796 



DISSECTION OF THE EYE. 



Iris Is vas- 

 cular and 

 muscular ; 



situation ; 

 form; 



attachment 

 anterior 

 surface ; 



posterior 

 surface. 



The pupil. 



convex, as required for vision at near distances. The ciliary muscle* 

 is therefore the muscle of accommodation. 



The IRIS (fig. 289, e) is a vascular and muscular structure, about 

 half an inch in diameter, the vessels of which are continuous withi 

 those of the choroid coat. Its position and relations may bo 

 observed in the different dissections that have been prepared. 



Placed within the ring of the ciliary muscle, it is suspended in> 

 front of the lens (fig. 288, /), and is pierced by an aperture for the 

 transmission of the light. It is circular in form, is variously 

 coloured in different persons, and is immersed in the aqueous humour. 

 ; By its 'circumference, it is connected with the choroid coat, and by 

 the ligamentum pectin atum with the cornea. The anterior surface 



is free in the aqueous 

 humour, and is marked 

 by lines converging to- 

 wards the pupil. The 

 posterior surface is 

 covered with a thick 

 layer of pigment (fig. 

 291), to which the term 

 t't-ea has been applied. 



The aperture in it 

 (fig. 289) is the pupil- 

 of the eye ; this is 

 slightly internal to the 

 centre, and is nearly 

 circular in form ; but 

 its size is constantly 

 varying (from ^th to 

 ^rd of an inch) by the 

 contraction of the mus- 

 cular fibres, according 

 to the degree of light 

 acting on the retina. 



Membrane 

 of the pupil 

 in the fo?tus: 



situation ; 



time of dis- 

 appearance. 



Component 

 structures. 



Sphincter 



FIG. 291. ENLARGED REPRESENTATION OF THE 

 PARTS OF THE EYEBALL ON ONE SIDE 

 OPPOSITE THE LENS : THE LETTERS REFER 

 TO THE SAME PARTS AS IN FIG. 288. 



d. Cornea. 



e. Ciliary muscle, radiating fibres. 

 /. Iris. 



g. Lens. 



i. Posterior chamber. 



j. Ciliary part of the retina. 



m. Circular bundle of the ciliary muscle. 



n. Front of vitreous body. 



o. Canal of Petit. 



>. Canal of Schlemm. 



s. Inner end of ciliary process. 



t. Suspensory ligauient of the lens. 



Membrane of the pupil. 

 In the foetus the aper- 

 ture of the pupil is closed 



by a vascular transparent membrane, which is attached to the edge of the 

 iris, and divides into two distinct chambers the space in which the iris is 

 suspended. The vessels in it are continuous behind with those of the iris 

 and the case of the lens. About the eighth month the vessels become 

 impervious, and at the time of birth only fragments of the structure remain. 



Structure. The stroma of the iris is composed of connective tissue, 

 the fibres of which are directed for the most part radially towards 

 the pupil. In it are involuntary muscular fibres, both circular and 

 radiating, together with pigment-cells ; and vessels and nerves ramify 

 through the tissue. 



Muscular fibres. The sphincter of the pupil (fig. 290, d) is "a 

 narrow band about -g^th of an inch wide, which is close to the 



The dilator of the pupil (e) 



and dilator pupil, on the posterior aspect of the iris, 

 of pupil. 



