950 THE FIBRES OF THE BULB. [BOOK in. 



of the connections of which in the front part of the brain we shall 

 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 interolivary 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. 109, f. a. e.) pursue a course which is 

 at first ventral along the side of the anterior fissure and then 

 lateral over the ventral surface of the pyramid and olivary nucleus, 

 by which path they reach the lateral surface of the bulb, and so 

 the restiform body and cerebellum. In this way, the two nuclei 

 in question contribute to the restiform body of the opposite side 

 of the bulb. These external arcuate fibres, which as they sweep 

 round the ventral surface of the pyramid traverse the arcuate 

 nucleus, though they vary much in individual brains, form a 

 considerable portion of the white matter seen on the ventral and 

 lateral surfaces of the bulb ; it is by them that the olivary nucleus 

 is covered up. 



The cuueate and gracile nuclei, besides this crossed and 

 somewhat roundabout connection with the restiform body of the 

 opposite side, are believed to have more direct connection with 

 the restiform body of the same side by means of fibres which 

 pass by a more or less direct lateral path from them to it. 

 Accepting this view we may say that the two nuclei are connected 

 with the opposite side of the cerebellum by external arcuate 

 fibres, and with the same side of the cerebellum by the other 

 ^fibres just mentioned. In any case the connection between the 

 two nuclei and the cerebellum is large and important. 



Thus the important strand of fibres which is called in the bulb 

 the restiform body, &nd higher up the inferior peduncle of the 

 cerebellum, is connected with the spinal cord in two chief ways : 

 directly by means of the cerebellar tract, and indirectly by means 

 of the cuneate and gracile nuclei which, as we have said, diminish 

 in bulk forwards as the restiform body increases. By the relay of 

 the gracile nucleus it is brought into connection with the median 

 posterior column along the whole length of the cord, and so with 

 that division of the posterior roots which ( 577) in each of the 

 several spinal nerves goes to form that column. By the relay of 



