THE MEDULLA OBLONGATA. 



283 



features displayed by sections of the medulla at different levels depend upon 

 the changes induced chiefly by four factors: fi) the decussation of the 

 pyramidal tracts, (2) the appearance of the dorsal nuclei, (3) the production 

 of the formatio reticularis, and (4) the opening out of the fourth ventricle. 



The decussation of the pyramidal tracts, assuming for conven- 

 ience that all fibres pass from below upwards, produces conspicuous changes 

 when followed in consecutive sections from the spino-bulbar junction upwards. 

 The decussation first appears as strands of fibres that pass from the field of 

 the lateral pyramidal tract, in the lateral column of the cord, obliquely 

 through the adjacent anterior horn of gray matter and across the bottom of 

 the anterior median fissure to gain the opposite anterior area of the medulla. 

 At a slightly higher level (Fig. 330), where the decussation is fully estab- 



Nucleus gracilis 



Isolated 

 anterior 

 cornu 



Pyramidal decussation 



FIG. 330. Section across medulla at level A, Fig. 329 ; fibres of pyramidal decussation almost fill anterior 

 median fissure; posterior horns are displaced laterally by increased posterior columns. X5J4- (Prepa- 

 ration by Professor Spiller.) 



lished, the large strands of obliquely sectioned fibres are seen cutting 

 through the gray matter, partly filling the median fissure, and collecting on 

 either side of the latter as the large ventral bundles which thence upwards 

 constitute the prominent pyramidal fields. In consequence of the greater 

 space required by the pyramids, from 8090 per cent, of the fibres having 

 crossed, the isolated anterior horns of the gray matter (cut off by the 

 crossing strands) and the adjacent anterior ground-bundles are displaced 

 laterally. Higher, the ground-bundle assumes a position behind the pyramid 

 and eventually becomes continuous with the posterior longitudinal fasciculus. 

 The detached anterior cornu is pushed outwards and backwards and gradually 

 becomes broken up by and interspersed among the fibres of the formatio 

 reticularis. 



The posterior nuclei include two new masses of gray matter, the 

 nucleus gracilis and the nucleus cuneatus (Fig. 331), into which the long 

 posterior tracts of the cord (Goll and Burdach) are prolonged. The 

 gracile nucleus, the first encountered in passing upwards, begins on a level 

 with the pyramidal decussation and rapidly increases in bulk until it invades 

 the entire fasciculus gracilis. The superficial stratum of spinal fibres, pro- 

 longed from Coil's tract, gradually diminishes as more and more of its com- 

 ponents end in arborizations around the cells of the gracile nucleus, until, 

 finally, all are interrupted. These neurones are multipolar and of varying 



