The Eye of the God-fish. 205 



delicate membrane almost entirely made up of minute, closely- 

 aggregated granules. 



Turning now to the consideration of the retina, I may, in 

 the first place, observe that it is well-nigh impossible to obtain 

 a thin vertical section of this membrane, unless the eyeball has 

 been previously immersed in a strong acid solution ; at least 

 this is the case with the posterior division commonly described 

 as Jacob's membrane. Very soon after death, the relations of 

 the delicate and complex elements of this structure are lost by 

 disintegration, but a careful examination of the broken-up 

 tissues themselves affords a clearer insight into their true 

 histological character than can possibly be obtained from the 

 artificially consolidated section. Whilst the latter method, 

 therefore, demonstrates the actual relations of the component 

 tissues of the organ, the former conveys a truthful conception 

 of the nature of these elementary particles. 



Under ordinary circumstances, when a small portion of the 

 retina of the cod is subjected to microscopic examination, with 

 the one-fourth or one-fifth objective, all that we see is a more 

 or less confused mass of semi-transparent tissues, in which, 

 however, the following elements may be distinctly recognized : 

 a fibrous matrix inclosing oval nuclei, dense layers of granules, 

 nerve filaments with or without ganglionic enlargements (Fig. 8) ; 

 rod-like fragments which are portions of the well-known bac- 

 cillce variously twisted, and frequently tapering to a narrow 

 point at one end (Fig. 9) ; and large oval corpuscular bodies 

 which are neither more nor less than the so-called cones, whose 

 character varies considerably in different members of the ver- 

 tebrata. If the eye be not perfectly fresh, the cones display 

 the utmost irregularity of outline, some being cylindrical, some 

 club-shaped (Fig. 10) ; many of them split up longitudinally 

 (Fig. 11), and showing a central cavity (Fig. 12) ; a few per- 

 fectly spherical (Fig. 13), and others oval (Fig. 14), in which 

 case the contents of the corpuscles are usually confined within 

 a second investing membrane, the latter being more or less 

 widely separated from the outer covering. 



These appearances, though in part abnormal, are not alto- 

 gether uninstructive ; but when the retina of a fish more re- 

 cently killed is examined, it will then be seen that all the fore- 

 going illustrations represent only the separated halves of the 

 cones which are double in the cod (Fig. 15) and its allies, as 

 indeed has also been shown to obtain in the perch by the 

 researches of Kolliker and H. Muller. Although the twin- 

 cone last referred to does not exhibit the true normal con- 

 dition of these corpuscles, yet, before going farther, I may here 

 remark that these various demonstrations seem to prove the 

 cones to possess a double envelope, the inner one inclosing a 



