THE BULB. 597 



a. 1. 7i.) and which is probably a detached portion of the base of the anterior 

 horn or of the lateral region of the gray matter, is called the lateral or 

 antero-lateral nucleus. 



Hence, besides the diffuse reticular formation, this ventral part of the 

 bulb contains more sharply defined collections of gray matter in the olivary 

 nucleus, and the other bodies just mentioned. 



523. We must now turn to the dorsal part of the bulb. Here in the 

 first place we must distinguish between the portions of gray matter which 

 are more immediately connected with the cranial nerves taking origin from 

 this part of the bulb, and the portions which have no such obvious connec- 

 tion. In the spinal cord, the anterior horns supply, as we have seen, the 

 origins of the successive anterior motor nerves ; but in the transformation of 

 the cord into the bulb the anterior horns have been broken up or displaced ; 

 and the parts of the anterior horns serving as the nuclei of origin for motor 

 nerves have been translated from the ventral to the more dorsal regions. 

 Hence, it is in the more dorsal part of the gray matter that we have to seek 

 for the nuclei of origin not only of afferent but also of motor cranial nerves. 

 It will be convenient to consider all these nuclei of origin of cranial nerves 

 by themselves, and we may here confine ourselves to the gray matter of other 

 nature. We may, however, say that these nuclei from that of the third 

 nerve backward are more or less closely associated with the gray matter 

 immediately surrounding the central canal. This central gray matter, in the 

 narrow sense of the term, is marked out somewhat low down (Fig. 132, 3) 

 by the fibres of the sensory decussation which sweep round it ; it appears in 

 sections higher up as a fairly distinct region (Fig. 132, 4) ; and it is this 

 part of the gray matter which is exposed on the floor of the fourth ventricle 

 when the central canal (Fig. 132, 5, 6) opens out into that space. We say 

 exposed ; but in reality the true gray matter is covered by a superficial layer 

 of tissue of a peculiar nature (indicated in Fig. 132, 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. 132, 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 decus- 

 sation for example, an islet of gray matter (Fig. 132, 2, m. 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. 132, 3) that this islet 

 is the hind end of an invasion from the more centrally placed gray matter, 

 and that at the same time there has taken place a similar inroad of gray 

 matter into the external posterior column (Fig. 132, 3, e. p. n.) ; indeed, a 

 slight extension of gray matter into the external posterior column may be 

 seen even before this (Fig. 132, 2, e.p. n.). It will further be observed that 

 these gray 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 gray matter covered by a relatively thin layer of white fibres. 

 These columns of gray matter are now called respectively the median poste- 

 rior 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 larger number of fibres 

 pass ventrally, with a more or less curved course, to form, as we have seen 

 ( 518), the superior decussation and to pursue certain paths through the 



