540 BULBUS OCULI 



corona ciliaris, where they end in bulbous extremities. The 

 bulbous ends occupy the space between the peripheral margin 

 of the iris and the margin of the anterior surface of the 

 crystalline lens, and they form the peripheral boundary of the 

 posterior chamber of the eyeball. The //foe ciliares are much 

 less prominent than the ciliary processes. Both the processes 

 and the folds are in relation, posteriorly, with the hyaloid 

 membrane, which separates them from the vitreous body, 

 and with the peripheral part of the zonula ciliaris (see p. 

 544) to which they are attached. 



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, it is coloured differently in 

 different subjects, and it has a central perforation termed the 

 pupil. Its anterior surface is faintly striated in a radial 

 direction. 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. The 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. The circular fibres, which act as a 

 sphincter, are supplied by the oculo-motor nerve. The 

 dilator fibres are supplied by sympathetic nerve fibres. 



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 forwards, between 

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

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



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. 



