60 Da KNOX on the Comparative Anatomy of the Eye. 



the dissections, I find it stated, that the membrane is sometimes 

 partially absent, but, perhaps, never entirely ; that it seems to be 

 destined for the conveyance of bloodvessels, and, possibly, lym- 

 phatics *, and is apparently connected with the secretion of the 

 pigmentum nigrum ; for, in those animals in which the pigment 

 is very fluid and abundant, the membrane seemed most distinct ; 

 and, on the contrary, was altogether deficient, in those in which 

 the pigment seemed to form a part of the choroid itself. It is 

 difficult to say precisely where the membrane terminates ; but 

 there is little doubt of its being expanded over the membranes 

 forming the canal of Petit, and that it may even extend to the 

 edge of the pupil. 



We may now proceed to examine the retina, because the 

 anatomy of its termination is intimately connected with a very 

 beautiful and very singular structure, not yet sufficiently de- 

 scribed. On its external aspect the retina is apparently inclosed 

 by a membrane, adhering to it so closely, that it cannot be de- 

 monstrated apart. From some successful dissections, I am in- 

 clined to consider the numerous bloodvessels seen on the inter- 

 nal surface of the retina, as expanded on an excessively deli- 

 cate tunic. These fix down the terminating edge of the retina 

 all round the anterior edge of the vitreous humour, and adhere 

 firmly to the capsules forming the external paries of the canal of 

 Petit. Whether or not the external parietes of the canal of Pe- 

 tit be a mere continuation of these membranes, conjoined with 

 the hyaloid, is a matter of little importance ; it is sufficient for 

 our purpose, that they seem continuous, and are firmly connect- 

 ed to each other. In whatever way formed, the membrane, or 

 assemblage of membranes, proceeds forwards, to be inserted into 



* Dr PORTAL, in one case, found hydatids situate between the choroid and re- 

 tina. Unfortunately, he does not describe the pathology of the case with sufficient mi- 

 nuteness ; the fact, however, is valuable. 



