798 



THE NERVOUS SYSTEM 



pyramidal tract. The majority of the latter fibres decussate gradually in the com- 

 missural bundle and in the ventral white commissure of the cord as they approach 

 the levels of their termination. The proportion of fibres crossing in the chief decus- 

 sation varies. Cases have been noted in which apparently the entire pyramids 

 decussate at this level. In other cases the direct or ventral pyramidal tract may be 



CENTRAL GREY SUBSTANCE 

 CENTRAL CANAL 



FIG. 592. TRANSVERSE SECTION OF MEDULLA OBLONGATA AT THE LEVEL OK THE DECUSSATION 



OF THE PYRAMIDS. 



NUCLEUS OF FUNICULUS GRACILIS 

 FUXK'ULUS CUXKA TUS 



SUBSTANT1A GELAT1XOSA (ROLANDI) 



. SPINAL TRACT OF TRIGEMIXfS 



SI'IX. I /. i -KREBELLO- 

 FASC1CULU8 



GI> WEBS' TRACT 



LATERAL CF.RRHRO-SP1NAL 

 FASCICU1. I S 



VENTRAL HORN 

 DECUSSATION OF PYRAMIDS 

 PYRAMID 



much larger than usual, at the expense of the lateral. The decussation usually 

 appears to be symmetrical and it occurs so suddenly that the fibres, in coursing from 

 the ventral to the lateral positions, detach the tips of the ventral horns of the spinal 

 cord from the remainder of the grey figure, and these appear as isolated, irregu- 

 larly shaped masses of grey substance in transverse sections of the medulla. .From 

 this level upwards the outline of the grey figure of the cord is lost, and the cell- 



FIG. 593. TRANSVERSE SECTION OP MEDULLA OBLONGATA AT LEVEL OF THE DECUSSATION OF 



THE LEMNISCI. 



POSTERIOR MEDIAN FISSURE 



CENTRAL GREY SUBSTANCE 

 NUCLEUS OF HYPOGLOSSUS -, ' 

 INTERNAL ARCUA TE FIBRES 



ROOT FILUM OF 

 HYPOGLOSSUS -' 



NUCLEI'S III-' INFERIOR 

 1 1 LIVE 



MEDIAL ACCESSORY OLIVARY' 

 NUCLEUS 



NUCLEUS OF FUNICULUS GRACILIS 



i COM3IISSURAL NUCLEUS OF ALA CINEREA 

 M'CI.EUS OF FUXICULUS CUNEATUS 



, DORSAL EXTERNAL ARCUA TE 

 FIBRES 



/NUCLEUS OF SPINAL 



TI:ACT OF TRHH-:MI.\US 



SPINAL TRACT OF 



TBIQBMINVB 



... RESTIFORM BODY 

 XUCLEUS LATER ALIS 



- St'BKTANTIA RETICULARIS 



*- VENTRAL l-:.\'ll'l:\.\L ARCUATE 

 FIBRES 



DECUSSA TION OF LEMNISCI 



RAPHE 



PYRAMID 



columns of the ventral horns occur in more or less detached groups as the motor 

 nuclei of the cranial nerves. 



The origin and decussation of the lemnisci (fillet) begins immediately above 

 the decussation of the pyramids, and here the arrangements characteristic of the 

 spinal cord are further modified. The dorsal portion of the grey figure of the cord 

 is manifest up to this level, but here, after a considerable increase in its thickness, 



