1070 



HI MAN ANATOMY. 



gracilis and, even after the disappearance of the latter, continues as a striking collec- 

 tion of gray matter beneath the dorsal surface of the medulla, from which it is 

 separated by the posterior superficial arcuate fibres. Within the upper part of the 

 fasciculus cuneatus the gray matter becomes subdivided into two masses (Fig. 924), 

 the more superficial and continuous of which is called the nucleus cuneatus externus, 

 and the deeper and more broken one, the nucleus cuneatus intcrnus. 



Owing to the increased bulk of the fasciculi of the posterior area occasioned by the 

 appearance and expansion of the contained nuclei, the dorsal horns of the gray matter 

 are displaced laterally and forward, so that they come to lie on a level with the central 

 canal. Meanwhile the posterior cornua themselves, especially the capping substantia 

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

 matter that cause the dorso-lateral projections of the Rolandic tubercles seen on the 



FIG. 922. 



Nucleus gracilis 



Funiculus gracilis 

 Funiculus cuneatus _, / 



Spinal root of V IHTVC _^ 



^^9j 



Substantia gelatinosa-j3B^H 



Accessory olivary nucleus 





Antero-lateral ground-bundle^ 



Nucleus cuneatus 



rCentral gray matter 

 arcuate fibres 



Fibres of XII nerve 



Sensory decussation 



Anterior superficial arcuate fibres' 



Pyramidal tracts 



Transverse section of medulla at level C, Fig. 919, showing sensory decussation, posterior nuclei and 

 pyramidal tracts. X 5%. Preparation by Professor Spiller. 



surface. Beneath the latter and closely overlying the outer border of the extensive area 

 of the substantia gelatinosa, a crescentic tract of the longitudinally coursing nerve- 

 fibres marks the position of the descending root of the trigeminal nerve (Fig. 922). 

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

 long sensory tracts continued from the cord and the distribution of impulses so 

 received to the cerebellum and to the higher centres, it is evident that new paths of 

 the second order must arise within these nuclei. About on a level with the- upper limit 

 of the pyramidal or motor decussation, fibres emerge from the gracile and cuneate 

 nuclei, sweep forward ami inward in bold curves and cross the median raphe to the 

 opposite side < >f the medulla, immediately behind the pyramids (Fig. 922). They 

 then turn sharply upward and form the beginning of the important sensory pathway 

 known as the median fillet (Icmnisctis nicdialis i that connects the medullary nuclei 

 with the higher centres, as the superior corpora quadrigfemina and the optic thalamus. 

 The first fibres that emerge in this manner from the gracile and cuneate nuclei 

 constitute a fairly well delined strand to which the name sensory decussation or 

 decussation of the fillet is given. It must not be supposed, however, that with 

 this decussation the crossing ceases, for, quite the contrary, it is only the beginning 

 of an extended series of sensory fibres that pass across the raphe at various levels 

 throughout the brain-stem. As many longitudinally coursing fibres are encountered 

 by tho>e sleeping from side- to side, an interweaving of vertical and horizontal fibres 

 occurs, which results in the production of the characteristic formatio reticularis that 

 constitutes a large part of the medulla, as well as of the dorsal or tegmental portions 



