DR KNOX on the Comparative Anatomy of the Eye. 49 



latter opinion has been observed in the eye of the lion. In 

 birds and fishes, a reflected portion of the membrane of the 

 aqueous humour seems to cover the whole anterior surface of the 

 iris, and is the cause of that peculiar elasticity observable in the 

 iris of birds *, by which the relation of the iris to the cornea is 

 changed, when life is extinct, from a right angle to a very acute 

 one. In the eye of the deer, the ox, &c. the anterior layer of 

 the iris is connected to the inner membrane of the cornea, by 

 numerous short and seemingly tendinous fibres, and these mem- 

 branes are evidently distinct. 



The inner membrane of the sclerotic is simply a reflected 

 membrane of the choroid. Near to the origin of the retina, or 

 at least to its point of union with the optic nerve, the inner 

 membrane of the sclerotic can hardly be distinguished from the 

 choroid, which may here be divided into as many layers as the 

 anatomist chuses. It is curious enough to trace the use which 

 anatomists, from preconceived notions, have been induced to 

 make of the pia mater. The celebrated ZINN and WRISBERG 

 considered the inner layer of the sclerotic to be a continuation 

 of the pia mater ; others, equally celebrated, imagined the outer 

 layer of the choroid to be the true continuation of the cerebral 

 membrane. It was soon transferred to the inner layer of the 

 choroid ; and, lastly, to the membrane immediately in contact 

 with the retina, and which has lately been described by Dr JA- 

 COB in the Philosophical Transactions. This latter opinion was 

 held by the late Dr MONRO. I do not consider the inquiry as 

 being of the least importance. 



From the flexible nature of the sclerotic coat in man and 

 quadrupeds, and even in birds, it has been supposed that com- 

 pression exercised upon it by the muscles of the eye-ball, may 



' 



* This is not present in the eye of the cassowary. 

 VOL. X. P. I. 



