124 



HUMAN PHYSIOLOGY 



taught us that this stereoscopic vision indicates solidity 

 (Fig. 61). 



Corresponding Areas of the Two Retinae. In order that 



Fio. 61. Stereoscopic 

 Vision. 



Fio. 62. Corresponding Areas of the 

 two Retinae in Binocular Vision. 

 The upper and outer area of the 

 right retina corresponds to the 

 upper and inner area of the left 

 retina, and the other areas corre- 

 spond as shown by the shading. 

 In each pair of areas definite points 

 correspond with one another, 



with the two eyes single 



vision may occur, the eyes 



must be directed to the same 



place, so that the image 



of that place falls on each 



central spot. If this does 



not occur, double vision results. If the central spot of one 



eye corresponds to the central spot of the other, certain 



points in each retina will have corresponding points in the 



other which will be stimulated by the same part of the 



picture when the eyes are working together (Fig. 62). 



Movements of Eyeballs. To 



secure this harmonious action of 



1 the two retinae, it is necessary 



that the eyes should be freely 

 movable. Each eye in its orbit 

 is a ball and socket joint in 

 which the eyeball moves round 

 every axis (Fig. 63). The axis 

 of the eye (a) is set obliquely to 

 the axis of the orbit (6), and the 

 centre of rotation is behind the 

 Fm. 63. The left Eyeball in the centre of the ball. The move- 

 Orbit, with the Muscles acting ments are produced by three 



upon it. i* 



pairs of muscles. 



1. The internal and external recti (LR. and Ex. R.). 



2. The superior and inferior recti acting along the lines 

 indicated (S.R.). 



3. The superior and inferior obliques acting in the line (S.ob.). 

 The internal rectus rotates the pupil inwards. 



external outwards. 



S.ob. 



