304 THE RHOMBENCEPHALON. 



The nucleus lateralis inferior (Figs. 92 and 94) is situated in 

 the reticular substance of the lateral area. Though it is made 

 up of large cell-bodies, they are so scattered among the fibers of 

 this region that the nucleus is invisible to the naked eye. It prob- 

 ably gives origin to descending fibers which are continued both 

 in the anterior and lateral column of the spinal cord (Tschermak). 

 It is known to receive fibers from the anterior ascending cerebello- 

 spinal tract and from the nuclei funiculi gracilis and funiculi 

 cuneati, and it gives rise to the ascending tract already traced to 

 the cerebellum, which probably transmits tactile, muscular, pain 

 and temperature impressions to the cerebellar cortex. 



The nucleus ambiguus (Fig. 93) forms an irregular sheet of 

 gray substance, pear-shaped in section, which extends longitu- 

 dinally through two-thirds of the medulla. It is prolonged from 

 near the ventricle ventro-lateralward into the substantia reticu- 

 laris. It is not visible to the naked eye. Its axones form the 

 cerebral root of the eleventh nerve and probably a part of the motor 

 roots of the tenth and ninth nerves. The root fibers of the acces- 

 sory nerve run out through the posterior lateral sulcus below the 

 level of the olive. 



The nucleus of the ala cinerea (Fig. 93) is in part derived from 

 the base of the anterior columna; this part is motor in func- 

 tion and its neurones resemble those of the anterior columna in 

 the cord. It belongs to the ninth and tenth nerves. It is situated, 

 above, close to the floor-ependyma of the fourth ventricle under 

 the ala cinerea; and it extends, inferiorly, into the closed medulla 

 nearly as far as the hypoglossal nucleus. It is immediately lateral 

 to the nucleus of the hypoglossal. Its axones run in a curve, 

 convex toward the median plane, between the restiform body 

 and the olive. They are joined medially by those axones of the 

 nucleus ambiguus which enter the roots of the ninth and tenth 

 nerves. By this nucleus and the nucleus ambiguus, many dis- 

 tinct fascicles are formed, belonging to the roots of the ninth, tenth 

 and the cerebral part of the eleventh nerves. They run in slightly 

 different planes, but all of them emerge in the region of the posterior 

 lateral sulcus of the medulla (Fig. 85). Intermingled with the 

 motor cells of the nucleus alse cinereae, there are the small spindle 



