INDEPENDENT EFFECTORS 51 



> ^ 



the hemisphere containing the iris still continued to ex- 

 hibit this contraction. He concluded, though on insuf- 

 ficient grounds, that the sphincter of the pupil was acted 

 on directly by the light. The belief that this reaction 

 was due to an intraocular reflex and not to the direct 

 action of the light on the muscle concerned was shown to 

 be extremely improbable by Steinach ( 1892 ) , who worked 

 on a number of lower vertebrates, but especially on the 

 frog and eel. Steinach found that when the half of the 

 bulb containing the iris was cut down to the extreme by 

 removing the edges of the retina and the ciliary body, the 

 reaction still took place. Moreover, if a very small in- 

 tense point of light was focussed on the iris, the portion 

 of this organ thus illuminated was the first part to begin 

 to contract and this activity spread from the region thus 

 stimulated to the adjacent regions. A histological in- 

 vestigation of the contracted iris showed that the ele- 

 ments concerned in this contraction were the pigmented 

 smooth muscle cells of the sphincter pupillae, whose pig- 

 ment absorbed precisely those rays, the shorter wave 

 lengths, that had been found to be especially stimulating. 

 Steinach, therefore, concluded that in fishes and amphibi- 

 ans the smooth muscle elements of the sphincter pupillae 

 may be directly stimulated by light. 



The objection to this conclusion raised by Magnus 

 (1899) to the effect that the reaction could be prevented 

 by atropin, though the sphincter would still respond to 

 electrical stimulation, was shown by Guth (1901) not to 

 be well founded. Guth, moreover, was able to demon- 

 strate a contraction in the sphincter nearly two weeks 

 after the eye had been cut out, a period much too long for 

 the persistence of an intraocular reflex mechanism in an 

 organ thus removed from the body. He also was able to 



