284 



NORMAL HISTOLOGY. 



size; some are small, with dendrites ramifying close to the cell-body, and 

 others, both large and small, with widely spreading branched processes. 

 Meanwhile the cuneate nucleus appears within the fasciculus cuneatus as 

 a club-shaped mass of gray matter, which soon becomes a prominent mottled 

 area. This nucleus, also composed of neurones with contracted or with widely 

 spreading dendrites, extends to a higher level than the nucleus gracilis. 



Owing to the increased bulk of the fasciculi of the posterior area occa- 

 sioned by the appearance and expansion of the gracile and cuneate nuclei, 

 the dorsal horns of the gray matter are displaced laterally and ventrally, so 

 that they come to lie on a level with the central canal. Moreover, the 

 posterior cornua themselves, particularly the capping substantia gelatinosa, 



FIG. 331. Section across medulla a few millimeters above level A, Fig. 329, showing increased pos- 

 terior nuclei and substantia gelatinosa, sensory decussation and pyramidal tracts, i, fasciculus gracilis; 

 a, fasciculus cuneatus ; 3, spinal root of fifth nerve ; 4, substantia gelatinosa ; 5, accessoiy olivary nucleus; 

 6, fasciculus antero-lateral ground-bundle ; 7, superficial arcuate fibres ; 8, pyramidal tracts ; 9, sensory 

 (fillet) decussation ; 10, fibres of twelfth nerve ; n, deep arcuate fibres ; 12, central gray matter, surround- 

 ing canal ; 13, nucleus cuneatus ; 14, nucleus gracilis. X 5X- (Preparation by Professor Spiller.) 



gain materially in bulk and now appear as two club-shaped masses of gray 

 matter (Fig. 331) that cause the dorso-lateral projections, the tubercula, 

 Rolandi, seen on the surface. Beneath these elevations and closely over- 

 lying the areas of the substantia gelatinosa, crescentic tracts of longitudinally 

 coursing nerve-fibres mark the position, one on each side, of the spinal roots 

 of the trigeminal nerves. 



The chief purpose of the gracile and cuneate nuclei being the reception 

 of the long afferent tracts continued from the cord and the distribution of 

 impulses so received to the cerebellum and to the higher centres, new paths 

 arise within these nuclei. About the level of the upper limit of the pyramidal 

 decussation, axones of their neurones emerge from the gracile and cuneate 

 nuclei as the deep arcuate fibres, sweep forwards and inwards in bold curves 

 and cross the mid-line to the opposite side of the medulla. Most of them 

 then turn sharply upwards and form the beginning of the important sensory 

 pathway known as the median fillet (lemniscus media/is}. The lowest 

 fibres that cross in this manner constitute a fairly well defined strand, the 

 sensory decussation or decussation of the fillet. The crossing does not cease 



