576 THE NERVOUS SYSTEM. 



its section paralyzes these fibers, causing contraction of the pupil, 

 while its stimulation produces dilatation. 



Mydriatics are drugs which cause the pupil to dilate ; atropin 

 is a well-known mydriatic. Cocain, daturin, and hyoscyamin 

 likewise produce dilatation of the pupil, and are therefore mydri- 

 atics. 



Myotics are drugs which produce contraction of the pupil. 

 Prominent in the list of myotics are eserin, pilocarpin, and 

 morphin. The mydriatics probably act by paralyzing the oculo- 

 motorius and stimulating the sympathetic. When large doses are 

 used, the circular fibers may be paralyzed directly. Myotics 

 paralyze the sympathetic and stimulate the oculomotorius. 



In addition to producing dilatation of the pupil, the mydriatics 

 paralyze the accommodation, so that as long as their effect lasts it 

 is impossible to focus the eye for near objects. Myotics, besides 

 contracting the pupil, cause a contraction of the ciliary muscle, 

 and thus the lens is adjusted for near objects. 



When light falls upon the retina this portion of the eye is 

 stimulated, and the impression is carried by the optic nerve to the 

 brain, and- there motor impulses are generated which are trans- 

 mitted through the third nerve to the sphincter of the iris, causing 

 it to contract ; this is, therefore, a reflex act. When one goes 

 from the dark into the light, the pupil contracts, but this contrac- 

 tion lasts only a short time, and is followed by dilatation, and, in 

 a few minutes the size of the pupil is about the same as at first. 

 If, on the other hand, one goes from the light into the dark, there 

 is at first a dilatation of the pupil, then a contraction, and in about 

 twenty minutes the pupil is as it was before the dilatation. These 

 observations demonstrate that there are other influences than the 

 incidence of light upon the retina to produce the changes in the 

 pupil. 



The Retina. As odors excite the olfactory apparatus and savors 

 excite the gustatory, so does light excite the retina. As neither 

 odors nor savors reach the brain, where smell and taste are pro- 

 duced, but only the nerve-impulses which they excite and which 

 the olfactory and gustatory nerves transmit, so when the light- 

 waves fall upon the retina they go no farther ; but the nerve-im- 

 pulses which they there excite are carried to the brain by the optic 

 nerve and produce the sensation called " light." Thus it is that a 

 blow upon the eye or an injury to the optic nerve produces in the 

 brain the impression of a flash of light, although the room in 

 which the blow or injury was received may be absolutely 

 dark. 



That the optic nerve is itself insensitive to light is shown by 

 the fact that at the point where it enters the eye, forming the optic 

 disk, is the " blind spot," at which there is an entire absence of 

 sight. This fact may be demonstrated in the following simple 



