ON THE MECHANISM OF THE EYE. 



601 



fishesjthc consideration of its redness will no 

 longer have any weight. Stripped of the 

 membrane which loosely covers its internal 

 surface, (Fig- 103.) it seems to have trans- 

 verse divisions, somewhat resembling those of 

 muscles, and to terminate in a manner some- 

 what simihir; (Fig. 104.)but, when viewed in 

 a microscope, tlie transverse divisions ap- 

 pear to be craciis, and the whole mass is evi- 

 dently of a uniform texture, without the 

 least fibrous appearance : and; if a particle 

 of any kind of muscle is compared with it, 

 the contrast becomes very striking. Besides 

 it is fixed down, throughout its extent, to 

 the posterior lamina of the choroid, and has 

 no attachment capable of directing its cflcct; 

 to say nothing of the diflSculty of conceiving 

 what that effect would be. Its use must 

 remain, in common with that of many other 

 parts of the animal frame, entirely' concealed 

 from our curiosity. 



The bony scales of the eyes of birds, which 

 were long ago described in the Memoirs of 

 the Academy, by Mery *, in the Philosophi- 

 cal Transactions, by Mr. Ranby fj -Tid by 

 Mr. Warren ;|:, afterwards in two excellent 

 Memoirs of M. Petit on the eye of the tm- 

 key and of the owl ^, and lately by Profes- 

 sor Blumenbach ||, Mr. Pierce Smith ^, and 

 Mr. Home **, can, on any supfiosition, have 

 butlittle concern in the accommodation of the 

 eye to different distances : they rather seem 

 to be necessary for the protection of that 

 organ, large and prominent as it is, and un- 



«n. 15. 



t Phil. Trans. XXXIII. 223. Abr. VII. 435. 



: Ptiil. Trans. XXXIV. 113. Abr. VII. 43?. 



§ Mem. de 1' Acad. 1735. 163. 1730. 166. Ed. Amst. 



II Comm. Gott. VII. 62. 



^ Phil. Trans. 1795. 263. 



•• Phil. Trans. 1786. 14. 



VOL. II. 



supported by any strength in the orbit, 

 against the various accidents to wliich the 

 mode of life and rapid motion of those ani- 

 mals must expose it ; and they are much less 

 liable to fracture than an entire bony ring 

 of the same thickness would have been. The 

 marsupium nigrum appears to be intended to 

 assist in giving strength to the eye, to prevent 

 an J' change in the place of the lens, by exter- 

 nal force : it is so situated as to intercept but 

 little light, and that little is principally what 

 would have fallen on the insertion of the 

 optic nerve: and it seems to be too firmly 

 tied to the lens, even to admit any consider- 

 able elongation of the axis of the. eye, al- 

 though it certainly would not impede a pro- 

 trusion of the cornea. There is a singular ob- 

 servation of Poupart, respecting the eyes of 

 insects, which requires to be mentioned here. 

 He remarks, that the eye of the libellula is 

 hollow; that it communicates with an air 

 vessel placed longitudinally in the trunk of 

 the body; and that it is capable of being in- 

 flated from this cavity : he supposes that 

 the insect is provided with this apparatus, 

 in order i'or the accommodation of its eye 

 to the ])erception of objects at different dis- 

 tances*. There is no difficulty in supposing 

 that the means of producing the change of 

 the refractive powers of the eye, may be, in 

 different classes of animals, as diversified as 

 their habits, and the general conformation 

 of their organs. But an examination of the ~ 

 eyes of libellulae, wasps, and lobsters, in- 

 duces me not only to reject the suggestion of 

 Poupart, but to agree with those naturalists, 

 who have called in question the. pretensions 

 of these organs to the name usually a[)plied 

 to them. Cuvicr has given a very fair state- 



• rhil. Trans. XXII. 673. .\br. II. 762. 

 4H 



