24 GRAY SUBSTANCE OF THE MEDULLA OBLONGATA. 



CHAPTER V. 



THE VAGUS NUCLEUS AND HOOTS. 



The Nucleus. — Stilling^ was the first to point out the exact locality of this nucleus, 

 and to give an accurate description of its form. The vagal nuclei are, as both 

 Clarke and Stilling have shown, an upward extension of the vesicular columns from 

 which the spinal accessory nerves arise, and in the lower portion of the medulla 

 share so closely the characteristics of the latter, that it is quite impossible to deter- 

 mine any line of demarcation. 



The roots of the vagus appear to be first given ofi^, soon after the central canal 

 opens into the fourth ventricle, and the two nuclei, which are at first joined by the 

 transverse commissure forming the posterior boundary or roof of the central canal, 

 are shortly after entirely separated, and he on each side of the hypoglossal nuclei 

 on the floor of the ventricle. 



The form of the vagus nucleus is much the same as that of the hypoglossal, with 

 the exception that it is bifurcated at the apex, forming two spurs or processes, 

 between which run numerous thick and very conspicuous longitudinal fasciculi, 

 which are the upward extension of those penetrating the spinal accessory nucleus. 

 Most of the nerve roots appear to be given oif from the inner process, a few only 

 passing from the outer or posterior spur, which is brought into very close connection 

 with the post-pyramidal and restiform nuclei, into which it is as it were wedged, 

 forming in the upper part of the medulla a starting point for numerous arciform 

 fibres (Plate XIII, Fig. 6"). 



The changes in form which the vagal nuclei undergo as they ascend are but slight, 

 and may be readily studied in Plate XV from the human medulla, and in Plate 

 XIII from that of the sheep. 



The changes in position of the nuclei have already been sufficiently dwelt upon 

 in the chapters on the morphology of the medulla, the most important of these 

 changes consisting in the gradual development of a new pyramidal mass formed 

 partly out of the substance of the vagus, and partly from the post-pyramidal nucleus, 

 by means of which the nucleus of the vagus is pushed more and more forwards, till 

 it becomes blended with the mass noAv constituting the great pyramidal nucleus of 

 the auditory, the base of which forms almost the entire floor of the fourth ventricle. 



The ceUs of the vagus nucleus closely resemble those of the spinal accessory, 

 being rather more oval or fusiform than those of the hypoglossal. They are exceed- 



* Medulla Oblongata.. 



