DISSECTION OF THE EYEBALL. 



261 



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 anmdus alhidus. 



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 j and a posterior, behind it. In the 

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

 lens-capsvile, 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 cui*tain extended across the interior 

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

 By variations in the size 

 of this aperture, the 

 amount of light trans- 

 mitted to the retina is 

 regulated. It varies 

 somewhat in colour, but 

 is most frequently of a 

 yellowish-brown tint. Its 

 anterior surface, which 

 shows some lines con- 

 verging to the pupil, in 

 bathed by the aqueous 

 humour, as is also its 

 posterior surface immedi- 

 ately internal to its 

 attachment. The gi'eater 

 part of the posterior sur- 

 face, however, is in con- 

 tact with the capsule of 

 the lens, und glides on it 

 during the movements of the curtain. The circumferential border is 

 attached within the junction of the sclerotic and cornea. The inner 

 border circumscribes the pupil, which varies in outline according to its 

 size. When much contracted, the pupil is a very elongated ellipse, the 

 long axis of which is in the line joining the nasal and temporal canthi of 



Choroid membrane and Iris exposed by the removal 

 OF THE Sclerotic and Cornea (Quain after Zinn). 



R. One of the segments of the sclerotic thrown back ; h. Ciliary 

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

 vorticosa or choroidal veins. 



