184 CLINICAL APPLIED ANATOMY. 



quadrigemina, the nucleus lies in the front part of the floor of 

 the iter, and the nerve emerges from the inner side of the crus. 

 Tumours of the crus therefore tend to paralyse this nerve as 

 already indicated. (See Fig. 17, p. 191.) 



The fourth nerve nucleus also lies in the floor of the iter, 

 immediately behind the nucleus of the third, and corresponds to 

 the fore part of the posterior corpora quadrigemina. The nerve 

 emerges dorsally from ihe valve of Vieussens, and winds round 

 the outside of the crus. It may suffer in association with the 

 third when tumours occupy the quadrigeminal bodies or be com- 

 pressed by a tumour near the anterior end of the cerebellar vermis. 



The sixth nerve nucleus lies beneath the eminentia teres 

 in the anterior part of the floor of the fourth ventricle; the 

 nerve fibres emerge in the groove between the top of the pyramid 

 and the lower border of the pons. Owing to its long intracranial 

 course, paralysis of this nerve has little localising value, but the 

 nerve lies between the ventral surface of the pons and the basilar 

 process, and is likely to be compressed in subtentorial lesions. 

 Paralysis of all the ocular muscles of one eye, often associated 

 with blindness of the same eye, is likely to be caused by a tumour 

 of the sella turcica, which lies adjacent to the cavernous sinus, 

 and contains the pituitary body. 



The sixth nerve nucleus may be considered to be the centre for 

 conjugate deviation of the eyes towards its own side, its crossed 

 connection with the nucleus of the opposite third nerve accounts 

 for this. The communication is effected through the posterior 

 longitudinal bundle. A tumour in the pons or corpora quad- 

 rigemina affecting the sixth nucleus will render impossible con- 

 jugate deviation of the eyes towards the same side, since it will 

 paralyse the associated actions of the external rectus of its own 

 side and the internal rectus of the opposite eye. If both sixth 

 nuclei be involved the eyes will remain fixed in the mid position. 

 When the facial nerve is paralysed by tumours within the pons, 

 the nucleus of the sixth nerve around which the genu of the 

 facial root turns, may also suffer. Outside the pons facial paralysis 

 is more likely to be associated with paralysis of the auditory 



