

THE HIND-BRAIN OR RHOMBENCEPHALON 



779 



this strand is named the medial longitudinal fasciculus, and will be again re- 

 ferred to. 



Gray Substance of the Medulla Oblongata (Figs. 694, 695). In addition to the 

 gracile and cuneate nuclei, there are several other nuclei to be considered. Some 

 of these are traceable from the gray substance of the medulla spinalis, while others 

 are unrepresented in it. 



1. The hypoglossal nucleus is derived from the base of the anterior column; 

 in the lower closed part of the medulla oblongata it is situated on the ventro- 



lateral aspect of the central canal; but 

 in the upper part it approaches the 

 rhomboid fossa, where it lies close to 

 the middle line, under an eminence 

 named the trigomim hypoglossi (Fig. 709). 

 Numerous fibers connect the two nuclei, 

 both nuclei send long dendrons across the 

 midline to the opposite nucleus; commis- 

 sure fibers also connect them. The nucleus 

 measures about 2 cm. in length, and con- 



8 



FIG. 692. Superior terminations of the posterior fas- 

 ciculi of the medulla spinalis. 1. Posterior median 

 sulcus. 2. Fasciculus gracilis. 3. Fasciculus cuneatus. 

 4. Gracile nucleus. 5. Cuneate nucleus. 6, 6', 6". 

 Sensory fibers forming the lemniscus. 7. Sensory 

 decussation. 8. Cerebellar fibers uncrossed (in black). 

 9. Cerebellar fibers crossed (in black). (Testut.) 



FIG. 693. Transverse section passing through the 

 sensory decussation. (Schematic.) 1. Anterior median 

 fissure. 2. Posterior median sulcus. 3, 3. Head and 

 base of anterior column (in red) . 4. Hypoglossal nerve. 

 5. Bases of posterior columns. 6. Gracile nucleus. 7. 

 Cuneate nucleus. 8, 8. Lemnisous. 9. Sensory decus- 

 sation. 10. Cerebrospinal fasciculus. (Testut.) 



sists of large multipolar nerve cells, similar to those in the anterior column of the 

 spinal cord, whose axons constitute the roots of the hypoglossal nerve. These 

 nerve roots leave the ventral side of the nucleus, pass forward between the white 

 reticular formation and the gray reticular formation, some between the inferior 

 olivary nucleus and the medial accessory olivary nucleus, and emerge from the 

 antero-lateral sulcus. 



2. The nucleus ambiguus (Figs. 696, 697), the somatic motor nucleus of the glosso- 

 pharyngeal, vagus and cranial portion of the accessory nerves, is the continuation 

 into the medulla oblongata of the dorso-lateral cell group of the anterior column 

 of the spinal cord. Its large multipolar cells are like those in the anterior column 

 of the cord ; they form a slender column in the deep part of the f ormatio reticularis 

 grisea about midway between the dorsal accessory olive and the nucleus of the 

 spinal tract of the trigeminal. It extends from the level of the decussation of the 

 median fillet to the upper end of the medulla oblongata. Its fibers first pass back- 

 ward toward the floor of the fourth ventricle and then curve rather abruptly 

 lateralward and ventrally to join the fibers from the dorsal nucleus. 





