414 



ELEMENTARY PHYSIOLOGY 



use convex glasses. In such persons the bulb of the 

 eye is generally too short. 



A kind of long-siglitednesa also comes on in old people ; 

 but this is different from the above, and is simply due, in 

 the majority of cases at all events, to a loss of power of 

 adjustment. The refractive power of the eye remains the 

 same, but the ciliary muscle fails to work and the lens 

 has become less elastic with years ; and hence adjust- 

 ment for near objects becomes impossible, though distant 



Ch. m 



Fio. 132. 



B 



A, the muscles of the right eyel'all viewed from above, and B of the 

 left eyeball viewed from the outer side ; H.R, the superior rectus ; Inf. R, 

 the inferior rectus ; E.K, In.R, the external rectus ; S.Ob, the superior 

 oblique; Inf.Oh. the inferior oblique; Ch. the chiasma of the optic 

 nerves (ll.) ; ///, the third nerve which supplies all the muscles except 

 the superior oblique and the external rectus. 



objects are seen as before. For near objects such persons 

 have to use convex glasses. They should perhaps be 

 called " old-sighted " rather than "long-sighted." 



5. The Muscles of the Eyeball.— The muscles which 

 move the eyeball are altogether six in number — four 

 straight muscles, or recti, and two oblique nmscles, the 

 obliqui (Fig. 132). The straight muscles are attached to 

 the back of the bony orbit, round the edges of the hole 

 through which the optic uerve passes, and run straight 



