VISION. 463 



the orbit, arising from the margin of the aperture 

 through which the optic nerve passes, and being 

 inserted by a broad tendinous expansion into the 

 fore-part of the sclerotic coat. Three of these 

 are marked a, b, and c in the figure ; and the 

 edge of the fourth is seen behind and above b. 

 These, straight muscles, as they are called, sur- 

 round the optic nerve and the eye-ball, forming 

 four longitudinal bands ; one (a) being situated 

 above, for the purpose of turning the eye up^ 

 wards ; a second (c), situated below, for turning 

 it downwards ; and the two others, on either side, 

 for performing its lateral motions to the right or 

 left. The cavity of the orbits being considerably 

 larger than the eye-ball, the intervening space, 

 especially at the back part, is filled up by 

 fat, which serves as a soft cushion for its pro- 

 tection, and for enabling it to roll freely in all 

 directions. 



Besides these straight muscles, there are also 

 two others (s and i), termed the oblique muscles, 

 which give the eye-ball a certain degree of rota- 

 tion on its axis. When these act in conjunction, 

 they draw the eye forwards, and serve as anta- 

 gonists to the combined power of the straight 

 muscles. The upper oblique muscle (s) is re- 

 markable for the artificial manner in which its 

 tendon passes through a cartilaginous pulley (p) 

 in the margin of the orbit, and then turns back 

 again to be inserted into the eye-ball ; so that the 



VOL. II. H H 



