336 APPLIED PHYSIOLOGY 



lens brings the light to a focus so as to form an image of 

 an object upon the retina. In the cells of the retina are 

 particles of brown coloring matter in which light produces 

 an instant change. This excites in the optic nerve an 

 impulse which the brain interprets as sight. 



607. Coverings of the eye. The eyeball is loosely situ- 

 ated in a deep depression of the skull, called the orbit. 

 The space between it and the bone is padded with fat and 

 crossed by numerous muscles, nerves, and blood tubes. 

 Thus it is thoroughly protected from injury. It can be 

 freely turned at will in all directions by six slender mus- 

 cles which rise from the back part of the orbit. It is pro- 

 tected in front by two thin but strong lids, which can be 

 moved up and down at will. From the edges of the lids 

 there project two or three rows of stiff curved hairs, which 

 still further protect the eye. The lids can be closed by a 

 flat circular muscle which completely surrounds them. 

 The insides of the lids and the front side of the eyeball, 

 except the cornea, are covered with a soft mucous mem- 

 brane, called the conjunctiva. 



608. Tears. The conjunctiva and cornea are mois- 

 tened by a saltish fluid called tears. Tears are secreted by 

 a gland called the lachrymal gland, which is situated just 

 above and to the outer side of the eyeball. At the inner 

 end of the edge of each lid is the opening of a small tube 

 which unites with the tube from the other lid and forms a 

 single tube called the nasal duct, leading to the nose. 

 Ordinarily the nasal duct drains away the tears as fast 

 as they are formed, and sometimes, as in crying, their 

 salt taste can be noticed in the mouth. Often they are 

 produced so fast that some run over the lids and fall down 

 the face. The uses of tears are to wash away particles of 

 dust which fall upon the eyeball, and to moisten its surface. 



