152 COORDINATION OF OCULAR MOVEMENTS. [Boon in. 



contraction by impulses passing down the right sixth nerve and 

 left third nerve ; the efferent impulses therefore cross in the case 

 of one nerve but not in the case of the other. Similarly, when 

 the right hemisphere is stimulated, impulses pass down the right 

 third nerve and left sixth nerve. Stimulation of the occipital 

 region ( 671) also leads to a similar conjugate turning of both 

 visual axes to the opposite side, accompanied in certain cases 

 by a raising or lowering of the visual axes. So far artificial 

 stimulation of the cortex has not been found to give rise to lateral 

 deviation towards the side stimulated, impulses have not been 

 excited so as to pass down to the external rectus of the same 

 side ; but, as we have already urged ( 657), little weight can be 

 given to the negative results of what is at best a very rough mode 

 of stimulation. 



We have already ( 671) urged that the ocular movements 

 which result from the stimulation of the occipital region cannot 

 be due to an indirect stimulation of the ' motor ' frontal area, and 

 that probably they are carried out through some special ties 

 between the occipital cortex and the primary visual centres ; and 

 in this relation, it is worthy of notice that ocular movements, 

 similar to those obtained by stimulating the cortex, may be 

 obtained by stimulating the anterior corpora quadrigemina. 

 Stimulation of these structures on one side leads to conjugate 

 lateral movement of the visual axes to the opposite side and it is 

 stated that a more median stimulation leads to a downward 

 movement of both sides with convergence, or in cases where con- 

 vergence previously existed, to an upward movement with a return 

 to parallelism. If on the other hand the stimulus is brought 

 to bear directly on the nucleus of the third nerve the movements 

 excited are said to be limited entirely to the eye of that side. 

 From this we may infer perhaps that some, at least, of the coordi- 

 nation of which we are speaking, takes place in some part or other 

 of the anterior corpora quadrigemina. 



Lastly we may remark that the tract of fibres which we de- 

 scribed ( 634) as the posterior longitudinal bundle, uniting as 

 it does the three ocular nuclei, has been supposed to be an instru- 

 ment assisting in coordination by serving as a tie between the 

 several nuclei ; and it has been especially urged that some of the 

 fibres of this tract cross over from the sixth nucleus of one side 

 and joining the tract of the opposite side pass to the third nucleus 

 of the opposite side, thus affording an anatomical basis for what 

 we have seen to be one of the most frequent associations in 

 ocular movements, that of the external rectus of one eye with the 

 internal rectus of the other eye. In connection with these nuclei 

 it is worthy of note that while all the fibres proceeding from the 

 sixth nucleus of one side pass into the nerve of the same side, 

 some of the fibres issuing from the third nucleus appear to 

 decussate ( 623) and the whole of the fourth nerve ( 622) 



