CHAP. IL] THE BRAIN. 947 



(Fig. 109, 3) by the fibres of the sensory decussation which sweep 

 round it ; it appears in sections higher up as a fairly distinct 

 region (Fig. 109, 4) ; and it is this part of the grey matter which 

 is exposed on the floor of the fourth ventricle when the central 

 canal (Fig. 109, 5, 6) opens out into that space. We say exposed ; 

 but in reality the true grey matter is covered by a superficial 

 layer of tissue of a peculiar nature (indicated in fig. 109, 5, 6, 

 by a thick black line) similar to that which is found at the hind 

 end of the conus medullaris in the spinal cord. 



We saw that at the level of the first cervical nerve coincident 

 with the horizontal flattening out of the posterior horns the 

 posterior columns assumed very large dimensions. In this region 

 (Fig. 109, 1) they consist entirely of white matter, that is, of 

 longitudinal fibres. 



At a little higher level, however, at the level of the middle of 

 the decussation for example, an islet of grey matter (Fig. 109, 

 2, ra. p. n.) makes its appearance in the median posterior column. 

 A little further forward, at the level of the established pyramids, 

 it will be seen (Fig. 109, 3) that this islet is the hind end of 

 an invasion from the more centrally placed grey matter, and that 

 at the same time there has taken place a similar inroad of grey 

 matter into the external posterior column (Fig. 109, 3, e. p. n.); 

 indeed a slight extension of grey matter into the external pos- 

 terior column may be seen even before this (Fig. 109, 2, e. p. n.). 

 It will further be observed that these grey masses have so largely 

 encroached on the white matter, that both the median posterior 

 or fasciculus gracilis and the external posterior column or 

 fasciculus cuneatus, instead of being simply tracts of white fibres 

 as they were in the hinder part of the bulb and in the cord, have 

 now become columns of grey matter covered by a relatively thin 

 layer of white fibres. These columns of grey matter are now 

 called respectively the median posterior nucleus, or nucleus 

 fasciculi gracilis, or more shortly, the gracile nucleus; and the 

 external posterior nucleus, or nucleus fasciculi cuneati, or the 

 cuneate nucleus. From the ventral aspect of these nuclei a 

 large number of fibres pass ventrally with a more or less 

 curved course to form as we have seen, 606, the superior decus- 

 sation and to pursue certain paths through the reticular formation, 

 of which we shall speak later on. It is at this level and for 

 some little distance above (Fig. 109, 4, 5), that these nuclei 

 acquire their greatest development. Farther forward (Fig. 109, 

 6), when the fourth ventricle has opened out and the nuclei 

 of the cranial nerves are becoming conspicuous, and the posterior 

 columns have been thrust aside laterally, both these nuclei have 

 diminished in size ; still farther forward they become still smaller, 

 and towards the pons they gradually disappear. 



The mass of gelatinous substance, forming at the level of 

 the first cervical nerve the swollen caput of the horn close to 



