620 TEXT-BOOK OF PHYSIOLOGY. 



the circular fibers of which alternately contract and relax with in- 

 creasing and decreasing intensities 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 enlarged. 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 into the 

 fovea. Contraction of this muscle also occurs as an associated move- 

 ment 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 contractions 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 sentient surface, the 

 retina ; an afferent nerve, the optic ; a central emissive center situated 

 in the gray matter beneath the aqueduct of Sylvius; and an efferent 

 nerve, the motor oculi. 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 con- 

 tinuously 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. 245, page 534.) These 

 two muscles appear to bear an antagonistic relation to each other, for 

 section of the motor oculi is followed by relaxation of the sphincter 

 muscle and dilatation of the pupil. Stimulation of the sympathetic 

 is followed by a more pronounced dilatation. The size of the pupil 

 is the outcome of a balancing of these two forces. 



OPTIC DEFECTS. 



Presbyopia. This is a condition of the eye characterized by a 

 defective or diminished accommodative power. As age advances the 

 lens loses its elasticity and the. power to increase its refraction, and 



