120 TOPOGRAPHICAL ANATOMY OF THE 



eyeball than it is elsewhere. A portion of the hyaloid membrane 

 that surrounds the vitreous body is applied to the ciliary processes, 

 and is radially thickened and folded so as to present grooves and 

 elevations that are adherent to the processes and the depressions 

 between them. 



Immediately behind the ciliary processes there is a circular area, 

 almost entirely smooth, known as the orbiculus ciliariK This is 

 directly continuous with the choroid, of which it might be considered 

 a part, and from which it is distinguishable only by its greater thick- 

 ness and the absence of a lamina choriocapillaris. 



The ciliary muscle (m. ciliaris) consists of involuntary fibres 

 arranged, for the most part, in a meridional or radiating manner. 

 There is, however, some admixture of oblique and circular fibres. 



Sclera, s 



Choroid. 



. Ciliary processes. 



Crystalline lens. 



Retina.' 

 Fig. 50.— The ciliary body and crystalline lens viewed trom behind. 



The radiating fibres arise from the corneo-scleral junction and the 

 ligamentum pectinatum iridis, and are attached to the ciliary processes 

 and orbiculus ciliaris. 



Dissection. — The posterior surface of the iris can be studied in the 

 preparation naade to show the ciliary body ; but to reveal the anterior 

 surface it is necessary to remove the cornea from another eyeball. 



The iris ^ is a muscular diaphragm placed immediately in front 

 of the crystalline lens. About its centre is an elliptical opening, the 

 pupil (pupilla), which during life is capable of variation in size to 

 regulate the amount of light entering the eye. When the pupil is 

 fully dilated it approaches the circular in outline ; but when strongly 

 contracted it is little more than a narrow linear opening. These 

 variations in size are governed by muscular fibres in the substance 

 of the iris. Some of them are arranged in a circular manner around 

 the pupil (m. sphincter pupillce), and to them motor impulses are 



1 Ipis (iris) [Gr.], a rainbow, a bright-coloured circle surrounding another body, 

 Galen was apparently the first to apply the name to the iris of the eye. 



