REFLEXES OF THE PUPIL 



647 



two images are produced, the lower one g really appears in the position Q, 

 while the upper one appears in the position P. This may be readily verified 

 by covering the holes in succession. 



During accommodation two other changes take place in the eyes. The 

 eyes converge by the action of the extra-ocular muscles, chiefly by the internal 

 and inferior recti or internal and superior recti. The pupils contract. 



Movements of the Eye. The eyeball possesses movement around three axes indicated 

 in figure 462, viz., an antero-posterior, a vertical, and a transverse, passing through a 

 center of rotation a little behind the centre of the optic axis. The movements are ac- 

 complished by pairs of muscles. 



Direction of Movement. 

 Inward, .... 



Outward, .... 



Upward, 



Downward, .... 

 Inward and upward, 

 Inward and downward, . 

 Outward and upward, 

 Outward and downward, 



By what muscles accomplished. 



Internal rectus. 



External rectus. 

 ( Superior rectus. 

 | Inferior oblique. 

 j Inferior rectus. 

 / Superior oblique. 



Internal and superior rectus. 



Inferior oblique. 

 j Internal and inferior rectus. 

 ( Superior oblique, 

 j External and superior rectus. 

 \ Inferior oblique, 

 j External and inferior rectus. 

 ( Superior oblique. 



The contraction of all of the muscles during the act of accommodation, viz., 

 of the ciliary muscle, of the recti muscles, and of the sphincter pupillae, is 

 under the control of the fibers of the third nerve. But the superior oblique 

 may also be employed, in which case the fourth nerve is concerned. 



Reflexes of the Pupil. Contraction of the iris may occur under the 

 following circumstances: i, On exposure of the eye to a bright light; 2, On 

 the local application of eserine (active principle of Calabar bean); 3, On the 

 administration internally of opium, aconite, and in the early stages of chloro- 

 form and alcohol poisoning; 4, On division of the cervical sympathetic or of 

 stimulation of the third nerve. Dilatation of the pupil occurs, i, in a dim 

 light; 2, when the eye is focussed for distant objects; 3, on the local applica- 

 tion of atropine and its allied alkaloids; 4, on the internal administration of 

 atropine and its allies; 5, in the later stages of poisoning by chloroform, 

 opium, and other drugs; 6, on paralysis of the third nerve; 7, on stimulation 

 of the cervical sympathetic, or of its center in the floor of the front of the 

 aqueduct of Sylvius. The contraction of the pupil is under the control of 

 a center in the floor of the aqueduct beneath the anterior corpora quadri- 

 gemina. This center is reflexly stimulated by a bright light, and the dilata- 

 tion when the center is not in action is due to the stimulation of the radial 

 fibers of the iris by sympathet* nerves. In addition, it appears that both 



