734 THE SENSES 



the local application of eserine (active principle of Calabar bean). 3. On the 

 administration internally of opium, aconite, and in the early stages of chloro- 

 form and alcohol poisoning. 4. On division of the cervical sympathetic or of 

 stimulation of the third nerve. Dilatation of the pupil occurs, i, in a dim 

 light; 2, when the eye is focused for distant objects; 3, on the local applica- 

 tion of atropine and its allied alkaloids; 4, on the internal administration of 

 atropine and its allies; 5, in the later stages of poisoning by chloroform, 

 ether, and other drugs; 6, on paralysis of the third nerve; 7, on stimulation 

 of the cervical sympathetic, or of its center in the floor of the front of the 

 aqueduct of Sylvius. The contraction of the pupil is under the control of 

 a center in the floor of the aqueduct beneath the anterior corpora quadri- 

 gemina. This center is reflexly stimulated by a bright light, and the dilata- 

 tion when the center is not in action is due to the stimulation of the radial 

 fibers of the iris by sympathetic nerves. In addition, it appears that both 

 contraction and dilatation may be produced by a local action of certain drugs 

 which is independent of, and probably often antagonistic to, the action of the 

 central apparatus of the third and sympathetic nerves. 



The close co-ordination between the two eyes is nowhere better shown 

 than by the condition of the pupil. If one eye be shaded by the hand its 

 pupil will of course dilate; the pupil of the other eye will also dilate, though 

 unshaded, due to crossed reflex action. 



Defects in the Optical Apparatus. Under this head we may con- 

 sider the defects known as: i, Spherical Aberration; 2, Chromatic Aberra- 

 tion; 3, Astigmatism; 4, Myopia; 5, Hypermetropia. 



The normal or emmetropic eye is so perfect that parallel rays are brought 

 exactly to a focus on the retina without any effort of accommodation, figure 

 466. Hence all objects except near ones (in practice all objects at a distance 

 of twenty feet or more) are seen without any effort of accommodation; in 

 other words, the far-point of the normal eye at rest is at an infinite distance. 

 In viewing near objects we are conscious of the effort (the contraction of 

 the ciliary muscle) by which the anterior surface of the lens is rendered 

 more convex, and rays which would otherwise be focused behind the retina 

 are converged upon the retina. 



Spherical Aberration. The rays in a cone of light from a point on an 

 object situated in the field of vision do not all meet in the same point in 

 the retina, owing to the greater refraction of the rays which pass through 

 the margin of a lens than those traversing its central portion. This defect 

 is spherical aberration. In the camera, telescope, microscope, and other 

 optical instruments it is remedied by the interposition of a screen with a 

 circular aperture in the path of the rays of light, cutting off all the marginal 

 rays and allowing the passage only of those near the center. Such cor- 

 rection is effected in the eye by the iris, which forms a diaphragm to cover the 

 circumference of the lens, and prevents the rays from passing through any 



