316 Dr. Darwin’s Experiments on the 
either remain uniformly, or gradually diminifh ; and neither 
their alternate prefence and evanefcence as in the fi.rft experi- 
ment, nor the perpetual changes' of their colours as in the 
fecond, nor the tlafh of light or colour^ in the p re (Ted eye as 
in the third, nor the rotation or libration of the fpeftra as in 
the fourth, could exift. 
It is not abfur'd to conceive, that ; the retina may be ftlmu- 
lated into motion, as well as the red and white mufcles which 
form our limbs and veflels; fince it confifts of fibres, like 
thofe, intermixed with its medullary fubftance. To evince 
this ftrudture, the retina . of Jin ox’s eye was fufpended in a 
giafs 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 fimple mucus, when it is 
diftended till it breaks; which (hews that it confifts of fibres ; * 
and this its fibrous conftruclion became ftill more diftiiidt to the 
fight, by adding fame cafiftic alkali to the water, as the ad- 
hering raucusi was firft eroded, and the hair-like fibres remained 
floating in the vefiel. Nor does the degree of tranfparency of 
the retina invalidate the evidence, of its fibrous ftrufture,. fince 
Leeuwenhoek has fhewn that the cry ft al line humour itfelf 
confifts of. fibres.^ (Ardajia Naturae, V. : ?t. p. 70.) 
Hence it appears, that as the mufcles have larger fibres. 
intermixed with ia fmaller quantity of nervous medulla, 
the organ of viftoh has a .greater quantity of nervous me- 
dulla intermixed with fmaller fibres; and it is probable, that 
the locomotive mufcles, as well, as the vafcular ones, of mi- 
crofcopic animals have much greater tenuity than thefe of the 
retina. 
And befides the fimilar laws, which will be fhewn in this 
Paper to govern alike the; aclions of the retina and of the. 
mufcles^ 
