70 Da KNOX on the Comparative Anatomy of the Eye. 



There is still another mode of displaying the structure of the 

 eye in birds, equally advantageous with that described. Make 

 an incision through the ciliary body, immediately anterior to the 

 ciliary muscle, and a cavity will be opened into, lying imme- 

 diately over the ciliary plexus. This cavity is bounded internally 

 by the ciliary body ; externally by the ciliary muscle and nerves, 

 and their investing membrane ; posteriorly, by the connection of 

 the ciliary muscle with the choroid ; anteriorly by the membra- 

 nous expansion connecting the iris, ciliary body, cornea, and an- 

 terior termination of the ciliary muscle, to each other. This ca- 

 vity may, for short distances, be filled with air by the blowpipe. 

 It has never been described, though it exists in the eyes of the 

 mammalia. Where the ciliary nerves assume the ribbon form, 

 or become a plexus, the canal is interrupted by the passage of 

 the nerve across it, and by cellular membrane ; but it is to be 

 observed, that, in most of the mammalia, in which there is no true 

 plexus of ciliary nerves, the canal is small, and partly filled with 

 a very rare cellular tissue. The anatomy of this cavity displays 

 the whole anatomy of the anterior part of the eye-ball ; it shews 

 that the anatomist has confounded the ciliary nerves with the ci- 

 liary muscle, and it demonstrates the influence which the mo- 

 tions of the ciliary muscle and iris must have over the internal 

 configuration of the eye-ball. Numerous branches of nerves pro- 

 ceed to the ciliary muscle and iris, corresponding to the exer- 

 tions they are required to make in this class of animals. 



The anatomical distribution of these parts differs but little 

 in the mammalia from that just described, and the analogy 

 may be deemed correct. The triangular cavity immediately 

 anterior to the insertion of the ciliary muscle into the sclero- 

 tic, is not so capacious, and the muscle is but indirectly connect- 

 ed with the internal layer of the cornea. It may be readily 

 imagined, that certain physiological differences will arise out of 

 the anatomical deviations just described. It has been often as- 



