1891.] On the Fossa Patellaris of the Corpus Wtreutn. 1.39 



lens the fossa patellaris and the posterior wall of the canal of 

 Petit (now opened up) would be bounded or lined immediately by 

 the substance of the corpus vitreum. According to the other side 

 with which I entirely agree there stretches from side to side a 

 distinct membrane, so that iu no part of its extent does the substance 

 of the corpus vitreum reach the surface. Let me now proceed to the 

 proofs, which are of various kinds chemical, optical, and mechanical. 



Chemical Pigments. Aniline dyes and picrocarmine stain the 

 capsule of the lens, the hyaloid, and other such elastic membrane?. 

 When the fresh corpus vitreum is so stained and I prefer strong 

 picrocarmine for some three minutes,, then washing in copious water 

 the hyaloid is perfectly well seen floating in water, with its wrinkles 

 on distortion and its well-defined free edge at a puncture. Exactly 

 the same appearance is seen on the front of the corpus vitreum here 

 there is something that stains deeply and that wrinkles. Moreover 

 picrocarmine stains the hyaloid membrane and the vitreous substance 

 differently : the former is red and the latter is yellow. The same 

 difference is seen at the edge of a puncture in the floor of the patellar 

 fossa: the red membrane is quite distinct from the yellow vitreous 

 substance. 



Optical. If, by means of a lens, the sun's rays be concentrated 

 upon the hyaloid membrane, it is seen to have a fluorescent appear- 

 ance, somewhat as if the surface had been bathed in a solution of 

 quinine sulphate. That this fluorescent appearance is due to the 

 lyaloid is obvious when the concentrated rays are made to fall on a 

 puncture in the hyaloid membrane. The vitreous substance itself 

 has no such appearance, but is clear and glassy, .so that the puncture 

 is beautifully seen, and the edge of the hole is sharp and well defined. 

 Exactly the same appearances are obtained when we examine the 

 front of the vitreous ; the fluorescence is here too, and the difference 

 between this appearance and that of the vitreous substance showing 

 through the puncture is very marked. 



Mechanical. When a blunt-pointed instrument is gently pressed 

 upon the hyaloid membrane and then removed, the substance recoils 

 simply, perhaps leaving a dimple for a little time; but, on- pressing 

 more firmly, there comes an instant when the instrument suddenly 

 sinks ; one has the impression that a membrane has been punctured, 

 and that behind the membrane the substance is soft and inelastic.. This 

 impression is at once supported on squeezing the mass of the vitreous 

 between the fingers ; a little elevation of the vitreous substance is 

 projected like a pimple through the opening in the membrane, and 

 recedes when the pressure is withdrawn. When this is repeated in 

 the front of the vitreous, the results are identical. 



So far, I have not mentioned anything which might not be equally 

 well explained by supposing the existence of a dense superficial layer 



