288 THE SPECIAL SENSES 



part of the body, in front, and near the brain. They also com- 

 mand a wide range of view, being 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 envelop- 

 ing 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 perspira- 

 tion from running 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 considerable 

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

 cushion, the eye rests. 



54. The Eyelids. The eyelids are two movable curtains, or 

 folds, which, when shut, cover the front part of the orbit, and 

 hide the eye from view. The upper lid is the larger, has a 

 curved margin, and moves freely, while the lower lid is com- 

 paratively short and straight, and has but a slight degree of 

 motion (Fig. 63). Skin covers the exterior of the lids, while a 

 fine mucous membrane lines their inner surface, and is likewise 

 spread out over the entire front of the eyeball. This membrane, 



63. The protection of the eyeball against injury ? The overhanging brow ? The opening 

 for the optic nerve ? 



54. What are the eyelids ? The upper lid ? The lower one ? The mucous membrane of 

 the eye ? 



