MOVEMENTS OF THE IRIS 685 



a woman eighteen minutes after decapitation, produced dilatation of the 

 pupil (Wagner). 



There seem to be two distinct nerve-centres corresponding to the two 

 sets of nerves that regulate the movements of the iris. One of these 

 centres presides over the reflex contractions of the iris, and the other is 

 the centre of origin of the nervous influence through which the pupil 

 is dilated. 



The mechanism of reflex contraction of the iris under the stimulus 

 of light is sufficiently simple. An impression is made on the retina, 

 which is conveyed by the optic nerves to the centre, and in obedience 

 to this impression, the sphincter of the iris contracts. If the optic nerves 

 are divided, so that the impression can not be conveyed to the centre, 

 or if the third nerve is divided, no movements of the iris can- take place. 

 The centres which preside over the reflex phenomena of contraction of 

 the pupil are situated in the bulb. The action of these centres is crossed 

 in animals in which the decussation of the optic nerves is 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. It has been observed 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 dilatation of the pupil of the eye which is exposed 

 to the light, and an indirect, or "consensual" movement of the iris on 

 the opposite side. The consensual contraction occurs about f- of a sec- 

 ond later than the direct action, and the consensual dilatation, about J- of 

 a second later. The condition of the pupillary reflexes often gives im- 

 portant information in cases of cerebral disease. They are particularly 

 important in the early stages of paresis. 



The filaments of the sympathetic that produce dilatation of the 

 pupil take their origin from the spinal cord. In the spinal cord, be- 

 tween the sixth cervical and the second thoracic nerves, is the inferior 

 cilio-spinal centre. When the cord is stimulated in this situation, both 

 pupils are dilated. If the cord is divided longitudinally and the two 

 halves are separated from each other by a glass plate, stimulation of the 

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

 does not occur when the sympathetic in the neck has been divided. In 

 addition to the inferior cilio-spinal centre, there is a superior centre, 

 which is in communication with the superior cervical ganglion and is 

 situated near the sublingual nerve. The influence of this centre over 

 the pupil can not be demonstrated by direct stimulation, because it is 

 too near the origin of the fifth, irritation of which affects the iris ; but 

 it is shown by division of its filaments of communication with the iris. 



