818 



THE SENSE OF SIGHT 



On excitation of the trunk of the third nerve, we obtain a constric- 

 tion of the pupil, while the stimulation of the sympathetic nerve in 

 its cervical portion, gives rise to pupillar dilatation. Obviously, there- 

 fore, the division of the former must evoke a dilatation of the pupil, 

 and that of the latter, pupillar constriction. Under normal conditions, 

 these two mechanisms are tonically set and oppose one another. Con- 

 sequently, the removal of the constrictor impulses must allow the 

 dilator impulses to gain the upper hand, while the division of the sym- 

 pathetic nerve must permit the constrictor influences to exert their full- 



l 



FIG. 425. DIAGRAMMATIC REPRESENTATION OP THE NERVES GOVERNING THE PUPIL. 



(After Foster.) 



II, optic nerve; eg, ciliary ganglion; rb, its short root from ///, motor oculi nerve; 

 sym., its sympathetic root; rl, its long root from V, ophthalmonasal branch of oph- 

 thalmic division of fifth nerve; sc, short ciliary nerves; Ic, long ciliary nerves. 



est power. It appears, therefore, that the constrictor and dilator 

 muscles of the iris are arranged antagonistically to one another, in 

 a manner similar to that of the flexor and extensor muscles of the ex- 

 tremities (Sherrington). Thus, inasmuch as it has been shown that 

 the contraction of one set of skeletal muscles is usually facilitated by 

 the inhibition of the other set, it may be assumed that a similar recipro- 

 cal relationship exists between the muscle fibers of the iris. Certain 



