410 PHYSIOLOGY 



upwards and inwards, with contraction of the pupil and spasm of accommo- 

 dation. 



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 hori- 

 zontally, decussating with the nerve of the opposite side. The superficial 

 origin is therefore from the valve of Vieussens. This nerve supplies the 

 superior oblique muscle of the eyeball. Its stimulation causes the eyeball to 

 look downwards and outwards. 



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. 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 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. Sherring- 

 ton 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 Sherrington 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 pro- 

 prioceptive sense-organs and the afferents of the third, fourth, and sixth 

 nerve pairs themselves. 



The fifth nerve, or trigeminus, resembles a spinal nerve in that it has a 

 motor as well as a sensory root. The motor root is much the smaller of the 

 two. The fibres of the sensory root take their origin in the cells of the 

 Gasserian ganglion, which is in all respects similar to the ganglion of a 

 posterior spinal nerve root. The sensory root represents the somatic 

 afferent part of all the motor cranial nerves from the third to the hypoglossal 

 and has a correspondingly wide field of termination in the brain stem. The 

 afferent fibres of the fifth nerve, as they enter the pons, bifurcate, like a spinal 

 afferent nerve, into ascending and descending branches. The ascending 

 branches are short and pass to an upper sensory nucleus, situated below the 

 lateral part of the fourth ventricle in the upper part of the pons. The 



