SECT. III. 3. MOTIONS OF THE RETINA. 19 



body in its ftruclure, we may conclude, that it muft 

 refemble them in poflefling a power of being excit- 

 ed into animal motion. -The fubfequt-nt experi- 

 ments on the optic nerve, and on the colours re- 

 maining in the eye, are copied from a paper on ocu- 

 lar fpecira publifhed in the feventy-fixth volume of 

 the Philof. Tranf. by Dr. R. Darwin of Shrewf- 

 bury ; which, as I mail have frequent occafion to 

 refer to, is reprinted in this work, Seel:. XL. The 

 retina of an ox's eye was fufpended in a glafs of 

 warm water, and forcibly torn in a few places ; the 

 edges of thefe parts appeared jagged and hairy, and 

 did not contract and become fmooth like (imple 

 mucus, when it is diftended till it breaks ; which 

 evinced that it confifled of fibres. This fibrous 

 conftruction became ftill more diftincl to the fight 

 by adding fome cauftic alcali to the water ; as the 

 adhering mucus was firft eroded, and the hair-like 

 fibres remained floating in the veffeL Nor does the 

 degree of tranfparency of the retina invalidate this 

 evidence of its fibrous ftrudure, fince Leeuwenhoek 

 has fliewn, that the cryftaJline humour itfelf confilb 

 of fibres. Arc. Nat. V. I. 70. 



Hence it appears, that as the mufcles confift of 

 larger fibres intermixed with a fmaller quantity of 

 nervous medulla, the organ, of vifion confift-s of a 

 greater quantity of nervous medulla intermixed 

 with fmaller fibres. It is probable that the locomo- 

 tive mufcles c/f microfcopic animals may have 

 greater tenuity than thofe of the retina ; and there 

 is reafon to conclude from analogy, that the other 

 immediate organs of fenfe, as the portio molHs of 

 the-auditory nerve, and. the rete mucofum of the 

 ikin, poffefs a fimilarity of ftruclure with the reti- 

 na, and a fimilar power of being excited into anU 

 mal motion. 



III. The fubfequent articles {hew, that neither 

 mechanical impreffions, nor chemical combinations. 



of 



