152 DK. COBBOLD ON THE EYE OF THE COD-FISH. 



III. The normal condition of the retinal cones in the Cod is 

 double, and the phenomena exhibited by these twin-cones, under 

 the action of water, seem to indicate that the bacillar prolon- 

 gations {Zapfenstabcheii) of the cones are not persistently formed 

 appendages, as the representations of Kolliker, H. Miiller, and 

 Nunneley would lead us to suppose, but bear more the character 

 of tactile bodies capable of protruding filaments under the influ- 

 ence of stimuli. 



The cones and bacilli of Jacob's membrane are not true nerve- 

 structures in the sense maintained by Kolliker and Miiller, neither 

 can they properly be called " percipients of light ;" but they are, 

 in point of fact, special tactile organs, like the Pacinian corpuscles 

 of the skin, and are destined to receive and convey impressions 

 produced by pencils of light impinging upon and reflected from 

 the internal limiting membrane of the choroid, — the impressions 

 being subsequently and finally transferred to the true nervous 

 elements of the inner layer of the retina by the intervention of 

 the granular layers, which are held together by the delicate Miil- 

 lerian filaments. 



Although I have arrived at the above conclusion respecting the 

 Pacinian-corpuscular character of the cones from independent and 

 oft-repeated examinations, I may observe that a similar opinion 

 had been previously recorded by Professor Groodsir of Edinburgh, 

 who, from considerations affecting the development of the verte- 

 brate eyeball, goes even further, and asserts that the bacillar 

 layer, with its rods and cones, " belongs morphologically to the 

 transparent humours of the eye." 



All observations made on the retina after it has been immersed 

 in solutions of chromic acid, in so far as the intimate histological 

 characters are concerned, should be received with extreme caution, 

 because the normal characters of its component parts are at once 

 destroyed by the addition of coagulating reagents. The same 

 remark is equally applicable to the vitreous body and other soft 

 tissues of the eyeball. Strong acid solutions, however, are useful 

 in determining the relations of the ultimate elements of any given 

 compound tissue, as has been abundantly proved by recent inves- 

 tigations, and more particularly by those of Hannover, Kolliker, 

 and Ileinricli Midler. 



