DIOPTRICS OF THE EYE. 329 



unable to use his eyes for near work reading, for example until 

 the effect of the drug has worn off. In ophthalmological literature 

 this condition of paralysis of the ciliary muscle is spoken of as 

 cycloplegia, and most of the mydriatic drugs are also cycloplegics. 

 On the contrary, the stronger miotics stimulate the ciliary muscle, 

 and therefore during their period of action throw the eye into a 

 condition of forced accommodation. 



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 result either from a con- 

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

 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 (law of reciprocal in- 

 nervation). Anderson* 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 contracting first and subsequently 

 the tone of the constrictors suffering 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 cir- 

 cumstances, normal and pathological. In sleep, for instance, 

 the eyes roll upward and outward and the pupils are constricted. 

 The miosis in this case may be due to a cessation in tonic activity 

 on the part of the dilator muscle, or to an increased tonus of the 

 sphincter muscle. The latter explanation is strengthened by the 

 fact that experiments indicate that during the waking period the 

 tonus of the cranial center controlling the sphincter is kept under 

 inhibition by the inflow of sensory impulses. During sleep this 

 central inhibition diminishes or disappears. Emotional states also 

 affect the size of the pupil and thus aid in giving the facial expres- 

 sions characteristic of these conditions. Writers speak of the 

 eyes dilating with terror or darkening with emotions of deep pleas- 

 ure. This pupillary accompaniment of the emotional states may 

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



