SPINAL CORD AND COMMENCEMENT OF MEDULLA OBLONGATA. 193 



sl 10 wing that these fibres come from the opposite layer of the 

 fillet. Thus the diagram of a sensory spinal nerve will also 

 represent the course of the sensory vagus (and the glosso-phar- 

 yngeal as well) nerve, first nucleus in spinal ganglion ; root, 

 second nucleus (sensory vagus-nucleus), crossed ascending 

 central tract. 



The nucleus of the hypoglossal consists of several collec- 

 tions of ganglion-cells, which are united by a fine net-work. 

 From the large multipolar cells are given off fine processes, 

 which converge to form a number of nerve-rootlets. 



From the nucleus, just as from the anterior horn, are devel- 

 oped afferent fibres, which pass over across the median line. 

 They do not continue their course far on the opposite side, but, 

 after decussating, run toward the brain in the raphe and become 

 united with other fibres (from the facial nucleus) in the pons. 

 The whole fasciculus then passes into the pyramid of the crusta. 

 Although this course differs somewhat from that of the central 

 motor tracts in the spinal cord, still it is essentially the same 

 root, nucleus, crossed tract to the pyramids. 



The net-work connecting the different divisions of the hypo- 

 glossal nucleus is of peculiar interest ; we meet with nothing 

 like it in any of the other nerve-nuclei. We should also notice 

 that there is no other nerve whose fibres must act so harmoni- 

 ously and so nearly at the same instant, as the fibres of the 

 hypoglossal in the act of swallowing. Inasmuch as I have been 

 compelled to place before you to-day figures which are largely 

 diagrammatic, it will be well for you, at the close of this lecture, 

 to study carefully the accompanying representation of the hypo- 

 glossal nucleus. It was drawn by Koch, in all its wealth of 

 cells and fibres. He it was who first described the net-work. 

 Ventrad of this- nucleus lies another collection of cells (Roller's 

 hypoglossal nucleus), from which, however. I do not think that 

 hypoglossal fibres arise ; at least, not in man. 



The pyramidal decussation was discovered as early as 1710, by Francois 

 Petit. The olivary bodies were first described by Vieussens. The macroscopic 



