304 THE SPECIAL SENSES. 



In the above description of the innervation of the iris and the causes of 

 mydriasis and miosis the simplest explanations offered have been adopted. 

 It should be added, however, that some facts are known which indicate that 

 the conditions are more complex. Thus, Meltzer and Auer* have shown 

 that in mammals the application of solutions of adrenalin to the eye has no 

 effect on the iris and the same is true after section of the cervical sympathetic. 

 But if the superior cervical ganglion is removed the adrenalin causes a maxi- 

 mal mydriasis. This paradoxical dilatation indicates that the ganglion 

 has some specific influence upon the iris in addition to serving as part of the 

 pathway for the pupillodilator fibers, since as long as it is present it prevents 

 the adrenalin from acting upon the musculature of the iris. 



The Balanced Action of the Sphincter and Dilator Muscles 

 of the Iris. It would seem that under normal conditions both the 

 sphincter and the dilator muscle are kept more or less in tonic 

 activity by impulses received through their respective motor fibers. 

 They thus balance each other, to speak figuratively, and a mechan- 

 ism of this kind in which two opposing actions are in play is in a 

 condition to respond promptly and smoothly to an excess of stimu- 

 lation from either side. The two muscles, in fact, act as antago- 

 nists in the same manner as the flexor and extensor muscles around 

 a joint. At the same time this relation adds some difficulties to 

 the explanation of specific reactions, since it is evident that a dila- 

 tation of the pupil may be caused either by a contraction of the 

 dilator muscle or a loss of tone (inhibition) in the sphincter, while 

 in constriction of the pupil the effect may resuit either from a con- 

 traction of the sphincter or an inhibition of the dilator; or, last, 

 the contraction of one muscle may always be accompanied by an 

 inhibition of its antagonist, as is supposed to be the case with the 

 flexor and extensor muscles of the limbs. Andersonf has given 

 some evidence to show that the dilatation of the pupil in cats is 

 due to a double action of this sort, the pupillodilator muscle con- 

 tracting first and subsequently the tone of the constrictors suf- 

 fering an inhibition. Alterations in the size of the pupil take place 

 not only under the conditions described above namely, the light 

 and the accommodation reflex and the action of drugs, but also 

 under many other circumstances, normal and pathological. In 

 sleep, for instance, the eyes roll upward and inward and the pupils 

 are constricted. It would seem probable that the miosis in this case 

 is due to a cessation in tonic activity on the part of the dilator 

 muscle rather than to an active contraction of the sphincter muscle, 

 the state of sleep being characterized by a diminution in activity 

 in the central nervous system. Emotional states also affect the 

 size of the pupil and thus aid in giving the facial expressions char- 

 acteristic of these conditions. Writers speak of the eyes dilating 

 with terror or darkening with emotions of deep pleasure. This pupil- 



* Meltzer and Auer, "American Journal of Physiology," 11, 28, and 40, 1904. 

 t " Journal of Physiology," 30, 15, 1903. 



