52 THE ELEMENTARY NERVOUS SYSTEM 



produce contraction by the strong illumination of small 

 groups of muscle fibers isolated from the sphincter. The 

 application of Steinach's conclusions to the iris of the 

 cephalopod eye, to that of birds, in which the sphincter is 

 composed of cross-striped muscle fibers, and even to mam- 

 mals was made by Nepveu (1907). 



Steinach's investigations, however, received their full- 

 est confirmation from the work of Hertel (1906), who 

 studied not only the eyes of the lower vertebrates but 

 those of the higher forms including even man. As a 

 means of stimulation Hertel used light from an electric 

 arc, from a gas flame, and diffuse daylight. After the 

 optic nerve had been cut, the iris in the eye of the eel and 

 of the frog contracted to all three lights, whereas that in 

 the eye of the cat and the rabbit contracted only to the 

 arc light, a condition also observed in human beings who 

 had suffered a degeneration of the optic nerve. In the 

 three mammals thus tested a pencil of strong light could 

 be thrown through the pupil into the fundus of the eye 

 without producing any contraction, though as soon as 

 this pencil was brought to bear on the edge of the iris a 

 local contraction was observed. Hertel, therefore, con- 

 cluded that the contraction of the pupil in blinded eyes 

 was not due to an intraocular reflex but to direct stimu- 

 lation and that this occurred in the eyes of mammals as 

 well as in those of the lower vertebrates. The mammal 

 iris, however, appeared to be less easily stimulated in this 

 way than that of the fish or the amphibian, for it did not 

 react to the weaker lights, gas light and daylight, to which 

 the others were freely responsive. Thus Hertel 's ob- 

 servations completely confirmed as well as extended those 

 of Steinach. 



The sphincter pupilltf of the vertebrate eye, and prob- 



