102 SPECIAL SENSES. 



over these reflex phenomena are situated in the tubercula 

 quadrigemina. In the remarkable experiments of Flourens 

 upon the encephalic centres, it was shown that the iris loses 

 its mobility after destruction of : the tubercula. 1 This fact 

 has been repeatedly confirmed by later experimenters. In 

 birds, in which the decussation of the optic nerves is com- 

 plete, this action is crossed, destruction of the tubercle upon 

 one side producing immobility of the iris upon the opposite 

 side ; but in man, where the anatomical relations of the optic 

 nerves upon the two sides are more complex, the crossed ac- 

 tion is probably not so complete. In man, the axes of both 

 eyes are habitually brought to bear upon objects, and it is 

 well known that there is a physiological unity in the action 

 of the two eyes in ordinary vision. We also observe that, 

 when one eye only is exposed to light, the pupil becoming 

 contracted under this stimulus, the pupil of the other eye 

 also contracts. There is, indeed, a direct contraction and di- 

 latation of the pupil of the eye which is exposed to the light, 

 and an indirect, or "consensual" movement of the iris upon 

 the opposite side. The consensual contraction occurs about 

 f- of a second later than the direct action, and the consensual 

 dilatation, about -J of a second later. 2 



Budge and "Waller have shown that the filaments of the 

 sympathetic which produce dilatation of the pupil take their 

 origin from the spinal cord. In the spinal cord, between the 

 sixth cervical and the second thoracic nerves, is situated the 

 inferior cilio-spinal centre. When the spinal cord is stimu- 

 lated in this situation, both pupils become dilated. If the 

 cord be divided longitudinally and the two halves be sepa- 

 rated from each other by a glass plate, stimulation of the right 

 half produces dilatation of the right pupil, and vice versa. 9 



1 FLOURENS, Recherches experimentales sur les proprietes et les fonctions du 

 systeme nerveux^ Paris, 1842, p. 144, et seq. 



2 BONDERS, Anomalies of Accommodation and Refraction of tJie Eye, The 

 New Sydenham Society, London, 1864, p. 573. 



3 Brown-Sequard assigns wider limits to the cilio-spinal centre. He states 

 that " section of a lateral half of the spinal cord at the level of the fifth, the 



