MOVEMENTS OF THE IRIS. TOT 



the pupil. That these fibres are subjected to nervous influence, is rendered 

 certain by experiments upon the sympathetic system. There can be no doubt 

 that the action of the sympathetic upon the pupil is directly antagonistic to 

 that of the third pair, the former presiding over the radiating muscular fibres ; 

 and the only question to determine is the course taken by the sympathetic 

 filaments to the iris. Experiments on the influence of the fifth pair upon the 

 pupil have been somewhat contradictory in different animals. In rabbits sec- 

 tion of this nerve in the cranial cavity produces contraction of the pupil ; 

 but in dogs and cats the same operation produces dilatation. In the human 

 subject, of course, it is impossible to determine this point by direct experi- 

 ment ; and the varying results obtained in observations upon different ani- 

 mals probably depend upon differences in the anatomical relations of the 

 nerves. It is probable, however, that the filaments of the sympathetic which 

 animate the radiating fibres join the fifth nerve near the ganglion of Gasser, 

 and from this nerve pass to the iris. 



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

 of nerves which 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 upon the retina, which is 

 conveyed by the optic nerves to the centre, and in obedience to this impres- 

 sion, the sphincter of the iris contracts. If the optic nerves be divided, so 

 that the impression can not be conveyed to the centre, or if the third nerve 

 be 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 medulla oblongata. 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 " consen- 

 sual " movement of the iris upon the opposite side. The consensual con- 

 traction occurs about f of a second later than the direct action, and the con- 

 sensual dilatation, about ^ of a second later (Bonders). 



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 the inferior cilio-spinal centre. When the spinal cord is stimulated in this 

 situation, both pupils become dilated. If the cord be divided longitudinally 

 and the two halves be 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 ad- 

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



