CH. XLIV.] 



INTERNAL STRUCTURE OF BULB 



633 



nucleus cuneatus. These nuclei form a most important position of 

 relay in the course of the afferent fibres from cord to brain. The 

 new fibres (the second relay of the sensory spinal path) arising from 

 the cells of these nuclei pass in a number of different directions, and 



n.c 



n.c' 



FIG. 462. Transverse section of the medulla oblongata in the region of the superior decussation. a.mj., 

 anterior median fissure ;/.., superficial arcuate fibres; py., pyramid; n.a.r., nuclei of arcuate 

 fibres ; /.ai, deep arcuate fibres becoming superficial; o, o', lower end of olivary nucleus; n.l., 

 nucleus lateralis; f,r., formatio reticularis; /.a 2 , arcuate fibres proceeding from the formatio 

 reticularis; g, substantia gelatinosa of Rolando; d.V., descending root of fifth nerve; /.c., 

 funiculus cuneatus; n.c., nucleus cuneatus; n.c.', external cuneate nucleus; n.g., nucleus 

 gracilis; f.g., funiculus gracilis; p.mj., posterior median fissure; c.c., central canal surrounded 

 by grey matter, in which are n.XL, nucleus of the eleventh and n.XII., nucleus of the twelfth 

 nerve ; s.d., superior decussation (decussation of fillet). (Modified from Schwalbe.) 



break up the rest of the grey matter into what is called the formatio 

 reticularis. 



The nucleus gracilis and nucleus cuneatus are often spoken of as 

 the posterior column nuclei ; they do not receive all the ascending 

 branches of the posterior root fibres, for a number of these branches 

 have already entered the grey matter and arborised amongst its cells 

 in the spinal cord itself. The cells of the posterior column nuclei 

 are of moderate size, and their axons pass as internal arcuate fibres 

 into the reticular formation between the two olivary bodies, which 

 is known as the inter-olivary layer. They cross the median raphe 

 dorsal to the pyramids, and then turn upwards towards the upper 

 parts of the brain, and so constitute what is known as the fillet. In 

 the higher parts of the bulb and pons, this tract is reinforced by 

 fibres from the cells of the sensory nuclei of the cranial nerves. 

 The fillet becomes a longitudinal bundle, which passes upwards to 



