106 ANIMAL MECHANISM. 



narily correspond ,in color. Whenever the iris acts, as 

 for instance, il does, in going from a dark, into a light 

 room, the pupil is made smaller, acting uniformly in its 

 fibres, to keep it circular. On returning to the dark 

 apartment, the pupil enlarges again. A knowledge of 

 this fact, will explain the reason of a painful sensation in 

 \^'ie eye, caused by a strong and sudden light. As soon 

 as the iris has had time to diminish the size of its pupil, 

 we can endure the same luminous object with perfect 

 comfort. When we leave a well lighted room, on first 

 going into a dark street, everything appears lurid and 

 indistinct. The iris soon begins to enlarge the pupil, to 

 admit more light, and when that has been accomplished, 

 although in comparative darkness, we recognise objects 

 without an effort. Acting independently of the will, its 

 duties are like those of a faithful sentinel, always con- 

 sulting the safety of the splendid optical instrument con- 

 fided to its care, with reference to its subserviency to the 

 being for whose use it was exclusively constructed. Were 

 it otherwise, were it left to our own care, how often it 

 would be neglected, and indeed, totally ruined, solely for 

 the want of undivided attention. All that complex system 

 of machinery, on which life and existence are constantly 

 depending, (the vital organs,) are wisely placed beyond 

 the reach of the laws of volition. If the pulsation of the 

 heart, the function of the lungs, or the circulation of 

 blood in the brain, depended upon our attention, our 

 recollection of the fact, that they must be kept in motion, 

 or we could not live, we should be in great danger of 

 forgetting it, and therefore die in our first slumber ! 



Parrots have a voluntary control over the pupil, opening 

 and closing it at pleasure. How this is done, or why, in 

 the constitution of that bird, it is neces'sary, we cannot 

 determine. Cats, also, appear to have a similar power of 

 moderating or graduating the quantity of light, admitted 

 into their eyes, as it suits their own convenience. 



In carnivorous quadrupeds, the pupil is commonly oval, 

 and oblique, permitting them to look from the bottom to 

 the top of a tree, without much elevation of their heads. 

 Graminivorous quadrupeds have an oblong pupil, placed 

 horizontally, with respect to the natural position of the 



