EETINA OF THE BLOWELT. 



409 



Berger and Yiallanes trace the optic-nerve fibres througli a 

 series of small round ceils, very conspicuous in the outer half of 

 my retina, Hickson's periopticon. Hickson regards these cells 

 as of quite secondary import. They clearly belong to the sup- 

 porting tissue and are external to the sheaths of the retinal 

 elements, which are continuous with the perineurium of the optic 

 nerve. 



Dr. Hickson and Dr. G-renacher suppose the sheathing cells of 

 the great rods, retinulae of G-renacher, to be the nerve-terminals ; 

 and more recently Flatten pretends that the optic nerve ter- 

 minates in the crystalline cone. There is therefore no unanimity 

 amongst previous writers, especially in matters of detail ; as it is 

 impossible that they can all be right, it is quite possible, as I 

 assert, that they are all wrong. 



Dr. Hickson's neurospongium, or terminal anastomosis, which 

 is inadmissible on physiological grounds, is no nerve-plexus 

 at all, but the tracheal plexus and the sustentacular framework 

 of my retina. 



It is exceedingly difficult to prepare sections which show the 

 true retinal end-organs. This difficulty arises from the fact that 

 the chloroform and alcohol used in the process of imbedding 

 dissolve the fatty matters from the nerves, and the external 

 extremities of my retinal rods are completely dissolved or dis- 

 integrated by the action of aqueous media. 



I have, however, on many occasions succeeded in obtaining 

 sections in which both the inner and outer extremities of the 

 retinal rods, as well as the nerves, remain more or less unaltered. 

 Another difficulty arises from the extreme transparency of these 

 structures in very thin sections, and from the fact that they 

 cannot be stained by any of the stains used in such researches ; 

 the outer ends of the rods are not affected by strong solutions of 

 aniline dyes, except vesuvin *. 



In thicker sections the numerous round cells between the 

 retinal nerve-end organs, which are not connected with nerves, 

 but with the sustentacular framework, entirely conceal the outer 

 ends of the rods. 



There are two methods which give good results ; in both the 

 tissues must be fixed either with osmic acid and absolute alcohol 



* The best demonstration of these organs is obtained by staining with a 

 solution of vesuvin in aniline water. The solution must be quite freshly made, 

 and unfortunately such preparations fade rapidly when mounted in balsam. 



