600 



ON THE MECHANISM OF THE EYE. 



capsule of the lens; and if by the ciliary 

 processes we understand those fihmients which 

 are seen detached after tearing away tlie 

 capsule, and consist of ramifying vessels, the 

 observation is equally true of the common qua- 

 drupeds, and even of the human eye*. This 

 remark has indeed been made by Leroi, Aibi- 

 nus, and others, but the circumstance is not 

 generally understood. It is so difficult to ob- 

 tain a distinct view of these bodies, undis- 

 turbed, that I am partly indebted to acci- 

 dent, for having been undeceived respecting 

 them : but, having once made the observa- 

 tion, I have learnt to show it in an un- 

 questionable manner. I remove the posterior 

 hemisphere of the sclerotica, or somewhat 

 more, and also as much as possible of the 

 vitreous humour, introduce the point of a 

 pair of scissors into the capsule, turn out the 

 lens, and cut off the greater part of the pos- 

 terior portion of the capsule, and of the rest 

 of the vitreous humour. I next dissect the 

 choroid- and uvea from the sclerotica; and, 

 dividing the anterior part of the capsule into 

 segments from its centre, I turn them back 

 upon the ciliary zone. The ciliary processes 

 then appear, covered with their pigment, and 

 perfectly distinct both from the capsule and 

 from the uvea; (Plate 13. Fig. 101.) and 

 the surface of the capsule is seeij. shining, 

 "ind evidently natural, close to the base of 

 these substances. I do not deny that the 

 separation between the uvea and the pro- 

 cesses, extends somewhat further back than 

 the separation between the processes and the 

 capsule ; but the difference is inconsiderable, 

 and, in the calf, does not amount to above 

 half the length of the detached part. The 

 appearance of the processes is wholly irre- 



• Vid. Hall, Phyfiol. V. 433. It Duvemey, ibi citat 



concileable w ith muscularity ; and their be- 

 ing considered as muscles attached to the 

 capsule, is therefore doubly inadmissible. 

 Their lateral union with the capsule com- 

 mences at the base of their posterior smooth 

 surface, and is continued ueariy to the point 

 where they are more intimately united with the 

 termination of the uvea; so that, however this 

 portion of the base of the processes were flis- 

 posed to contract, it would be much loo short 

 to produce any sensii>le effect. What tiuir use 

 maj' be, cannot easily be determined : if it 

 were necessary to have any peculiar organs 

 for secretion, we might call them glands, 

 for the percolation of the aqueous humour; 

 but there is no reason to think them re- 

 quisite for this purpose. 



The marsupium nigrum of birds, and the 

 horseshoe like appearance of the choroid of 

 fishes, are two substances which have some- 

 limes, with equal injustice, been termed mus- 

 cular. All the apparent fibres of the marsu- 

 pium nigrum are, as Haller had very truly as- 

 serted, merely duplicatures of a membrane, 

 which,when its ends are cut off, may easily be 

 unfolded under the microscope, with the as- 

 sistance of a fine hair pencil, so as to leave 

 no longer any suspicion of a muscular texture. 

 The experiment related by Mr. Home*, can 

 scarcely be deemed a very strong argument 

 for attribuiing to this substance a faculty 

 which its appearance so little authorises us to 

 expect in it. The red substance, in the cho- 

 roid of fishes, (Plate 13. Fig. 102.) is more 

 capable of deceiving the observer ; its colour 

 gives it some little pretension, and I began 

 to examine it with a prepossession in favour 

 of its muscular nature. But, when we recol- 

 lect the general colour of the muscles of 



• Phil.TraiM. ireS. is. 



