682 THE SENSES 



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 accomplished by pairs of muscles. 



Direction of Movement. By what muscles accomplished. 



Inward Internal rectus. 



Outward External rectus. 



Upward { Superior rectus. 



L Inferior oblique. 



Downward.. ( Inf ^or rectus. 



1 Superior oblique. 



Inward and upward . . { J n r ternal a * d superior rectus. 



Inferior oblique. 



Internal and inferior rectus. 



Inward and downward < 



[ bupenor oblique. 



Outward and upward . . { External and superior rectus. 



( Inferior oblique. 



( External and inferior rectus. 



Outward and downward 



[ Superior oblique. 



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

 viz., of the ciliary muscles, 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. 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 focused 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 quadril 

 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 radia- 

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

 contraction and dilatation may be produced by a local action of certain drugs 

 which is independent of, and probably often antagonistic to, the action of the 

 central apparatus of the third and sympathetic nerves. 



The close co-ordination between the two eyes is nowhere better shown 

 than by the condition of the pupil. If one eye be shaded by the hand its 



