DIOPTRIC MECHANISMS. 753 



bloodvessels begin some time before the changes in the calibre of the blood- 

 vessels, and indeed may be over before these have arrived at their maximum. 

 Moreover, the fibres of the sympathetic, which, as we shall see, are con- 

 cerned in causing dilatation of the pupil, run a somewhat different course 

 from those which govern the bloodvessels 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. 



630. 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 temperature. 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 excep- 

 tional 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 quadri- 

 gemina 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 con- 

 traction 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 thor- 

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

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



631. 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 stimu- 

 lation 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 dilatation. 



In considering the movements of the pupil, however, 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 dilatation 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. 



632. The iris is supplied, in common with the ciliary muscle and 

 choroid, by the short ciliary nerves (Fig. 172, s. c.) coming from the oph- 

 thalmic or lenticular (ciliary) ganglion (/. c.) which is connected by its roots 

 with the third nerve (r. &.), the cervical sympathetic nerve (sym.), and 

 with the nasal branch of the ophthalmic division of the fifth nerve (r. I.). 

 The short ciliary nerves are, moreover, accompanied by the long ciliary 

 nerves (I. c.) coming from the same nasal branch of the ophthalmic division 

 of the fifth nerve. What are the uses of these several nerves in relation to 

 the pupil? 



633. If the cervical sympathetic in the neck be divided, all other por- 



48 



