578 BULBUS OCULI 



Iris. The iris lies anterior to the crystalline lens, and it 

 is separated from the cornea by the anterior chamber filled 

 with aqueous humour. By its circumference it is continuous 

 with the ciliary body, and it is connected by the ligamentum 

 pectinatum iridis with the margin of the cornea. 



The iris is circular in form, and has a central perforation 

 termed the pupil. Its anterior surface is faintly striated in a 

 radial direction. It is coloured differently in different 

 individuals. Its posterior surface is deeply pigmented. 

 The pupil presents a very nearly circular outline, 1 and 

 during life it constantly varies in its dimensions so as to 

 control the amount of light which is admitted into the 

 interior of the eyeball. These changes in the size of the 

 pupil are produced by the two groups of involuntary muscular 

 fibres which are present in the substance of the iris. One 

 group is composed of muscular fibres arranged circularly 

 around the pupil in the form of a sphincter; the second 

 group consists of fibres which have a radial direction, and 

 pass from the sphincter towards the circumference of the 

 iris, so as to constitute a dilatator muscle. By some 

 anatomists these radial fibres are considered to be elastic 

 and not muscular. 



Ciliary Nerves. The ciliary nerves arise from the ciliary 

 ganglion and the naso-ciliary nerve. They pierce the sclera 

 around the optic entrance, and extend anteriorly, between 

 the sclera and the chorioid, in the perichorioidal lymph space. 

 They will be seen, in the specimen in which the sclera has 

 been turned aside in separate flaps, in the form of delicate 

 white filaments (Fig. 253). In the posterior part of the 

 eyeball they occupy grooves on the deep surface of the sclera, 

 and can be separated from it only with difficulty. Reaching 

 the ciliary zone the ciliary nerves break into branches, which 

 join in a plexiform manner and send twigs to the ciliary 

 muscle, the iris, and the cornea. 



Ciliary Arteries. There are three groups of ciliary 

 arteries: (i) the short posterior ciliary arteries; (2) the long 

 posterior ciliary arteries ; and (3) the anterior ciliary arteries. 



The short posterior ciliary arteries, branches of the 

 ophthalmic, pierce the sclera around the optic entrance, 



1 It may be as well to mention here that the pupil in the ox and the 

 sheep is greatly elongated in the transverse direction. In the pig, how- 

 ever, it is approximately circular. 



