THE SENSE OF VISION. 771 



the outer border of the iris, it seems more reasonable to conclude that the 

 dilator nerves of the pupil act upon radial muscular fibres in the substance of 

 the iris, in spite of the fact that the existence of such fibres has not been uni- 

 versally admitted. 



Whatever view may be taken of the mechanism by which the sympathetic 

 nerves influence the pupil, there is no doubt that the iris is under the control 

 of two antagonistic sets of nerve-fibres, both of which are, under normal cir- 

 cumstances, in a state of tonic activity. Therefore, when the sympathetic 

 nerve is divided the pupil contracts under the influence of the motor oculi, and 

 section of the motor oculi causes dilatation through the unopposed- influence of 

 the sympathetic. 



The movements of the iris, though performed by smooth muscles, are more 

 rapid than those of smooth muscles found elsewhere e. g. in the intestines 

 and the arteries. The contraction of the pupil when the retina of the oppo- 

 site eye is illuminated occupies about 0.3" ; the dilatation when the light is cut 

 off from the eye, about 3" or 4". The latter determination is, however, diffi- 

 cult to make with precision, since dilatation of the pupil takes place at first 

 rapidly and then more slowly, so that the moment when the process is at an 

 end is not easily determined. After remaining a considerable time in absolute 

 darkness the pupils become enormously dilated, as has been shown by flash- 

 light photographs taken under these conditions. In sleep, though the eyes are 

 protected from the light, the pupils are strongly contracted, but dilate on 

 stimulation of sensory nerves, even though the stimulation may be insufficient 

 to rouse the sleeper. 



Many drugs when introduced into the system or applied locally to the con- 

 junctiva produce effects upon the pupil. Those which dilate it are known as 

 mydriatics, those which contract it as myotics. Of the former class the most 

 important is atropin, the alkaloid of the Atropa belladonna, and of the latter 

 physostigmin, the alkaloid of the Calabar bean. In addition to their action 

 upon the pupil, mydriatics paralyze the accommodation, thus focussing the eye 

 for distant objects, while myotics, by producing a cramp of the ciliary muscle, 

 adjust the eye for near vision. The effect on the accommodation usually 

 begins later and passes off sooner than the affection of the pupil. Atropin 

 seems to act by producing local paralysis of the terminations of the third pair 

 of cranial nerves in the sphincter iridis and the ciliary muscle. In large 

 doses it may also paralyze the muscle-fibres of the sphincter. With this para- 

 lyzing action there appears to be combined a stimulating effect upon the dilator 

 muscles of the iris. The myotic action of physostigmin seems to be due to a 

 local stimulation of the fibres of the sphincter of the iris. 



Although in going from a dark room to a lighter one the pupil at first con- 

 tracts, this contraction soon gives place to a dilatation, and in about three or 

 four minutes the pupil usually regains its former size. In a similar manner 

 the primary dilatation of the pupil caused by entering a dark room from a 

 lighter one is followed by a contraction which usually restores the pupil to its 

 original size within fifteen or twenty minutes. It is thus evident that the 



