APPLICATION OF THE ARGUMENT. 23 



exploring those which are rare and curious ; by which con- 

 duct of the understanding, we sometimes neglect the 

 strongest observations, being taken up with others, which, 

 though more recondite and scientific, are, as solid argu- 

 jnents, entitled to much less consideration. 



In order to keep the eye moist and clean, which qualities 

 are necessary to its brightness and its use, a wash is con- 

 stantly supplied by a secretion for the purpose ; and the 

 superfluous brine is conveyed to the nose through a perfora- 

 tion in the bone as large as a goose quiil. [Plate IV. fig. 1.] 

 When once the fluid has entered the nose, it spreads itself 

 upon the inside of the nostril, and is evaporated by the cur- 

 rent of warm air, which, in the course of respiration, is con- 

 tinually passing over it. Can any pipe or outlet for carry- 

 ing off the waste liquor from a dye-house or distillery, be 

 more mechanical than this is? It is easily perceived that 

 the eye must want moisture : but could the want of the eye 

 generate the gland which produces the tear, or bore the hole 

 by which it is discharged — a hole through a bone ? 



It is observable that this provision is not found in fish ; 

 the element in which they live supplying a constant lotion 

 .to the eye. 



It were, however, injustice to dismiss the eye as a piece 

 of mechanism, without noticing thrt most exquisite of all 

 contrivances, the nictitating membrane, which is found in 

 the eyes of birds and of many quadrupeds. [Plate IV fig. 

 2, 3, 4, 5.] Its use is to sweep the eye, which it does 

 in an instant ; to spread over it the lachrymal humour ; 

 to defend it also from sudden injuries ; yei: not totally, 

 when drawn upon the puuil, to shut out the light. The 

 commodiousness with which it lies folded up in the 

 upper corner of the eye, ready for use and action, and 

 the quickness with which it executes its purpose, are 

 properties known and obvious to every observer ; but, 

 what is equally admirable, though not quite so obvious, 

 is the combination, of two different kinds of substance, 

 muscular and elastic, and of two different kinds of ac- 

 tion, by which the motion of this mem^rance is performed. 

 It is not, as in ordinary cases, by the action of two 

 antagonist muscles, one pulling forward and the other 

 backward, that a reciprocal change it; effected ; but it is 

 thus : the membrance itself is an elastic substance, ca- 

 pable of being drawn out by force like a piece of elastic 

 gum, and hy its own elasticity returning, when the force 

 is removed, to its former position. Such being its nature. 



