VISION. 535 



tion and stimulation of its piaripheral end by dilatation of the 

 pupil. 



Trom such experiments it has been concluded that — 1. The 

 optic is the afferent nerve and the third nerve the efferent nerve 

 concerned in the contraction of the pupil ; and that the center 

 in the brain is situated as indicated above, so that the act is or- 

 dinarily a reflex. 3. That the cervical sympathetic is the path 

 of the efferent impulses regulating the action of the radiating 

 fibers of the iris. 



Its center has been located near that for the contraction of 

 the pupilrand it may be assumed to exert a tonic action over 

 the iris comparable to that of the vaso-motor center over the 

 blood-vessels. 



The impulses may be traced through the cervical sympa- 

 thetic and its ganglia back to the first thoracic ganglion, and 

 thence to the spinal cord and brain. There may be subsidiary 

 centers in the cervical spinal cord. 



It is to be remembered that, although the dilating center is 

 automatic in action, it may also act reflexly, or be modified by 

 unusual afferent impulses — as, e. g., the strong stimulation of 

 any sensory nerve which causes enlargement of the pupil 

 through inhibition of the center. To render the paths of 

 impulses affecting the iris somewhat clearer, it is well to bear 

 in mind the nervous supply of the part : 1. The third nerve, 

 through the ciliary (ophthalmic, lenticular) ganglion, supplies 

 short ciliary nerves to the iris, ciliary muscle, and choroid. 3. 

 The cervical sympathetic reaches the iris chiefly through the 

 long ciliary nerves and the ophthalmic division of the Efth. 

 3. There are sensory libers from the fifth nerve ; and, according 

 to some observers, also dilating fibers from this nerve inde- 

 pendent of the sympathetic, as well as those that may reach 

 the eye by the long ciliary nerves without entering the ciliary 

 ganghon. 4. The centers from which both the contracting and 

 dilating impulses proceed are situated near to each other in 

 the floor of the aqueduct of Sylvius. It is of practical im- 

 portance to remember the various circumstances under which 

 the pupil contracts and dilates. 



Contraction (Myosis).—!. Access of strong light to the 

 retina. 3. Associated contraction on accommodation for near 

 objects. 3. Similar associated contraction when the visual axes 

 converge, as in accommodation for near objects. 4. Reflex 

 stimulation of afferent nerves, as the nasal or ophthalmic divis- 



