CHAP, ii.] SIGHT. 501 



different course from those which govern the blood-vessels of the eye. 

 We may therefore adhere to the view that the main changes of the 

 pupil in the direction of narrowing and widening are brought about 

 by contractions of the plain muscular fibres in the iris. 



Muscular contractions leading to changes of the pupil may be 

 observed in the eye removed from the body, and indeed in the 

 extirpated iris. The plain muscular fibres of the iris like other 

 plain muscular fibres are remarkably sensitive to variations in tem- 

 perature. Besides this there seems to be in certain animals at 

 least a connection within the eye between the iris and retina of 

 such a kind, that light falling into an extirpated eye will lead to a 

 narrowing of the pupil. Putting aside however such exceptional 

 events we may lay down the broad principle that contraction of the 

 pupil, brought about by light falling on the retina, is a reflex act, 

 of which the optic is the afferent nerve, the third or oculo-motor 

 the efferent nerve, and the centre some portion of the brain lying 

 below the corpora quadrigemina in the front part of the floor of the 

 aqueduct of Sylvius. This is proved by the following facts. When 

 the optic nerve is divided, the falling of light on the retina no 

 longer causes a contraction of the pupil. When the third nerve is 

 divided, stimulation of the retina or of the optic nerve no longer 

 causes contraction ; but direct stimulation of the peripheral portion 

 of the divided third nerve causes extreme contraction of the pupil. 

 If the region of the brain spoken of above as a centre be carefully 

 stimulated contraction of the pupil will take place even in the 

 absence of light and after division of the optic nerve. After 

 removal of the same centre stimulation of the retina is ineffectual 

 in narrowing the pupil. But if the centre and its connections with 

 the optic nerve and third nerve be left intact and in thoroughly 

 sound condition, contraction of the pupil will occur as a result of 

 light falling on the retina, though all other nervous parts be 

 removed. 



The nervous centre is not a double centre with two completely 

 independent halves, one for each eye ; there is a certain amount 

 of functional communion between the two sides, so that when one 

 retina is stimulated both pupils contract. It might be imagined 

 that this cerebral centre acted as a tonic centre, whose action was 

 simply increased not originated by the stimulation of the retina ; 

 but this is disproved by the fact that, if the optic nerve be 

 divided, subsequent section of the third nerve produces no further 

 dilation. 



In considering the movements of the pupil, how r ever, we have 

 to deal not only with a narrowing of the pupil thus brought about, 

 in a reflex way by contraction of the circular sphincter fibres, and 

 with the absence of such a narrowing, but also with active dilation 

 due to a contraction of the radial dilator fibres, and this renders the 

 whole matter much more complex than might be supposed to be the 

 case from the simple statement just made. 



