484 TEE HUMAN BODY. 



which represents the orbits opened from above, the superior 

 rectus of the right side has been removed. The superior 

 oblique or pulley (trochlear) muscle, t, arises behind near 

 the straight muscles and forms anteriorly a tendon, 

 M, which passes through a fibre-cartilaginous ring, or 

 pulley, placed at the notch in the frontal bone where it 

 bounds superiorly the front end of the orbit. The tendon 

 then turns back and is inserted into the eyeball between 

 the upper and outer recti muscjes. The inferior oblique 

 muscle does not arise, like the rest, at the back of the orbit, 

 but near its front at the inner side, close to the lachrymal 

 sac. It passes thence outwards and backwards beneath the 

 eyeball to be inserted into its outer and posterior part. 



The inner, upper, and lower straight muscles, the inferior 

 oblique, and the elevator of the upper lid are supplied by 

 branches of, the third cranial nerve (see p. 168). The sixth 

 cranial nerve goes to the outer rectus; and the fourth to 

 the superior oblique. 



The eye may be moved from side to side; up or down; 

 obliquely, that is neither truly vertically nor horizontally, 

 but partly both; or, finally, it may be rotated on its antero- 

 posterior axis. The oblique movements are always accom- 

 panied by a slight amount of rotation. When the glance is 

 turned to the left, the left external rectus and the right in- 

 ternal contract, and vice versa; when up, both superior recti; 

 when down, both the inferior. The superior oblique muscle- 

 acting alone will roll the front of the eye downwards and 

 outwards with a certain amount of rotation; the inferior 

 oblique does the reverse. In oblique movements two of 

 the recti are concerned, an upper or lower with an inner 

 or outer; at the same time one of the oblique also always 

 contracts. Movements of :; rotation rarely, if ever, occur 

 alone. 



The natural combined movements of the eyes by which 

 both are directed simultaneously towards the same point 

 depends on the accurate adjustment of all its nervo-muscu- 

 lar apparatus. When the co-ordination is deficient the 

 person is said to squint. A left external squint would be 

 caused by paralysis of the inner rectus of that eye, for then, 



