VOL. LXXXIII.] PHILOSOPHICAL TRANSACTIONS. 310 



its axis, by a muscular power. But the ciliary processes neither appear to con- 

 tain any muscular fibres, nor have they any attachment by which they can be 

 capable of performing this action. Descartes imagined the same contraction and 

 elongation to be effected by a muscularity of the crystalline, of which he sup- 

 posed the ciliary processes to be the tendons. He did not attempt to demon- 

 strate this muscularity, nor did he enough consider the connection with the 

 ciliary processes. He says, that the lens in the mean time becomes more con- 

 vex, but attributes very little to this circumstance. De la Hire maintains that 

 the eye undergoes no change, except the contraction and dilatation of the pupil. 

 He does not attempt to confirm this opinion by mathematical demonstration; 

 he solely rests it on an experiment which has been shown by Dr. Smith to be 

 fallacious. Halier too has adopted this opinion, however inconsistent it seems 

 with the known principles of optics, and with the slightest regard to hourly ex- 

 perience. Dr. Pemberton supposes the crystalline to contain muscular fibres, by 

 which one of its surfaces is flattened, while the other is made more convex. 

 But, besides that he has demonstrated no such fibres. Dr. Jurin has proved that 

 a change like this is inadequate to the effect. Dr. Porterfield conceives that the 

 ciliary processes draw forward the crystalline, and make the cornea more convex. 

 The ciliary processes are, from their structure, attachment, and direction, utterly 

 incapable of this action; and, by Dr. 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 radi- 

 ated fibres, losing themselves, before the circle of adherence to the sclerotica, 

 in a brownish granulated substance, not unlike in appearance to capsular liga- 

 ment, common to the uvea and ciliary processes, but which may be traced sepa- 

 rately from them both. Now at the interior ring of the uvea, the appearance is 

 not absolutely inconsistent with an annular muscle. His theory of accommoda- 

 tion to distant objects is ingenious, but no such accommodation takes place. 



Musschenbroek conjectures that the relaxation of his ciliary zone, which ap- 

 pears 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 exam- 

 ple in the animal economy, and were it to take place, the coats of the eye 



