186 PHILOSOPHY OF ZOOLOGY. 
both, and thus alter the form of the humours, or change 
their relative position with regard to the retina. 
The crystalline lens has also been regarded as possessing 
the power of changing its form, and varying the focal dis- 
tance of the eye, either in consequence of the action of the 
ciliary processes, or of a change in the internal arrangement 
of its parts. Its structure, as displayed by LzuwennoExk *, 
Youne +, and others, after coagulation by heat or alcohol, 
is considered as muscular. But as it contains no fibrin, 
and is even soluble in water, with the exception of a small 
portion of extremely pellucid membrane, its muscular power 
is denied by some. Its increasing density towards its cen- 
tre, rather indicates a cellular structure, the cells being fil- 
led with pellucid matter of different degrees of concentra- 
tion. It may be added, as a still more decisive proof that 
this power of varying the focal distance is not seated in the 
crystalline, that when the lens is extracted in the disease 
termed the Crystalline Cataract, the limits of distinct vision 
suffer no diminution. 
When we look at objects within the limits of distinct vi- 
sion, the iris expands, so that the aperture of the pupil be- 
comes contracted. In this manner the least diverging rays 
only are admitted, and distinct vision obtained. 
By this arrangement, it is probable that the eye accom- 
modates itself’ to objects at different distances, within the 
limits of distinct vision, as it is known to do by the same 
means in reference to the quantity of light. 
The evidence in favour of this function of the iris ap- 
pears to me to be conclusive. The enlargement and con- 
traction of the pupil, are confined, indeed, within narrow 
limits, but so is the extent of distinct vision within the or- 
dinary limits. There is a particular range of minute in- 

* Oper. Om. p. 73. + Phil. Trans. 1793, p. 169. 
