662 TEXT-BOOK OF PHYSIOLOGY. 



From youth to old age, the elasticity of the lens steadily declines, and the 

 range of accommodation diminishes from the recession of the near point. 



Convergence of the Eyes during Accommodation. In binocular 

 vision of near objects the eyes are turned inward and the optic axis of each 

 a line passing through the center of the cornea and the center of the eye 

 turned toward the median line during accommodation. So long as the 

 eyes are directed toward the far point, 65 meters or beyond, the optic axes 

 are parallel. When the eyes are directed to any point within 65 meters the 

 optic axes are converged, the convergence increasing steadily as the near 

 point is approached. In this way the fovea of each eye is directed to the 

 same point and single vision made possible. Were this not the case, double 

 vision would result. 



Functions of the Iris. For purposes of distinct vision it is essen- 

 tial that the quantity of light entering the interior of the eye shall be so 

 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 

 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 pupilla 

 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. The 

 contraction of the dilatator pupillae 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 iris by 

 way of the fifth nerve. (See Fig. 270, page 582.) These two muscles 

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



