602 



THE EYE. 



the pupil, near wliich they join in a series of irregular elevations ; and, 

 internal to these, other finer lines pass to the pupil. The posterior 

 surface is covered with dark pigment, prolonged from the pigmentary 

 layer of the retina ; and, this being removed, there is seen at the 

 margin of the pupil a narrow circular baud of fibres (sphincter muscle 

 of the pu})il), with which the converging lines are blended. 



The pupil is nearly circular in form, and is placed a little to the 

 inner side of the centre of the iris. It varies in size according to the 

 contraction or relaxation of the muscular fibres, this variation ranging 

 from ^lyth to 4rd of an inch, and regulating the quantity of light 

 admitted to the eyeball. 



V 



STRUCTURE OF THE IRIS. 



Fibrous and muscular tissues form the framework of the iris, and 

 pigment cells ai'e scattered through the texture. It contains also 

 numerous vessels and nerves. In iVont and behind is placed a distinct 

 layer of pigmented cells. The delicate epithelioid layer of the membrane 

 of Descemet (fig. 403,2) is continued from the margin of the cornea over 

 the front of the iris ; its cells are smaller and more granular than those 

 which cover the membrane of Descemet, but are otherwise similar. 



The stroma consists of cells and fibres of connective tissue, the latter 

 directed radially towards the pupil, and circularly at the circumfer- 

 ence ; these, interweaving 

 Fig. 411. with one another, form a 



web which is more open in 

 the substance of the iris 

 than near the surfaces. 



The musctilar tissue is 

 of the non-striated kind, 

 and is disposed as a ring 

 {sphincter) around the pupil, 

 and as rays {dilatator) from 

 the centre to the circum- 

 ference. 



The sphncter (fig. 411, a) 

 is the flat narrow band on 

 the posterior surface of the 

 iris, close to the pupil, and 

 is about ^^yth of an inch 

 wide. At the edge of the 

 pupil the fibres are close 

 together, but at the periphe- 

 ral border they are separated, 

 and form less complete rings. 

 The dilatator {!>, h), less 

 apparent than the sphincter, 

 begins at the ciliary or outer 

 margin of the iris, and its 

 fibres, collected into bundles, 

 or forming a continuous membrane, situated near the posterior surface, 

 converge towards the pupil, and form a network by their intercom- 

 munications. At the pupil they blend with the sphincter, some reach- 



Ll 



.M 



Fig. 411. A SMALL PART OF THE IrIS, SHOWraG 



THE Muscular Structure (from Kolliker). 

 350 Diameters. 



The siiecimen is from an all:>ino-rabljit, and has 

 been treated ^vith acetic acid : a, the sjiliincter 

 muscle at the margin of the pupil ; 6, fasciculi of 

 the dilatator muscle ; c, connective tissue with 

 nuclei of cells rendered evident bv the acid. 



