THE SEKSE8. 7l7 



cils, they meet at a point behind the retina. The effect is the same 

 as if the retina were shifted forward to mm. Two images li.g. are 

 formed, one from each hole. It is interesting to note that when 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, 

 (1) The eyes converge by the action of the extra-ocular muscles chiefly 

 by the ipternal and inferior recti, or internal and superior recti. The 

 superior oblique and the inferior oblique may also be used to turn the 

 eye upward or downward. 



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

 indicated in fig. 427, viz., an an tero- posterior, a vertical, and a transverse, 

 passing through a centre 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. 



TT ( Superior rectus. 



U P ward \ Inferior oblique. 



-T. ( Inferior rectus. 



Downward ...... } Superior oblique. 



Til ( Internal and superior rectus. 



Inward and upward . . . . -j Inferior oblique' 



j Internal and inferior rectus. 

 Inward and downward ...... j Superior oblique. 



( External and superior rectus. 

 Outward and upward . . . } Inferio r oblique 



j External and inferior rectus. 

 Outward and downward . . . j Superior oblique. 



(2) The second change which takes place in the eyes is, that the 

 pupils contract. The contraction of all of the muscles which have to do 

 with accommodation, viz., of the ciliary muscle, of the recti muscles, 

 and of the sphincter pupillse is under the control of the third nerve. 

 But the superior oblique may also be employed, in which case the fourth 

 nerve is also concerned. 



Contraction of the pupil may also occur under the following circum- 

 stances: (1) 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 

 chloroform and alcohol poisoning; (4) on division of the cervical 

 sympathetic or stimulation of the third nerve, and dilatation of the pupil 

 occurs (1) in a dim light; (2) when the eye is focussed for distant ob- 

 jects; (3) on the local application 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 



