268 



THE ANATOMY OF THE HORSE. 



the scissors. Another incision may then be made parallel to the first, 

 and about half an inch from it. The piece of sclerotic between the 

 incisions may then be raised and turned backwards by destroying the 

 slender processes, nerves, and vessels that connect it to the choroid. At 

 the anterior edge of the piece of choroid thus exposed, and immediately 

 behind the rim of the iris, there will be seen a whitish zone — the ciliary 

 body, or annulus albidus. 



The Aqueous Humour occupies a chamber which is bounded in front 

 by the posterior surface of the cornea ; and behind by the capsule and 

 suspensory ligament of the lens, and by the ends of the ciliary processes. 

 It is across this chamber that the iris extends, and the chamber is some- 

 times described as being divided by the iris into two compartments, viz., 

 an anterior, in front of the iris ; and a posterior, behind it. In the 

 living eye, however, the posterior surface of the iris contacts with the 

 lens-capsule, so as to leave only a narrow chink behind the attachment 

 of the curtain to which the term posterior chamber may be applied. 

 The aqueous humour is composed of water with a small proportion of 

 common salt in solution. 



The Iris is a muscular pigmented curtain extended across the interior 

 of the eye, and having about its centre an aperture termed the pupil. 



By variations in the 

 size of this aperture 

 the amount of light 

 transmitted to the re- 

 tina is regulated. It 

 varies somewhat in 

 colour, but is most fre- 

 quently of a yellowish- 

 brown tint. Its an- 

 terior surface, which 

 shows some lines con- 

 verging to the pupil, is 

 bathed by the aqueous 

 humour, as is also its 

 posterior surface imme- 

 diately internal to its 

 attachment. The 

 greater part of the 

 posterior surface, how- 

 ever, is in contact with 

 the capsule of the lens, 

 and glides on it during the movements of the curtain. The circum- 

 ferential border is attached within the junction of the sclerotic and 

 cornea. The inner border circumscribes the pupil, which varies in 



Fig. 37. 



Choroid Mkmbrane and Iris exposed by the removal 

 of the Sclerotic and Cornea (Quain after Zinn). 



a. One of the segments of the sclerotic thrown back ; b. 

 Ciliary muscle ; c. Iris ; e. One of the ciliary nerves ; ./. One 

 of the vasa vorticosa or choroidal veins. 



