THE BULB. 599 



tract, the antero-lateral descending tract, our knowledge is very imperfect ; 

 nothing definite can be said at present. 



The cerebellar tract, occupying near to the surface a position which in the 

 series of sections (Fig. 132, (76.) appears now rather more ventral, now more 

 dorsal, eventually passes into the restiform body, of which it forms a large 

 part, and thus reaches the cerebellum. The antero-lateral ascending tract 

 possibly also takes the same course, but this is not as yet certain. 



The median posterior tract or column, becoming the fasciculus gracilis, 

 ends in the gracile nucleus ; and in a similar manner the external posterior 

 column, or fasciculus cuneatus, ends in the median and lateral masses of the 

 cuneate nucleus. As we have seen, the white matter of these columns dim- 

 inishes as the nuclei increase; and the nuclei after absorbing, so to speak, the 

 white matter diminish in turn ; the ascending degeneration observed in these 

 columns stops at these nuclei. It is a suggestive fact that as these nuclei 

 diminish forward the restiform body increases in bulk. 



The remaining fibres of the cord, belonging partly to the anterior column 

 and partly to the lateral column, not gathered into any of the above-men- 

 tioned tracts, appear to end, chiefly at all events, in the reticular formation 

 of the bulb itself, though some are carried on to the higher parts of the 

 brain. 



525. Thus of the various tracts or strands of the spinal cord two only 

 are known definitely and certainly to pass as conspicuous unbroken strands 

 through the bulb to or from higher parts ; namely, the pyramidal tract to 

 the cerebrum and the cerebellar tract to the cerebellum. All, or nearly all, 

 the rest of the longitudinal fibres of the cord reaching the bulb end, as far 

 as we know at present, in some part or other of the bulb ; and we may infer 

 that some or other nerve-cells of the bulb serve as relays to connect these 

 fibres of the cord with other parts of the brain. 



The gracile and cuneate nuclei stand out conspicuously as relays of this 

 kind, and through them the posterior columns of the cord make secondary 

 connections on the one hand with the cerebellum, and on the other hand 

 with various regions of the cerebrum. We have said ( 519) that fibres 

 passing ventrally from the gracile and cuneate nuclei sweep in a curved 

 course through the reticular formation as the internal arcuate fibres (Fig. 

 132, /. a. i.~). The hindmost of these form the superior decussation already 

 referred to, as seen in sections of the fore part of and in front of the pyram- 

 idal decussation (Fig. 132, 3, Supra Py. dec.}. After decussating ventral 

 to the central canal, these fibres form an area called the inter-olivary layer 

 (Fig. 132, 4, in. oL), lying dorsal to the pyramids between two of the olivary 

 nuclei. This layer may be regarded as the hind end or beginning on each 

 side of a remarkable longitudinal strand called the fillet (Figs. 131, B. F., 

 133, F.), of the connections of which in the front part of the brain we shall 

 speak hereafter. Thus these two nuclei are the source of fibres which cross 

 to the other side of the bulb, and reaching the inter-olivary layer dorsal to 

 the pyramids run up to higher parts of the brain by the fillet. We may 

 add that the formation of the fillet is also probably assisted by fibres from a 

 tract which lies just dorsal to the inter-olivary layer, and is derived from the 

 anterior columns of the cord. Besides its fibres of descending degeneration 

 the anterior column contains fibres of ascending degeneration, and these rise 

 dorsally in the bulb to form the tract in question. Though the whole tract 

 is of some length, the component fibres appear to be short. 



The gracile and cuneate nuclei give rise also to other fibres which, though 

 also sweeping ventrally and crossing to the other side, do not, when they 

 reach the inter-olivary region, assume a longitudinal direction, as do the 

 fibres forming the fillet, but as external arcuate fibres (Fig. 132, /. a. e.) 



