NATURAL, HISTORY. 223 



Ji/t'-tHH(/ht how thou dost me deliver 



My endl'esse lid and comfort-giver." SIR P. SIDNEY. 



666. Why are the eyes of eagles furnished with nictating 

 membranes ? 



Because sight is the chief sense upon which eagles depend for 

 their supply of food ; and in order to retain this organ as perfect as 

 possible, nature has accommodated it with this provision, which not 

 only tends to keep the eyes clean, but also shields them from strong 

 light when in sunny altitudes, and renders them more sensitive of 

 that reflected from below. 



667. The nictating membrane, which has already been repeatedly mentioned, is a 

 most curious apparatus, with which animals of very opposite natures are endowed, 

 but all of which have their eyes liable to certain conditions that render the pro- 

 tection afforded by this curious organ absolutely essential. The organ consists of 

 a thin semi-transparent membrane, which, when at rest, lies in the inner corner of 

 the eye, folded up like a drawn cu>-taia. By the combined action of two muscles, 

 it is capable of being drawn out, s>o us to cover the whole front of the eye-ball, like 

 a curtain , and its own elasticity restores it to the corner in which it previously 

 rested. The action of this beautiful membrane is very instantaneous. Wherevei 

 there are zoological gardens, it may- be observed with great effect in the eyes of tht 

 Great South American Eagte. 



This membrane is also found in the eyes of most quadrupeds, birds, and fishes , 

 but its development and use are beautifully proportioned to the necessities of each 

 animal. It is largely developed, as already stated, in those birds of prey which 

 seek sunny altitudes, and are said to " gaze at the sun." When thus apparently 

 gazing upward from their rocky heights, the membrane is probably drawn, to modify 

 the force of the intense rays. 



It is found, also, in the camel and the ostrich, because those aninv J5, being 

 frequenters of the sandy deserts, are liable to have their eyes injured y clouds 

 of sand. 



Many animals that obtain their food by pushing their heads among c ranches of 

 trees, bushes, rushes, and grass, are also provided with it. 



.5 .8 also found in fishes, which, having no eye-lids, because moisture is abun- 

 dantly supplied to their eyes by the element in which they live, yet need an 

 occasional protection from sea-clouds of sand, and drifting marine vegetation ; and 

 also from excess of light, when they approach the surface, quitting the depths 

 where the light is greatly modified. 



Man has no such membrane, because he has no occasion to thrust his head into 

 places which endanger his eyes ; and when, in pursuit of sport, he enters the woods 

 and jungles, he is able, by the momentary application of his hands, to guard LJ 

 eyes from Injury. 



