594 



THE NEKVOUS SYSTEM. 



Nervus Hypoglossus. The nucleus of origin of the hypoglossal nerve, the 

 motor nerve of the tongue, lies in the substance of the medulla oblongata. It is 

 composed of several groups of large multipolar cells, which closely resemble the 

 cells in the anterior column of gray matter in the spinal medulla, and is 

 pervaded by an intricate network of fine fibrils. In form it is elongated aiw 

 rod-like, and in length it is about 18 mm. It extends from a point immediately 

 above the decussation of the pyramids up to the level of the striae medullares. 

 The inferior portion of the nucleus is thus placed in the closed part of the medulla 

 oblongata (Fig. 494, p. 561), whilst its superior part is situated in the open part 

 (Fig. 495, p. 561). The former lies in that part of the central gray matter which 

 is continuous with the basal part of the anterior column of gray matter of the 



Optic nerve 



Optic chiasma 



Optic tract 

 Basislpedunculi cerebri^ 





xlnfundibulum (cut) 



Tuber cinereum "-.. 

 ^Corpus mamillare 



Substantia perforata posterior 

 Oculomotor nerve 



'Trochlear nerve 



Motor root of 

 trigeininal nerve\ 



Sensory root of^ 

 trigeminal ~ 



V-3/ 



Fasciculus obliquus pontis ~~~jf, "TJf 



Nervus intermedius "S. 

 Acoustic nerve 3 



Flocculus cerebelli- 

 Chorioid plexus in the 

 apertura lateralis of-j* 

 the fourth ventricle ' ' 



Lateral recess of /*- ^f* 

 fourth ventricle 



Facial nerve 



'Acoustic nerve 

 " Nervus intermedius 

 Glossopharyngeal nerve 



Vagus nerve 



Olive 

 Pyramid' 



Decussation of pyramidsr-~ 



Accessory nerve 

 Hypoglossal nerve 



^Spinal root of accessory nerve 

 ^First spinal nerve 



FIG. 527. 



-THE VENTRAL ASPECT OP THE MEDULLA OBLONGATA,: PONS, AND MESENCEPHALON, 

 showing the nerve roots. 



spinal medulla. It is thus placed on the anterior and lateral aspect of the central 

 canal, close to the median plane and the corresponding nucleus of the opposite side. 

 The superior part of the nucleus occupies a position in the gray matter on the floor 

 of the fourth ventricle, subjacent to the medial part of the surface area, which has 

 been described under the name of the trigonum hypoglossi. Within the nucleus 

 the axons of the cells arrange themselves in converging bundles of fine fibres, which 

 come together and leave the ventral aspect of the nucleus as the fila radicularia of the 

 nerve. The nerve bundles thus formed traverse the entire antero-posterior thickness 

 of the medulla oblongata, between the substantia reticularis grisea and the substantia 

 reticularis alba, and emerge on the surface, in linear order, at the bottom of the 

 furrow between the olive and the pyramid. After they emerge these fibres collect 

 to form three definite bundles like the anterior nerve-roots of three spinal nervee 

 (Fig. 527). In the substance of the medulla oblongata the fila radicularia of the 



