of the Canal of PETIT. 249 



as a structure of little moment, and had, indeed, been seen only 

 in detached portions, and had, moreover, been entirely misun- 

 derstood *. The value of the discovery may be best understood 

 by reflecting, that it immediately encloses the retina or sensitive 

 membrane of the eye, and hence becomes of the greatest inter- 

 xest to the physiologist. In my former paper, I was inclined to 

 view it as being perhaps the source of a portion at least of the 

 pigmentum nigrum, in which opinion I was strengthened by 

 viewing the membrane as extending in many animals as far as 

 the edge of the pupil, (a view which I still adopt relative to it) ; 

 but this opinion (in itself merely speculative, and connected 

 chiefly with the development of the membrane in the eyes of 

 fishes) a more deliberate and careful examination of the organ 

 compels me to abandon. It is not improbable that the gentle- 

 man to whom we owe the discovery, and our most correct views 

 of the subject, will himself resume the inquiry : in the mean 

 time, I shall take the liberty of stating a few observations I have 

 made relative to it, which seem hitherto to have escaped notice. 

 The membrane of Jacob is generally of a brownish colour, 

 and sufficiently opaque to arrest the rays of light, supposing 

 them to have passed through the semitransparent retina ; hence 

 we perceive that a part of the functions heretofore assigned by 

 physiologists to the choroid must belong to the membrane of Ja~ 



I am still inclined to think, that a very delicate membrane is detached from the 

 anterior termination of the canal of Petit, at the point where its parietes reunite, and 

 incloses the whole of the posterior surface of the capsule of the lens, closely adhering 

 to this capsule : it is very obvious that this membrane must be quite transparent, as 

 it is in the immediate line of the pupil. It would be extremely interesting to know, 

 whether it is this membrane, or the capsule itself of the lens, which, in certain dis- 

 eased states of the organ, becomes vascular, thickened, and opaque. 



* HALLEE saw portions of this membrane, which he describes as a sort of inor- 

 ganic mucus. His words are : " Ut magnas maculas nigras saepe retina tunicse ad 



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