192 CLINICAL APPLIED ANATOMY. 



presumption that the motor fibres to this muscle, which reach it 

 through the facial nerve, are really derived from part of the third 

 nerve nucleus. 



The nucleus of the fourth nerve innervates the superior oblique 

 muscle of the eye, and lies in the floor of the iter beneath the 

 posterior quadrigeminal bodies. This nucleus is in serial con- 

 tinuity with the nucleus of the third, and suffers in a similar way. 



The nucleus of the sixth nerve is some way behind that of the 

 fourth. It lies beneath the eminentia teres of the fourth ventricle, 

 immediately above the acoustic striae. The sharp bend which 

 the issuing fibres of the facial nerve make around the sixth 

 nucleus explains how a comparatively small lesion may involve 

 both the sixth and the facial nerves. The sixth nucleus is 

 important in that it is the nuclear centre which produces 

 conjugate deviation of the eyes to its own side, the associated 

 action of the opposite internal rectus being brought about by 

 fibres of the posterior longitudinal bundle which connect the 

 sixth nucleus with the appropriate part of the third nucleus of 

 the opposite side. 



In addition to being subject to degenerative lesions the ocular 

 nuclei may be damaged by vascular lesions of the ponsor crus, 

 and by gross lesions of the cerebral structures in their immediate 

 neighbourhood. Tumours of the pons, corpora quadrigemina, 

 pineal gland and optic thalamus may all be in a position to do 

 this, and tend to involve the nuclei of both sides. Again, all the 

 nuclei lie near the floor of parts of the ventricular system, being 

 situate in the grey matter of the third and fourth ventricles and 

 connecting iter, consequently they may be irritated by ventricular 

 inflammation and compressed by large ventricular effusions. 



Infranuclear ophthalmoplegia is produced by lesions of the 

 nerves below the level of the nuclei. 



After leaving its nucleus and before appearing at the base of 

 the brain, the third nerve traverses the crus cerebri. The motor 

 tract of the opposite side of the body also traverses the crus, so 

 the typical paralysis produced by a lesion of this part of the brain 

 is a crossed or alternate paralysis, the third nerve palsy being 



