THE MEDULLA OBLONGATA. 



1069 



The effects of the decussation of the pyramidal tracts, assuming for convenience 



that the latter pass from below upward, are conspicuous when followed in consecutive 



transverse sections from 



Fig. 920. 



Funiculus gracilis 



Funiculus 

 cuneatus 



Substantia 



gelatinosa 



of posterior 



cornu 



Decussating 

 fibres 



Transverse section of medulla at level A, Fig. 919; beginning of pvramidai 

 decussation. Weigert-Pal staining. X 5}4- Preparation made by Professor 

 SpiUer. 



the spino-bulbar junction 

 cerebralward. The first 

 suggestion of the decussa- 

 tion appears (Fig. 920) as 

 strands of nerve-fibres, that 

 pass from the field of the 

 lateral pyramidal tract in 

 the lateral column obliquely 

 through the adjacent ante- 

 rior horn of gray matter and 

 across the bottom of the an- 

 terior median fissure to gain 

 the opposite anterior col- 

 umn. At a slightly higher 

 level, where the decussation 

 is fully established (Fig. 

 921), the large strands of 

 obliquely sectioned fibres 

 are seen cutting through the gray matter, pardy filling the median fissure, and collecting 

 on either side of the latter as the large ventral bundles which thence upward constitute 

 the prominent pyramidal fields. In consequence of the greater space required by 

 the pyramids, the isolated anterior horns of the gray matter, cut of? by the crossing 

 strands, and the adjacent anterior ground-bundle are displaced laterally and at first 

 lie to the outer side of the decussated fibres. Later, the ground-bundle assumes a 

 position behind the pyramid and eventually becomes continuous with the posterior 

 longitudinal fasciculus (page 11 16 j. The detached anterior cornu of the gray 

 matter is pushed outward and backward and gradually becomes broken up by and 

 interspersed among the fibres of the formatio reticularis. 



The Posterior Nuclei and the Arcuate Fibres. — The robust tracts of 

 white matter (nerve-fibres) prolonged into the gracile and cuneate funiculi from the 

 tracts of GoU and of Burdach become invaded by new masses of gray matter, the 

 nucleus gracilis and cuneatus. The gracile nucleus, the first encountered, begins 



as a narrow area of gray 

 matter within the correspond- 

 ing strand, on a- level with 

 the pyramidal decussation 

 (Fig. 921). It rapidly in- 

 creases in bulk, until it 

 not only invades the entire 

 funiculus gracilis, but also 

 joins the gray matter sur- 

 rounding the central canal. 

 The superficial stratum of 

 spinal fibres gradually dimin- 

 ishes as more and more of its 

 components end around the 

 cells of the gracile nucleus, 

 until, finally, all are inter- 

 rupted. Meanwhile the 

 cuneate nucleus appears 

 within the funiculus cuneatus 

 as a dorsally directed club- 

 shaped mass of gray mat- 

 ter (Fig. 922") which soon 

 becomes a prominent mottled area, sharply defined by the overlying stratum of 

 Burdach fibres. The cuneate nucleus extends to a higher level than the nucleus 



Fig. 921. 



Nucleus gracilis 



Central 



gray 



matter 



Funiculus 

 cuneatus 



Spinal 

 root of V 

 nerve 



_\- Substantia 

 iS<,) gelatinosa 



Fibres of 

 lateral 

 pyramidal 

 tract 



Isolated 

 anterior 

 cornu 



Pyramidal decussation 



Transverse section of medulla at level B, Fig. 919; pyramidal decus- 

 sation well established ; posterior cornua are displaced laterally by 

 posterior columns. X 5!^. Preparation by Professor Spiller. 



