358 



GENEKAL SCIENCE 



is the window through which light enters the interior of the 



eye. 



The Choroid Membrane, The middle coat of the eye which 



lines the sclerotic coat is called the choroid membrane. Like 



the sclerotic it is pierced behind for the entrance of the optic nerve. 



The Aqueous Humor. 

 The cavity of the eye is filled 

 by three transparent bodies, 

 called humors. That occu- 

 pying mainly the space behind 

 the cornea and in front of the 

 iris is called the aqueous 

 humor. A very little of it 

 lies in the space behind the 

 iris, communicating with that 

 in front through the pupil. 

 It is a clear liquid, consisting 

 mostly of water with alkaline 



FIG. 243. A Section of the Human Eye. 

 A, cornea ; B, aqueous humor ; C, 

 pupil; D, iris; E, crystalline lens; K, 

 vitreous humor; L, optic nerve; F, 

 Sclerotic; G, choroid; H, retina. 



salts in solution. If the cor- 

 nea be punctured the aque- 

 ous humor escapes, the pro- 

 tuberant part of the eye collapses, and the sight is temporarily 

 lost. The wound will, however, heal in a short time and a new 

 supply of liquid will be secreted and sight restored. 



The Vitreous Humor. The large cavity enclosed mainly by the 

 retina contains a thick, jellylike transparent albuminous substance, 

 called the vitreous humor. 



The Protectors of the Eye. Not only is the eye well protected 

 by its location, but it has in addition certain guards and defenders. 

 The eyebrows shield it from excessive light and direct the per- 

 spiration to one side and possibly catch some particles of dust 

 that otherwise would get into the eye. The eyelids, composed 

 of a piece of cartilage covered with skin and lined with the mucous 

 membrane, act both reflexively and voluntarily, closing instantly 

 when danger approaches. Their edges contain small glands 

 (Meibomian glands) which secrete an oily fluid which prevents 

 the tears from overflowing on the face and keeps the lids from 



