XVIII 



THE SPECIAL SENSES AND SENSATIONS 



267 



blue, or gray. The third or inner coat is an extremely 

 thin and delicate film called the retina. It is really an 

 expansion of the optic nerve that forms a sensitive lining 

 for the inner surface of the eyeball. Before an object 

 can be seen it must form an image on the retina. 



184. Muscles of the Eye. Each eyeball has four 

 straight muscles attached to it, one above, one below, 



FIG. 142. Muscles of the eyeball. (From Blaisdell's "Life and Health." 

 Ginn & Co.) A, tendon of three recti muscles; B, external rectus, cut and 

 pulled downward; C, inferior rectus; D, internal rectus; , superior 

 rectus ; F, superior oblique ; H, pulley of the superior oblique ; K, inferior 

 oblique ; L, M, levator palpebrae ; -N, optic nerve. 



and one on either side ; besides these four there is an 

 oblique muscle attached to the upper part of the eyeball 

 and one to the lower. By means of these six muscles 

 the eye may be turned in any direction without mov- 

 ing the head at all. 



The muscles of the two eyes usually work in perfect 

 harmony, so that the eyes move together. Sometimes the 



