THE SENSE OF SIGHT 699 



adjusted that the formation and subsequent perception of the image shall be 

 sharp and distinct. This is accomplished by the iris, the circular fibers 



of which respectively contract and relax with increasing and decreasing in- 

 tensities of the light. The size of the pupil, therefore, through which the 



L light passes, will vary from moment to moment and in accordance with 



! variation in the light intensity. The quantity of light necessary to distinct 



' vision is thus regulated. 



In the total absence of light the sphincter pupillae muscle is relaxed and 

 the pupil widely dilated. With the appearance of light and an increase 

 in its intensity the muscle again contracts and the pupil progressively narrows. 

 With a given intensity in the light, the sphincter contraction is greater when 

 the light falls directly upon the fovea. Contraction of this muscle is an 

 associated movement in the convergence of the eyes during accommodation 

 and in consensus with the other eye. 



In addition to this function of the iris, it constitutes, by virtue of the 

 sphincter muscle contraction, an. important corrective apparatus. Being 

 non-transparent, it serves as a diaphragm intercepting those rays which 

 would otherwise pass through the peripheral portions of the lens and by 

 spheric aberration give rise to indistinctness of the image. The movements 

 of the iris by which the size of the pupil is determined are caused by the con- 

 tractions and relaxations of. the sphincter pupilla and dilatator ^puptila 

 muscles. The contraction of the sphincter is entirely reflex and involves 

 those structures necessary to the performance of any reflex act, viz.: a recep- 

 tive surface, the retina; afferent nerves, the pupillary fibers of the optic nerve; 

 a central emissive center situated in the gray matter beneath the aqueduct 

 of Sylvius; and efferent nerves, the motor oculi and the ciliary nerves. The 

 stimulus requisite to the excitation of this mechanism is the impact of light 

 waves or ether vibrations on the rods and cones. According to the intensity 

 of these vibrations will be the resulting contraction of the muscle. I he 

 contraction of the dilatator pupilte muscle is determined by the activity 

 of a continuously active nerve-center in the medulla oblongata which trans- 

 mits its nerve impulses through the spinal cord, along the first and second 

 dorsal nerves to the superior cervical ganglion, and thence to the ms by 

 way of the fifth nerve. (See Fig. 268, page 620.) These two muse 

 appear to bear an antagonistic relation to each other, or section of the motor 

 oculi is followed by relaxation of the sphincter muscle and dilatation of the 

 Jupil. Stimulation of the sympathetic is followed by a more >'? 

 dilatation. The size of the pupil is the resultant of a balancing o 

 two forces. 



OPTIC DEFECTS 



yopia Presbyopia may be defined as a condition of the normal 



ic^ 



impossible at ordinary distances. As age advances the lens , gradu- 

 atoSd* and hence its power to increase in convexity and thick- 

 ness to the s^e extent as in earlier life, in response to efforts of accommo- 

 da on The^efractive power is, thereby, lessened and the eye is no longer 

 ab e to see distinctly at the normal reading distances, viz. : 22 to 28 cm. 



rom a luminous point at the normal reading distances are 



and hence the diffusion circles mcrease 



