THE EYE AS AN OPTICAL INSTRUMENT. 509 



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 con- 

 tracts. 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 de- 

 pends on the accurate adjustment of all its nervo-muscular 

 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, 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-centres. Dropping 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. 142), 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 millimeters (^ inch), and from side to 

 side about 25 millimeters (1 inch). Except when looking at 

 near objects, the antero-posterior axes of the eyeballs are 

 nearly parallel, though the optic nerves diverge considerably 

 (Fig. 141); each nerve joins its- eyeball, not at the centre, but 

 about 2.5 mm. (^ inch) on the nasal side of the posterior end 

 of its antero-posterior axis. In general terms'the eyeball may 

 be described as consisting o three coats and three refracting 

 media. 



The outer coat, 1 and 3, Fig. 142, consists of the sclerotic 

 and the cornea, the latter oeing transparent and situated in 

 front; the iormei is opaque and white and covers the back 

 and sides of the globe and part of the front, where it is seen 



