464 PHYSIOLOGY 



levator palpebrarum, superior reetus, inferior rectus, internal rectus, 



and inferior oblique. 



Stimulation of the trunk of the third nerve causes the eyeball 

 to look upwards and inwards, with contraction of the pupil and 

 spasm of accommodation. By careful stimulation of various parts of 

 its nucleus the different movements of this muscle may be produced 

 separately. 



The nucleus of the fourth nerve is situated, just behind that for 

 the third, in the floor of the Sylvian aqueduct, on a level with the 

 inferior corpora quadrigemina. The fibres run from here down 

 towards the pons, then turn sharply backwards to pass into the 

 valve of Vieussens, which they cross horizontally, decussating with 

 the nerve of the opposite side. The superficial origin is therefore 

 k rom the valve of Vieussens, the thin plate of grey matter which 

 forms the roof of the fourth ventricle just in front of the cere- 

 bellum. This nerve supplies the superior oblique muscle of the 

 eyeball. Its stimulation causes the eyeball to look downwards and 

 inwards. 



The sixth nerve, the motor nerve for the external rectus muscle 

 of the eyeball, arises from a group of large multipolar cells lying 

 on each side of the middle line in the floor of the fourth ventricle 

 in its upper part. The fibres of the nerve pass directly outwards to 

 emerge from the anterior ventral surface of the medulla between 

 the pyramids and the olivary eminence, at the lower border of the 

 pons. Stimulation of this nerve causes the eyeball to look directly 

 outwards. All these three oculo-motor nuclei receive collaterals 

 from the longitudinal fibres forming the posterior longitudinal bundle, 

 many of which are axons of cells in Deiters' nucleus. It is by this 

 means that the contractions of the muscles moving the eyeball are 

 co-ordinated. Sherrington has shown that although the third, fourth, 

 and sixth nerves arise directly from the brain stem and have no 

 ganglion on their course, they are really mixed afferent- efferent 

 nerves. Their afferent fibres, which must arise from the cells in the 

 central nervous system itself, run to the receptor nerve- endings 

 with which all the extrinsic eye muscles are richly provided. They 

 are exclusively proprioceptive, and supply no organs outside the 

 muscles innervated by the motor fibres. The occurrence of afferent 

 fibres in these nerves explains the fact previously observed by Sher- 

 rington, that after total desensitisation of the eyeball by means of 

 cocaine, or by section of the first division of the fifth nerve, the ocular 

 movements are carried out with as much precision as in the normal 

 animal. As we have seen, such precision of movement requires the 

 co-operation of afferent impressions from the muscle, and the only 

 possible channels for these impressions are the proprioceptive sense- 



