VISION. 415 



provided within the orbits, for effecting the motions 



of the eye-ball. A view of 

 these muscles, with their at- 

 tachments to the ball of the 

 eye, but separated from the 

 other parts, is given in Fig. 

 410. Four of these proceed 

 in a straight course from the 

 bottom of 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, r., and c in the figure ; and the edge 

 of the fourth is seen behind and above b. These 

 straight muscles, as they are called, surround the 

 optic nerve and the eye-ball, forming four longitu- 

 dinal bands ; one (a) being situated above, for the 

 purpose of turning the eye upwards ; 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 

 orbit 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 protection, 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 rotation 

 on its axis. When these act in conjunction, they 

 draw the eye forwards, and serve as antagonists to 

 the combined power of the straight muscles. The 

 upper oblique muscle (s) is remarkable for the arti- 

 ficial manner in which its tendon passes through a 



