630 A TEXTBOOK OF PHYSIOLOGY 



SECTION V 

 BINOCULAR VISION VISUAL JUDGMENT 



THE formation of visual judgments depends largely upon the 

 eyes acting together, the increased visual field and the combinations 

 of eye movements greatly aiding in the formation of such judgments. 



Ocular Movements. The ocular movements depend upon the 

 action of six muscles. Of these, four arise from the back of the 

 eyeball, and are termed the superior, inferior, internal, and external 

 rectus, according to the site of their insertion into the side of 

 the eyeball. Contraction of the internal muscle turns the eye 

 directly inwards, of the external muscle directly outwards. The 

 superior and inferior muscles pass somewhat obliquely to their 

 insertion. On this account, during thsir action they pull the eyeball 

 somewhat inwards as well as upwards in the case of the superior, 

 and downwards in the case of the inferior muscle. This inward 

 deviation is corrected by the action of two oblique muscles. The 

 superior oblique passes along the inner wall of the orbit, and, after 

 passing through a fibrous ring, passes pulley-fashion backwards and 

 outwards, to be inserted into the upper surface of the eyeball. Acting 

 alone, this muscle rotates the eyeball downwards and outwards. The 

 inferior oblique takes origin from the front part of the inner wall of 

 the orbit, and passes beneath the eyeball backwards and outwards, 

 to be inserted into its outer part. By its action, the eyeball is turned 

 upwards and outwards. For direct upward movement, the superior 

 rectus and inferior oblique act together; for direct downward move 

 ment, the inferior rectus and superior oblique. For oblique move- 

 ments, two recti and one oblique act together, according to the direction 

 of the movement. Thus, in looking obliquely downwards and out- 

 wards, the external and inferior rscti and the superior oblique are 

 employed. When two eyes are used, the same muscles are employed 

 in both eyes for upward and downward movements; but for looking 

 sideways, the outer rectus of one orbit acts in conjunction with the 

 inner set of the other. When converging the eyes upon a near 

 object, or when intentionally turning both eyes inwards, the inner 

 muscles act together. 



All eye movements are normally so directed that the image of the 

 object looked at falls upon the yellow spot of both eyes; indeed, it 

 is impossible voluntarily to turn the eyes so that this is not the case. 

 It is not possible to turn one eye up and the other down, or both eyes 

 outwards. It is stated that the " seeing double " of intoxication is 

 due to the fact that, under this condition, the muscles of the eyeballs 

 are relaxed, the eyeballs diverge slightly. If we intentionally diverge 

 the eyeballs, as by pressing on the outer side of one, objects are seen 

 double. By shutting one eye, it will be found that the left image 

 belongs to the right eye, and the right image to the left eye. In death 

 the eyeballs diverge slightly. Such a divergence may be due to the 



