592 ANIMAL PHYSIOLOGY. 
swers to upper and lower to lower. This may also be made 
evident if two saucers (representing the fundus of each eye) be 
laid over each other and marked off, as in the figure. 
That such corresponding points do actually exist may be 
shown by turning one eye so that the image shall not fall, as 
indicated in the figure. Only now and then, however, is a per- 
son to be found who can voluntarily accomplish this, but it 
occurs in all kinds of natural or induced squint, as in alcohol- 
ism, owing to partial paralysis of some of the ocular muscles. 
Weare thus naturally led to consider the action of these muscles, 
Ocular Movements.—Upon observing the movements of an 
individual’s eyes, the head being kept stationary, it may be 
noticed that (1) both eyes may converge; (2) one diverge and 
the other turn inward; (3) both move upward or downward; 
ck 
Fia. 433.—View of the two eyes and related parts (after Helmholtz). 
(4) these movements may be accompanied by a certain degree 
of rotation of the eyeball. 
The eye can not be rotated around a horizontal axis without 
combining this movement with others. To accomplish the 
above movements it is obvious that certain muscles of the six 
with which the eye is provided must work in harmony, both as 
to the direction and degree of the movement—i. e., the move- 
ments of the eyes are affected by very nice muscular co-ordina- 
tions. 
