284 PHYSIOLOGY FOR DENTAL STUDENTS. 



The Function of the Pupil. Every optical instrument con- 

 tains a so-called diaphragm, which is a black curtain having a 

 central aperture, whose diameter can be altered to any required 

 size. The object of this is to prevent all unnecessary rays of 

 light from entering the optical instrument, thus materially in- 

 creasing the distinctness of the image. In the eye, this function 

 is performed by the iris with the pupil in its center. The size 

 of the pupil is altered by the action of two sets of muscle fibers 

 in the iris. One of these runs in a circular manner around the 

 inner edge of the iris; by contracting it causes constriction of 

 the pupil, an event which occurs, along with the bulging of the 

 lens, during accommodation for near vision. The other layer of 

 fibers runs in a radial manner, and by contracting causes dila- 

 tation of the pupil. This occurs in partial darkness, or when the 

 eye is at rest (although not during sleep). The circular fibers 

 are supplied by the third nerve, and the radial fibers by the 

 sympathetic. Under ordinary conditions both muscles are in a 

 state of tonic contraction (see p. 253), so that the actual si/.e 

 of the pupil at any moment is the balance between two opposing 

 muscular forces. This renders its adjustment in size very sensi- 

 tive. For example, it can become dilated either by stimulation 

 of the sympathetic (which occurs when any irritative tumor 

 effects the cervical sympathetic nerve), or by paralysis of the 

 third nerve (as by giving atropin). Conversely, constriction of 

 the pupil may be the result of stimulation of the third nerve (as 

 by a tumor at the base of the brain) or paralysis of the sympa- 

 thetic. 



These local conditions acting on the afferent nerves to cither 

 pupil are not nearly so often called into play as conditions acting 

 reflexly on both eyes at the same time. 



Certain of the afferent impulses which call these reflexes into 

 play travel by the optic nerve to the nerve centers for the pupil, 

 such for example as the stimulus set up by light falling on the 

 retina. The afferent pathway concerned in the contraction of 

 the pupil, which occurs in accommodation, must, OH the other 

 hand, be a different one because in the disease locomotor ataxia 

 (see p. 254), the pupil contracts on accommodation, but does not 



