THE STRUCTURE OF THE EYEBALL 



507 



THE FUNCTIONS OF THE IRIS. The iris contains two layers of 

 unstriated muscle fibres, the anterior which is circularly arranged so that 

 by its contraction it acts as a sphincter, while the posterior is arranged 

 radially, stretching from the attachment of the iris to the rim of the pupil 

 so that by its contraction it causes the pupil to open. Because of the 

 numerous pigment cells which it contains the. iris is opaque to 

 light. Contraction of the pupil thus causes the following 

 effects : (1) reduction in the amount of light entering the eye, 

 so that an image of less intensity is formed on the 



retina ; (2) 



Cornea 



Sinus venosus 



Conj 



iuncfiva 



I 

 I 



Retina 



FIG. 254. Section through anterior part of eyeball to show relations of iris and 

 ciliary bodies to corneo-scleral junction and lens. 



the use of the more central zones of the lens system only. The advantage 

 of this lies in the fact that, as will be described later, the more peripheral 

 zones suffer from errors of refraction to a much greater degree than do the 

 central ones : the contraction of the pupil therefore improves the definition 

 of the image ; (3) an increase in the depth of focus of the eye, which is 

 of great value for near vision. The way that depth of focus is obtained will 

 be described later (see page 530). 



CONTRACTION OF THE PUPIL occurs under the following cir- 

 cumstances : 



(1) When light falls on the retina. This movement, which is known 

 as * the light reflex,' is determined by a contraction of the sphincter pupillse, 

 together with a relaxation of the dilatator muscle. The contraction ensues 

 within a period of 0-04 to 0-05 sec. after the moment at which the light has 



