VISION. 329 



fore part of the sclerotic coat. Three of these arc 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, surround the optic nerve and the eye-hall, forming 

 four longitudinal 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 riglit or 

 left. The cavity of the orbits being considera])ly larger tlian 

 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 direc- 

 tions. 



J3esides these straight muscles, there are also two others 

 (s and i) termed the ohlique 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 an- 

 tagonists to the combined power of the straight muscles. 

 The upper oblique muscle (s) is remarkable 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 effect pro- 

 duced by ^he action of the muscle is a motion in a direction 

 exactly the reverse of that in which its fibres contract. This 

 mechanism, simple as it is, afibrds one of the most palpable 

 instances that can be adduced of express contrivance; for in 

 no other situation could the muscle have been so conve- 

 niently lodged as within the eye-ball; and in no other way 

 could its tendon have been made to pull in a direction con- 

 trary to that of the muscle, than by the interposition of a 

 pulley, turning the tendon completely round. 



The fore-part of the globe of the eye, which is of a white 

 colour, is connected with the surrounding integuments by a 

 membrane, termed the Conjunctiva* This membrane, on 



* An abundant supply of nerves has been bestowed on this membrane for 

 the purpose of conferring upon it that exquisite degree of sensibility which 

 Vol. II. 42 



