NERVE 



161 



5. Nervous Mechanism. — A somewhat complex nervous 

 mechanism presides over these various movements cf 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. 80). 



The centres for the third and fourth nerves are situated 

 in the floor of the aqueduct of 

 Sylvius under the corpora quadri- 

 gemina, while the centre for the 

 sixth is in the pons Varolii (fig. 

 80). 



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 harmoniously 

 with the other parts, thus pre- 

 sides over the ocular movements, 

 and this mechanism is controlled 

 by impulses constantly received 

 (a) from the two retinae ; (6) from 

 the ears ; and (c) 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 postulated. In lateral deviation of 

 the eyes, such as is reflexly produced by sudden auditory or 

 visual stimulation on one side, that part of the nucleus of 

 the third nerve which presides over the internal rectus of 

 one side acts harmoniously with the sixth nerve supplying 

 the external rectus of the other side, and hence it may be 

 supposed that a directing mechanism for lateral deviation 

 11 



Fig. 80. — The Nervous Mechan- 

 ism presiding over thecombined 

 movements of the two Eyes. 

 I.R., internal rectus; E.R., 

 external rectus; C.G., con- 

 vergent 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. 



