136 



VETERINARY PHYSIOLOGY 



When the eyes are allowed to sweep over a landscape or 

 any series of objects, or when these move rapidly past the eyes 

 or the eyes rapidly past them, as in travelling by train, the 

 axes are directed in a series of glances to different points, and 

 the succession of pictures thus got gives the idea of the con- 

 tinuous series of objects. This jerking movement of the eyes 



may be well seen in a passenger 

 looking out of a railway carriage 

 in motion. 



Nervous Mechanism. A some- 

 what complex nervous mechan- 

 ism presides over these various 

 movements of the eyes. All the 

 muscles are supplied by the third 

 cranial nerve, except the superior 

 oblique, which is supplied by the 

 fourth nerve, and the external 

 rectus, which is supplied by the 

 sixth nerve (fig. 66 ; see also fig. 

 95, p. 179). 



The centres for the third and 

 fourth nerves are situated in the 

 floor of the aqueduct of Sylvius 

 under the corpora quadrigemina, 

 while the centre for the sixth is 

 in the pons Varolii (fig. 96, p. 

 181, and fig. 95, p. 179). The 

 various centres are joined by 

 bands of nerve fibres which pass 

 between the sixth and fourth and 

 third centres, and in part at 

 least cross the middle line. 



A combined mechanism, each part of which acts harmoni- 

 ously with the other parts, thus presides over the ocular 

 movements, and this mechanism is controlled by impulses 

 constantly received from the two retinae, from the ear and 

 from the brain. Thus in convergence of the optic axes the 

 parts of the nuclei of the third nerves which supply the 

 internal recti muscles must act harmoniously together, and 

 hence a mechanism to direct this convergence may be 



FIG. 66. The Nervous Mechan- 

 ism presiding over the combined 

 movements of the two Eyes in 

 man and apes. I.E., internal 

 rectus ; E.E., external rectus ; 

 C.C., convergent centre acting 

 on the internal recti through 

 the nuclei of the third nerve ; 

 S.O., superior olive (centre for 

 lateral divergence) acting on the 

 external rectus of the same side 

 through the nucleus of the sixth, 

 and on the internal rectus of the 

 opposite side through the nucleus 

 of the third ; E., ear. 



