On the Mechanlfm of tht Eye. 33* 



but tlie circumftance is not generally underftood. It is (o difficult to obtain a diftima 

 view of thefe bodies, undifturbed, that lam partly indebted to accident, for having beett 

 ■undeceived refjiefting them : but, having once made the obfervation, I have learnt to (hew 

 it in an unqueftionable manner. I -remove the pofterior hemifphere of the fclerotica, or 

 ibmewhat more, and alfo as much as poffible of the vitreous humour, introduce the point 

 of a pair of fciflbrs into the capfule, turn out the lens, and cut off the greater part of the 

 pofterior portion of the capfule, and of the reft of the vitreous humour. I next difleft the 

 choroid and uvea from the fclerotica ;. and, dividing the anterior part of the capfule into 

 fegments from its centre, I turn them back upon the ciliary zone. The ciliary procefles 

 then appeaf, covered with their pigment, and perfedlly diftinft both from the capfule and 

 from the uvea; (Plate XV. Fig. 54.) and the furface of the capfule is feen fhining, and 

 evidently natural, clofe to the bafe of thefe fubftances. I do not deny that the feparation 

 between the uvea and the procefles, extends fomewhat further back than the feparation 

 between the procefles and the capfule ; but the difference is inconfiderable, and, in the 

 calf, does not amount to above half the length of the detached part. The appearance of 

 the procefles is wholly irreconcileable with mufcularity; and their being confidered as 

 mufcles attached to '*\e capfule, is therefore doubly inadmiflible. Their lateral union 

 with the capfule, commences at the bafe of their pofterior fmooth furfaccj and is conti- 

 nued nearly to the point where they are more intimately united with the termination of the 

 uvea ; fo that, however this portion of the bafe of the procefles were difpofed to contraft, 

 it would be much too (hort to produce any fenfible efi'efl. What their ufe may be, cannot 

 eafily be determined : if it were neceflary to have any peculiar organs for fecretion, we 

 .might call them glands, for the percolation of the aqueous humour; but there is no reafon 

 to think them requifite for this purpofe. 



The marfupium nigrum of birds, and the horfe-flioe-like appearance of the choroid of 

 fifties, are two fubftances which have fometimes, with equal injuftice, been termed muf- 

 cular. All the apparent fibres of the marfupium nigrum are, as Hallcr had very truly 

 aflerted, merely duplicatures of a membrane, which, when its ends are cut ofi^, may eafily 

 be unfolded under the microfcope, with the aflTiftance of a fine hair pencil, fo as to leave 

 no longer any fufpicion of a mufcular texture. The experiment related by Mr. Home *, 

 can fcarcely be deemed a very ftrong argument for attributing to this fubftance a faculty 

 which its appearance fo little authorifes us to expe£t in it. The red fubftance in the cho- 

 roid of fifties, (Plate XV. Fig. 55.) is more capable of deceiving the obferver ; its colour 

 gives it fome little pretenfion, and I began to examine it with a prepoflefllon in favour of 

 Its mufcular nature. But, when we recolletl the general colour of the mufcles of fifties, 

 the confideration of its rednefs will no longer have any weight. Stripped of the membrane 

 which loofely covers its internal furface, (Fig. 56.) it feems to have tranfverfi? divifions, 

 fomewhat refenibling thofe of mufcles, and to terminate in a manner fomewhat fimjlar; 



Phil. Tijnf, for ITSC* p. h8. 



(Fig. 



