CHAP, ii.] THE BRAIN. 965 



ordinary spinal nerve, the fibres of which, as we have seen, pass 

 into the grey matter without being obviously connected with 

 nerve cells. In its lower part at least it consists of extremely fine 

 fibres, and indeed looks very much like a continuation in the bulb 

 of the marginal (Lissauer's) zone of the spinal cord. 



622. The Fourth or Trochlear Nerve. The nucleus of this 

 nerve (Fig. 115, IV.) is a column of somewhat large multipolar ^ 

 cells on each side of the median line below the aqueduct (Fig. ' 

 113, IV. 7i.), reaching from the level of the junction of the 

 anterior and posterior corpora quadrigemina to the hinder level 

 of the latter body. 



The root, starting from the lateral surface of the nucleus, does 

 not take at first a ventral direction, but sweeps laterally and 

 dorsally in the outer layers of the central grey matter (Fig. 113), 

 and so curving round to the dorsal surface reaches the valve of 

 Vieussens, where in the median line it decussates with its fellow 

 in the substance of the valve; such a decussation at a distance 

 from the nucleus of origin is exceptional in the cranial nerves. 

 Leaving the surface of the brain in the valve, it takes a superficial 

 course curving (Fig. 108, B) laterally and ventrally, and makes 

 its appearance in a ventral view of the brain at the front edge of 

 the pons, on the lateral edge of the crus (Fig. 108, (7.). 



623. The Third or Oculomotor Nerve. The nucleus of this 

 nerve (Fig. 115, III., 114, III. n.) is a column of, for the most part, 

 fairly large multipolar cells lying on each side close to the median 

 line, in the grey matter of the central canal, just dorsal to a 

 bundle of fibres which we shall speak of as the longitudinal 

 posterior bundle; it reaches from the level of the posterior 

 commissure in the third ventricle to the level of the junction 

 of the anterior and posterior corpora quadrigemina. In a section 

 taken through its middle (Fig. 114) the nucleus is seen to give 

 off fibres which run vertically towards the ventral surface, 

 traversing the tegmentum and a body (Rn.) which we shall 

 presently speak of as the " red nucleus," but apparently making 

 no connections with these structures, and pierce the median edge 

 of the pes, emerging (Fig. 108, (7.) on the surface to the median 

 side of each crus. As we shall see later on, this nerve is now 

 exclusively efferent, whatever it may have been in more primitive 

 beings. We shall also see later on, that impulses starting from 

 the cerebrum of one side pass to the nerve of the other side, that 

 is to say decussate ; and this is also the case with the other 

 efferent cranial nerves. The fibres which appear to take origin 

 from the nerve cells of the nucleus do not cross over after 

 emerging from the nucleus, but keep to the same side; there 

 is no distant decussation as in the case just noted of the fourth 

 nerve. There are however fibres (Fig. 114, III.') which leaving 

 the nucleus cross the median raphe from one side to the other, 

 and these possibly are the paths for the decussation of the 



