668 



STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLTI. 



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 



n.e 



n.c 



a.m.f. 



PIG. 411. Transverse section of the medulla oblongata in the region of the superior decussation. a.m./., 

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

 fibres; /.a 1 , deep arcuate fibres becoming superficial; o, of, lower end of olivary nucleus; n.L, 

 . nucleus lateralis; f.r., fprmatio reticularis; /.o2, arcuate fibres proceeding from the formatio 

 reticulans; g, substantia gelatinosa of Rolando; d.V., descending root of fifth nerve; f.c., 

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

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

 Dy grey matter, in which are n.XL, nucleus of the eleventh and n.XIL, nucleus of the twelfth 

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



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 

 the optic thalamus, which forms the next cell-station on the path of 

 the sensory impulses to the cortex. 



Other points to be noticed in the section are the substantia 

 gelatinosa of Eolando (g) (representing the tip of the posterior cornu 

 of the cord), now separated from the surface by the descending root 

 of the fifth nerve (d. V.) ; the lateral nucleus (n.l.) (remains of the 



