THE EYE AND THE SENSE OF SIGHT 111 



the muscular action. The inferior oblique muscle arises 

 near the front of the orbit and at its inner side, whence it 

 passes under the ball and nearly halfway round it, to be 

 inserted into the back part on the temporal or outer side. 



148. The muscles of the two eyes act simultaneously 

 so that the two retinas may receive images from the same 

 objects at the same time and upon corresponding portions 

 of their surfaces. The external rectus of the right eye 

 contracts at the same time as the internal rectus of the left 

 eye, turning both eyes to the right, and vice versa. All 

 these muscles are supplied with nerves from the third pair 

 of cranial nerves, except the superior oblique, which is 

 supplied by the fourth, and the external rectus, which is 

 supplied by the sixth. 



149. The Ciliary Muscles and Nerves. Besides the mus- 

 cles which move the eyeball as a whole, certain muscles 

 within the eyeball have to do with vision. These are the 

 muscles of the iris, which vary the area of the pupil, and 

 the ciliary muscles, which accommodate the eye for differ- 

 ent distances by altering the shape of the lens, as will be 

 explained a little later. The ciliary nerves supply these 

 muscles. They are composed of fibers from the third and 

 fifth cranial nerves with others from the sympathetic sys- 

 tem. Branches from the ciliary nerves are distributed 

 also to the cornea. 



150. Other Appendages of the Eye ( Fig. 67). Each 

 eyeball lies in its orbit upon a soft cushion of fat, and the 

 cavity also contains connective tissue, blood vessels, and 

 nerves. The front of the eye is protected by the eyelids, 

 which are two folds of skin stiffened by thin plates of 

 fibrous tissue. Along the edges of the lids arise rows of 

 curved hairs, called eyelashes, which serve to protect the 

 eye from dust, and furnish a slight shade. On the inner 



