Mr. Young's Observations on Vision. 171 
Jurin’s calculations, there is not room for a sufficient motion 
of this kind, without a very visible increase in the length of 
the eye’s axis : such an increase we cannot observe. 
Dr. Jurin’s hypothesis is, that the uvea, at its attachment 
to the cornea, is muscular, and that the contraction of this 
ring makes the cornea more convex. He says, that the fibres 
of this muscle may as well escape our observation, as those of 
the muscle of the interior ring. But if such a muscle existed, 
it must, to overcome the resistance of the coats, be far stronger 
than that which is only destined to the uvea itself ; and the 
uvea, at this part, exhibits nothing but radiated fibres, losing 
themselves, before the circle of adherence to the sclerotica, in 
a brownish granulated substance, not unlike in appearance to 
capsular ligament, common to the uvea and ciliary processes, 
but which may be traced separately from them both. Now at 
the inferior ring of the uvea, the appearance is not absolutely 
inconsistent with an annular muscle. His theory of accommo- 
dation to distant objects is ingenious, but no such accommo- 
dation takes place. 
Musschenbroek conjectures that the relaxation of his ci- 
liary zone, which appears to be nothing but the capsule of the 
vitreous humour where it receives the impression of the ciliary 
processes, permits the coats of the eye to push forwards the 
crystalline and cornea. Such a voluntary relaxation is wholly 
without example in the animal economy, and were it to take 
place, the coats of the eye would not act as he imagines, nor 
could they so act unobserved. The contraction of the ciliary 
zone is equally inadequate and unnecessary. 
Some have supposed the pressure of the external muscles, 
especially the two oblique muscles, to elongate the axis of the 
