384 



PHYSIOLOGY AND HEALTH, 



896. The optic nerve (Fig. LXVI. g) passes from the base 

 of the brain forward through the socket and into the eyeball. 

 After passing the outer and middle coats, it is spread out on 

 their inner surface, and forms the retina, which receives the 

 rays of light. 



FIG. LXVIII. Muscles of the Eye. 



, Part of the bony socket. 



b, Optic nerve. 



c, Eyeball. 



d, Muscle that raises thie upper 

 lid. 



e, g, Muscle that passes from 

 the socket at a, through the loop 



/, and back to the ball. It rolls 

 the eye downward and outward. 

 h, Muscle attached to outer 

 edge of the bony socket, and to 

 the side of the ball. It rolls the 

 eye upward and inward. 



897. The eye is thus complicated, with many and various 

 parts, all of which are arranged and harmonized together, 

 and all acfapted to the action of light without, and to the per- 

 ceptive power of the brain within. 



The light is reflected from objects, and passes through the 

 transparent cornea and the pupil into the ball. The humors 

 and the lens refract these rays, and give them such a direction 

 that they fall upon the retina, where they form the image of 

 the object. This impression is carried along the optic nerve 

 to the brain, and there perception takes place and the object 

 is seen. 



