248 THE HUMAN BODY 



that is, neither truly vertically nor horizontally, but partly both; 

 or, finally, it may be rotated on its anteroposterior axis. The 

 oblique movements are always accompanied by a slight amount 

 of rotation. When the glance is turned to the left, the left external 

 rectus and the right internal contract, and rice versa; when up, 

 both superior recti; when down, both the inferior. The superior 

 oblique muscle acting alone will roll the front of the eye down- 

 wards and outwards with a certain amount of rotation; the infe- 

 rior 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. Move- 

 ments 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-muscular apparatus. 

 When the coordination 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, after the eyeball had been turned 

 out by the external rectus, it would not be brought back again 

 to its median position. A left internal squint would be caused, 

 similarly, by paralysis of the left external rectus; and probably 

 by disease of the sixth cranial nerve or its brain-centers. Drop- 

 ping of the upper eyelid (ptosis) indicates paralysis of its special 

 elevator muscle and is often a serious symptom, pointing to 

 disease of the brain-parts from which it is innervated. 



The Globe of the Eye is on the whole spherical, but. consists of 

 segments of two spheres (see Fig. 81), a portion of a sphere of 

 smaller radius forming its anterior transparent part and being 

 set on to the front of its posterior segment, which is part of a 

 larger sphere. From before back it measures about 22.5 milli- 

 meters (J inch), and from side to side about 25 millimeters (1 

 inch). Except when looking at near objects, the anteroposterior 

 axes of the eyeballs are nearly parallel, though the optic nerves 

 diverge considerably (Fig. 80); each nerve joins its eyeball, not 

 at the center, but about 2.5 mm. (^ inch) on the nasal side of the 

 posterior end of its anteroposterior axis. In general terms the 

 eyeball may be described as consisting of three coats and three 

 refracting media. 



The outer coat, 1 and 3, Fig. 81, consists of the sclerotic and the 



