NERVE 



159 



dog in certain position of the eyes, so that the image of that 

 place falls on each central spot {Practical Physiology). If 

 this does not occur, double vision results. The central 

 spot of one eye thus corresponds to the central spot of 

 the other, .and definite points in each retina have corre- 

 sponding points in the other, which, when the eyes are 

 working together, are stimulated by the same part of the 

 picture (fig. 77). 



2. Movements of Eyeballs. — To secure harmonious action 

 of the two retinas, it is necessary that the eyes should be 



Fig. 78.— The left Eyeball in the 

 Orbit from above, with the 

 muscles acting upon it. The 

 axis of the orbit, b ; the axis 

 of the eyeball, a. 



In 01 



£xR. 



IfiR. 



Fig. 79. — The Movements of the Eyeball 

 caused by the various Muscles of the 

 Eye. (Right Eye.) 



freely movable. Each eye in its orbit forms a ball and socket 

 joint in which the eyeball moves round every axis (fig. 78). 

 The orbit looks forward and outwards, but the axis of the eye 

 (a) is directed straight forward and is set obliquely to the 

 axis of the orbit (6). The centre of rotation is behind the 

 centre of the ball. The movements are produced by three 

 pairs of muscles. 



(1) The internal and external recti {l.P. 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,oh.). 



