416 THE SENSORIAL FUNCTIONS. 



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 produced by the 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, affords one of the most 

 palpable instances that can be adduced of express 

 contrivance ; for in no other situation could the 

 muscle have been so conveniently lodged as within 

 the eye-ball ; and in no other way could its tendon 

 have been made to pull in a direction contrary 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 Conjunc- 

 tiva* This membrane, on arriving at the base of 

 the eye-lids, is folded forwards so as to line their 

 inner surfaces, and to be continuous with the skin 

 which covers their outer sides. The surfaces of 

 the conjunctiva and of the cornea are kept con- 

 stantly moist by the tears, which are as constantly 

 secreted by the Lacrymal glands. Each gland, (as 

 shown at l. Fig. 417,) is situated above the eye, in 

 a hollow of the orbit ; and the ducts (o) proceed- 

 ing from it open upon the inner side of the upper 

 eye-lid (e). This fluid, the uses of which are ob- 

 viously to wash away dust, or other irritating sub- 



* An abundant supply of nerves has been bestowed on this mem- 

 brane for the purpose of conferring upon it that exquisite degree of 

 sensibility which was necessary to give immediate warning of the 

 slightest danger to so important an organ as the eye from the in- 

 trusion of foreign bodies. That this is the intention is apparent from 

 the fact that the internal parts of the eye possess but little sensibi- 

 lity compared with the external surface. 



