100 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



eyeball itself is a much more serious affair. From this 

 arrangement it is evident that motes in the eye can not 

 go beyond easy removal, as it can not go beyond a a', 

 Fig. 70. 



Muscles. — The nimble movements of the eye are 

 effected by six muscles in each eye. Four of these, the 

 straight muscles (recti), arise near together at the bottom 

 of the conical socket (Fig. 71), come forward diverging, 

 and are attached to the ball a little in front of the 



equator, one above 

 (superior rectus), 

 one below (inferior 

 rectus), one on the 

 outside (exterior 

 rectus), and one on 

 the inside (inte- 

 rior rectus). The 

 functions of these 

 are obvious. Each 

 turns the ball in the 

 direction of its pull. When we look upward the two 

 superior recti pull; when we look downward, the two 

 inferior recti. When we look to the right, the external 

 rectus of the right eye and internal rectus of the left 

 eye pull, and vice versa when we look to the left. When 

 we look at a very near object, then the two interior recti 

 pull so as to converge the eyes on the object looked at. 

 But we can not contract the two exterior recti so as to 

 turn both eyes outward, nor can we turn one eye upward 

 and the other downward, because these movements can 

 not serve any useful purpose, and therefore have never 

 been learned. 



There are two other muscles — the oblique. The supe- 

 rior oblique arises along with the recti at the bottom of 

 the socket, passes forward to the opening of the orbit, 



Fig. 71. — Muscles of the eyeball : a, optic nerve ; 

 b, superior oblique muscle ; c, pulley ; d, in- 

 ferior oblique. The other four are the recti. 



