ANIMAL MECHANISM. 99 



ratus, common to all others, probably see with peculiar 

 distinctness, in the darkest night, at unfathomable depths 

 of the ocean. 



With another alteration, not unlike changing the 

 distances between the lenses of a spy-glass, another fami- 

 ly of animals, as seals, &,c, see alternately in two elements. 

 Still further, ou the descending scale of creation, insects 

 are provided with motionless eyes, giving them the fa- 

 culty of seeing in every possible direction. And, lastly, 

 in snails and some kinds of worms, the eyes are fixed at 

 the extremity of a moveable feeler, adapting them to dif- 

 ferent focal distances, or they can be drawn entirely 

 within the head, for safe keeping, when not in use, pre- 

 cisely on the same principle of care that we draw out the 

 slides of an opera glass, and close them up again, when 

 no longer needed. 



Were we desirous of describing the nice variations in 

 the mechanism of the eyes of the several species of ani- 

 mals adverted to in this preliminary, however interesting 

 it might be to some, would, perhaps, appear tedious to 

 others. Confining ourselves, now, to the exclusive con- 

 sideration of the human eye, we shall proceed with an 

 orderly description of its several parts, hoping that the 

 few scientific terms which must necessarily be retained, 

 will not prove to be a serious embarrassment. 



THE SOCKET IN WHICH THE EYE ROLLS. 



Several thin pieces of bone assist in the formation of the 

 orbit, which, in a dry skull, is shaped much like a pear, 

 with its large end turned outward. The upper plate of 

 bone is arched, slightly resembling an arch of a bridge, 

 having the brain resting on it above, and the eye ball 

 moving under it below. Externally, the eyes are at con- 

 siderable distance, but the inner termination of the coni- 

 cal orbits, answering to the small end of the fruit, are 

 quite near together. At their points, is a ragged hole, 

 in each, through which the nerve of vision enters the 

 brain. A large quantity of fat is deposited in this socket, 

 between the bones and eye-ball, that the latter may always 

 move with perfect freedom, and without friction, in all 

 directions. After a long sickness, this cushion of fat is 



