48 DR KNOX on the Comparative Anatomy of the Eye. 



might be better stated, that as the eyes of the ostrich and casso- 

 wary have a strictly ornithological character, this fact may be of- 

 fered as an instance of the close observance which Nature pays 

 to her general laws. I have thought that the vitreous humour 

 in birds was less dense than in the mammalia, but I have no di- 

 rect experiments to prove this. 



Of the Sclerotic and Transparent Cornea. 



The sclerotic, or external covering of the eye-ball, is so inti- 

 mately connected with the dura mater, forming the sheath of 

 the optic nerve, that it may be considered, as in some measure, 

 an analogous membrane. Anteriorly, its connection with the 

 cornea presents a variety of forms, but there is this uniformity 

 in all the animals which I have examined, namely, that the in- 

 ternal layer of the cornea, that to which the name of the Tunic 

 of the aqueous humour has been given, does not unite with the 

 sclerotic, but with the iris. The mode in which this union takes 

 place is simply this : the whole external layer of the sclerotic 

 passes forwards beyond the circulus niger, and is inserted into 

 the edge of the cornea. The inner membrane of the sclerotic, 

 of the origin of which I shall presently speak, in its passage for- 

 wards, is interrupted by that portion of the annulus albas which 

 is left adhering to the sclerotic, when the choroid has been for- 

 cibly removed from it, but may be readily detected between this 

 portion of the annulus albus, and the posterior edge of the mem- 

 brane of the aqueous humour, to which, it seems to me, to at- 

 tach itself. Whether this inner membrane of the sclerotic, and 

 that of the aqueous humour, be really the one a continuation of 

 the other or not, I have, as yet, been unable satisfactorily to 

 make out. I am inclined to think that they are attached and 

 not continuous membranes, though an appearance supporting the 



