbed, t 



pigmentum m- win cones pr ject ' likepapille; a ^ ff) the small rods b ' eing 



he 



in a great measure lost at this place. And these (small 



f TT -,1 4 -, ,v> i bodies) are seen to become horizontal towards the extre- 



01 lUll, liaS affirmed, mity of the object, h, where some are in disorder. 



1. The nervous structure, viz., the nerve-fibres (&) be- 

 that they may Serve tO COn- twcennerve-oella (,). 2. Jacob's membrane. 3. Inner 

 T i i , ,1 surface of choroid. d. One of the small pointed bodies of 



duct back tO the Sensitive Jacob's membrane. 



portion of the retina those Fig B The Outer Surface of Jacob , s Membrane. 



rays of light which have tra- (From Hannover.) 



Versed that membrane, and Opposite e, the twin cones are obscurely seen, not being 



, r i in focus, while, at the lower part of the figure, near/, the 



not been entirely ab- sa me bodies are clearly discernible. Towards the riht 

 \\\r 



by 



Mr. GreOrge H. Field- 

 ing, 



that immediately behind the 

 retina, and in connexion with it, there is a peculiar membrane, separable 

 into distinct layers from the choroid, and supplied with bloodvessels, 

 which he proposes to name membrana versicolor. He presumes, that 

 it receives the vibrations of light, and communicates them to the retina: 

 the eyes, used for experiment, were those of the ox and sheep. 



The retina lines the choroid, and is a soft, thin, pulpy, and grayish 

 membrane, formed chiefly, if not wholly, by the final expansion of the 

 optic nerve. M. Kibes, 5 indeed, esteems it a distinct membrane, on 

 which the optic nerve is distributed; a structure more consistent 



1 Philosoph. Transact, for 1819; Medico-Chimrg. Transactions, xii., Lond., 1823, and Art. 

 Eye, in Cyclop, of Anat. and Phys., p. 186. 



2 Philosophical Transactions for 1829, p. 300. 



3 Manual of Physiology, American edition, p. 405, Philadelphia, 1849. 



4 Second Report of the British Association for the Advancement of Science; or Amer. 

 Journal of the Med. Sciences, Nov., 1833, p. 220. 



s Memoir, de la Societe Medicale d'Emulation, vii. 86. 



