ORGAN OF VISION. 225 



stances. In man, the pupil is circular; but it differs greatly in its 

 dimensions and shape in different animals. On the posterior surface 

 of the iris, the uvea, pigmentum nigrum exists, as on the choroid. 

 This layer has likewise some effect in giving colour to the eye; in blue 

 eyes, for instance, the tissue of the iris is nearly white, the pigmentum 

 which appears through it, being the chief cause of the coloration. 



At the point of junction between the iris and choroid coat, they are 

 united to the sclerotica by a band of cellular substance, called ciliary 

 ligament; and, from the anterior margin of the choroid, where it unites 

 with the base of the iris, numerous vasculo-membranous appendages 

 arise, which appear to be prolongations of the anterior margin of the 

 choroid, turning inwards towards the margin of the crystalline lens, and 

 terminating abruptly, without being attached to that body. They are 

 the ciliary processes. These beautiful appendages are from sixty to 

 eighty in number ; and resemble the disk of a radiated flower corpus 

 ciliare. On their posterior surface, they are covered by the same kind 

 of pigment as that on the choroid and uvea; and they impart a stain 

 to the membranes of the crystalline and vitreous humours. The great- 

 est diversity of opinion, here again, exists regarding both structure and 

 function. By some, the processes have been esteemed nervous; by 

 others, muscular; 1 glandular ; and vascular. Sir Everard Home asserts, 

 on the authority of microscopic observations by Mr. Bauer, 2 that be- 

 tween the processes are bundles of muscular fibres of considerable 



Fig. 105. Fig. 106. 





Anterior Segment of a Transverse Choroid and Iris, exposed by turning aside ihe 



Section of the Globe of the Eye seen Sclerotica. 



from within. 



c, c. Ciliary nerves branching in the iris. d. Smaller 



1. Divided edge of the three tunics; ciliary nerve, e, e. Vasa vorticosa. h. Ciliary ligament 



sclerotic, choroid (the dark layer), and and muscle, k. Converging fibres of the greater circle of 



retina. 2. Pupil. 3. Iris, the surface the iris. I. Looped and knotted form of these near the 



presented to view in this section being the pupil , with the converging fibres of the lesser circle of the 



uvea. 4. Ciliary processes. 5. Scalloped iris within them. o. The optic nerve. (From Zinn.) 

 anterior border of the retina. 



1 Hyrtl, Lehrbuch der Anatomic des Menschen, &<-., s. 408, Prag., 1846. 

 a Op. citat, and Philosoph. Transact, for Ib32, p. 78. 

 VOL. I. 15 



