THE SPECIAL SENSES 



337 



610. Oculo-motor Muscles (Fig. 213). The eyeball is 

 capable of being turned in all directions by means of six 

 slender muscles which begin in the back part of the orbit. 

 Four of them are straight. The one above turns the eye 

 upward, the one below turns it downward, the one toward 

 the nose turns it inward, and the one toward the temple 

 turns it outward. The other two are oblique. The supe- 

 rior oblique muscle passes forward through a loop which 

 serves as a pulley 



near the inner upper 

 front part of the orbit 

 (Fig. 213). It rotates 

 the eye in one direc- 

 tion, and its antagonist, 

 the inferior oblique 

 muscle, rotates it in 

 the opposite direction. 

 "Cross eyes" are 

 caused by too great 

 contraction of the in- 

 ternal straight mus- 

 cles, and "wall eyes" 

 are caused by too great 

 contraction of the ex- 

 ternal straight muscles. The defects may be remedied by 

 a skillful surgeon, who cuts the proper muscle with a 

 suitable instrument, and permits it in healing to become 

 attached to another point farther back. 



611, A person is blind while the eyes are moving. 

 Watch while some one in front of the class tries to 

 move the eyes gradually and uniformly across the field 

 of vision. Do the eyes move by jumps or steadily? The 

 motions of the lids and eyeball give the expression of the 

 eye. The eyeball itself has hardly more expression than 

 a glass eye. 



z 



FIG. 213. A, the Muscles of the Right Eyeball 

 (viewed from above). B, the Muscles of the 

 Left Eyeball (viewed from the outer side) . 



S.R, superior rectus; Inf.R, inferior rectus; E.R, 

 external rectus; S.Ob, superior oblique; Inf. Ob, 

 inferior oblique; //, the optic nerves; ch, their 

 crossing or chiasma; ///, the third cranial nerve. 



