DR KNOX on the Comparative Anatomy of the Eye. 59 



immediately beneath them, to form a portion of the parietes of 

 this canal. If we now examine the eye in the opposite direc- 

 tion, f. e. by dissecting off the sclerotic and choroid tunics, and 

 the retina, we may perceive a similar range of fibres proceeding 

 from the anterior edge of the vitreous humour, and from the 

 point where the retina terminates, forwards and upwards, to be 

 in like manner connected with the ciliary processes, by passing 

 in between each. If a lateral view be taken *, by making a very 

 delicate section of the eye, and gently raising the cut edges of 

 the ciliary body, there is still the same appearance, viz. of ante- 

 rior and posterior fibres, which have a common insertion be- 

 tween the ciliary processes near their base, and which form, in 

 conjunction with a transparent membrane, the external paries of 

 the canal of Petit. When we attempt to remove the ciliary pro- 

 cesses, and to detach them from the lens, some force is required, 

 and it not unfrequently happens, that the processes themselves 

 are torn, and a vast quantity of the pigmentum nigrum effused ; 

 at other times they may be detached, leaving the whole of the 

 semitransparent fibres lying over the outer paries of the canal of 

 Petit, and the canal itself perfectly untouched. The same re- 

 sult may be obtained by maceration ; but as it was evident that, 

 in separating the transparent humours from the dark tunic of 

 the choroid, a connection had been destroyed essential towards 

 understanding the anatomy of one of the most important parts 

 of the eye-ball, I re-examined the whole with great attention, in 

 numerous specimens, macerated for a long tune in spirits. 



In order fully to describe the anatomy of these parts, we 

 ought to commence with the retina, or, perhaps, with the mem- 

 brane sometimes found between the retina and choroid coat. 

 This membrane appears to have been known, and partly de- 

 scribed, by the late Dr MONRO ; its existence has been often de- 

 nied and re-asserted ; on referring to my Notes, taken during 



* See Plate III. Fig. 2. 



H 2 



