98 SPECIAL SENSES. 



observations are exceedingly interesting in connection with 

 some of the contradictory experiments of physiologists upon 

 the action of the motores oculornm upon the iris. Harless 

 noted, in subjects dead of various diseases, from five to thirty 

 hours after death, that the iris contracted under the stimulus 

 of light ; and he justly remarks that this is probably due to 

 direct action upon its muscular tissue, and that it is not 

 reflex, for the reason that the irritability of the nerves in 

 warm-blooded animals disappears certainly in twenty hours 

 after death. The experiments of Harless were made upon 

 the two eyes, one being exposed to the light with the other 

 closed. The contraction, however, took place very slowly, 

 requiring an exposure of several hours. 1 This mode of con- 

 traction is very different from the action of the iris during 

 life, but is precisely like the contraction observed by Bernard 

 after division of the motor oculi communis, which is slow and 

 gradual and undoubtedly depends upon the direct action of 

 light upon the muscular fibres. 2 



Action of the Nervous System upon the Iris. This sub- 

 ject, as far as it relates to the third pair, has been pretty fully 

 considered in connection with the physiology of these nerves ; 

 and it is unnecessary to refer again in detail to the experiments 

 which have already been cited. The reflex phenomena ob- 

 served are sufficiently distinct. When light is admitted to 

 the retina, the pupil contracts, and the same result follows 

 mechanical irritation of the optic nerves. When the third 

 pair of nerves has been divided, no such reflex phenomena 

 are observed. It is well known, also, that division of the 

 third nerves in the lower animals or their paralysis in the 

 human subject produces permanent dilatation of the pupil, 

 the iris responding, only in the slow and gradual manner 

 already indicated, to the direct action of light. 



The experiments made by direct stimulation of the third 



1 HARLESS, Die MusMirritabilitat, Munchen, 1848. 



2 See vol. iv., Nervous System, p. 133. 



