324 THE SPECIAL SENSES. 



sensations. Under normal conditions the light reflex is bi- 

 lateral,— that is, light thrown upon one retina only will cause 

 constriction of the pupil in both eyes. In the lower animals, 

 whose optic nerves cross completely in the chisrna, the light 

 reflex, on the contrary, is unilateral, affecting only the eye that 

 is stimulated.* We may conclude, therefore, that the bilaterality 

 of the reflex in the higher animals is dependent upon the partial 

 decussation of the optic fibers in the chiasma, a sensory stimulus 

 upon one retina giving rise to impulses which are conveyed to the 

 two sides of the brain. It is possible, however, that in addition 

 commissural connections may exist between the central connections, 

 — the motor centers in the midbrain. It is usually stated that 

 the effect of the light upon the sphincter muscle is greatest when 

 the retina is stimulated at or near the fovea and that it varies 

 directly with the intensity of the light and the area illuminated.f 

 The Action of Drugs upon the Iris. — The condition of con- 

 striction of the pupil is frequently designated as miosis (mi-o'-sis) 

 and the condition of dilatation as mydriasis (myd-ri'-as-is). Many 

 drugs are known which, when appHed directly to the absorptive 

 surfaces of the eye or when injected into the circulation, affect 

 the muscles of the iris and therefore vary the size of the pupil. 

 Those drugs that cause miosis are spoken of as miotics, and those 

 that produce m3Tlriasis as mydriatics. Atropin, the active prin- 

 ciple of belladonna, homatropin, and cocain are well-known myd- 

 riatics, while physostigmm (eserin) and muscarin or pilocarpin are 

 examples of the miotics. There has been much question as to the 

 precise action of these drugs. For an adequate discussion of this 

 question the student is referred to works on pharmacology; but 

 it may be said that the evidence from the physiological sidej indi- 

 cates that atropin causes mydriasis by paralyzing the endings of 

 the constrictor nerve fibers in the sphincter muscle, while phy- 

 sostigmin and muscarin cause miosis by stimulation of the endings 

 of these same fibers. § In the case of cocain it is probable that the 

 drug first stimulates mainly the endings of the dilator fibers in the 

 dilator muscles, and in stronger doses causes additional mydriasis 

 by paralyzing the constrictor fibers. The stronger mydriatics 

 paralyze not only the sphincter pupillse, but also the similarly 

 innervated ciliary muscle, thus destroying the power of accom- 

 modation. When atropin is applied to the eye the individual is 



* Steinach, "Archiv f. d. gesammte Physiologie," 47, 313, 1890. 



fSee Abelsdorff and Feilchenfell, "Zeitsohrift f. Psychologie und Phys- 

 iologie des Sinnesorgane," 34, 111, 1904. 



JSchultz, "Archiv f. Physiologie," 1898, 47. 



I According to Langley, "Journal of Physiology," 39, 235, 1909, the stim- 

 ulating or paralyzing effect of such drugs is due to an action not on the nerve 

 terminals, but on a special receptive substance in the muscle-fibers. 



