THE SPECIAL SENSES. 199 



sight is that there are two eyes, which may either act to- 

 gether as one, and be fixed upon one object, or one eye may 

 be used independently of the other. In consequence of 

 this arrangement the loss of one eye does not necessitate 

 blindness, and, in fact, it not infrequently happens that the 

 sight of one eye may be long impaired or lost before the 

 fact is discovered. We next notice that it is placed at the 

 most elevated part of the body, in front, and near the brain. 

 It also commands a wide range of view, being itself moved 

 with great rapidity, and being further aided by the free 

 motion of the head and neck. The organ of vision consists 

 essentially of two parts : the optical instrument itself the 

 eyeball and its enveloping parts, or the case in which the 

 instrument is kept free from harm. The latter, which are 

 external, and which we shall first consider, are chiefly the 

 Orbits, the Eyelids, and the apparatus for the Tears. 



53. The Orbits. The eyeball, which is a delicate 

 organ, is well defended against external injury within the 

 orbits or bony sockets of the head. These are deep conical 

 hollows, bounded in part by the bones of the skull, and in 

 part by those of the nose and cheek. The orbit juts out 

 beyond the most exposed portion of the eyeball, as may be 

 seen by laying a book over the eye, when it will be found 

 that no part of the eyeball, unless it be very prominent, 

 will be touched by the book ; so that the only direction in 

 which an injury is liable to be received is immediately in 

 front of the eye. The overhanging brow is itself covered 

 by a layer of thick skin, studded with short, stout hairs, 

 which are so bent as to prevent the perspiration from run- 

 ning into the eye and obscuring vision. Through a hole 

 in the bottom of the orbit, the nerve of sight passes out- 

 ward from the brain. The orbit also contains a considera- 

 ale amount of a fatty tissue, upon which, as upon an elastic 

 cushion, the eye rests. 



53. The protection of the eyeball against injury? The overhanging brow? 

 The opening for the optic nerve ? 



