576 MANUAL OF PHYSIOLOGY. 



relax the suspensory ligament, makes the sphincter of the iris 

 contract, and also stimulates the internal rectus to move the eye 

 inwards. The voluntary nerve centre must be in intimate rela- 

 tion to the reflex centre, which keeps up the tonic action of the 

 sphincter iridis. 



We have then central nerve governors for the ciliary and iris 

 movements. The ciliary muscle and sphincter of the pupil are 

 together stimulated by the will, and the sphincter alone is excited 

 by means of a centre which reflects the stimulus arriving from 

 the retina by the optic nerve along the branches of the third 

 nerve. The dilator of the pupil, if a muscle, is also kept in gentle 

 tonic action by the impulses sent from the spinal marrow, via 

 the sympathetic, with the vaso-motor impulses ; but some think 

 that the blood supply and tissue elasticity explain the dilatation. 



From the facts (1) that reflex contraction of the pupil may be 

 produced by stimulating the retina, even when the eye is cut off 

 from the brain centres, and (2) that the local effect of atropia in 

 dilating, and calabar bean in narrowing the pupil, seem in a 

 measure independent of the central nerve organs, it has been 

 concluded that there must be some local nerve mechanism in the 

 eye itself capable of reflecting nerve stimuli and being affected 

 by these poisons. 



The student must never lose sight of the circumstances under 

 which the pupils contract, namely : 



1. The application of strong light even to one retina, causes 

 reflex stimulation of the ciliary nerves. 



2. Stimulation of the nasal and ophthalmic branches of the 

 fifth afferent nerve excites the sphincter. 



3. Contraction of the pupil is "associated" with accommoda- 

 tion for near objects. 



4. Similar " associated " contraction, accompanies inward move- 

 ment of the eyeball. 



5. During sleep, or as the result of vaso-motor disturbances in 

 the brain (auiemia), the pupil contracts. 



6. Under the influence of physostigmatin, nicotin, and morphia. 



7. From any stimulation of the optic or third nerves or the 

 corpora quadrigemina. 



