MEDULLA, PONS, AND MIDBRAIN. 289 



together with bundles from the olive and the vestibular nuclei, 

 make the inferior peduncle. It passes upward on the surface 

 of the medulla, between the cochlear and vestibular nuclei, to 

 the lower end of the pons, where it turns dorsalward, enters 

 the cerebellum, and decussates in the roof of the fourth ven- 

 tricle. In passing dorsalward it lies just outside the dentate 

 nucleus, which receives the superior peduncle. The superior 

 peduncle starts in the red nucleus. Its fibres decussate in the 

 dorsal part of the pons, and enter the cerebellum just internal 

 to the inferior peduncle. The fibres of the middle peduncle 

 come from the pontal nuclei. They decussate in the pons 

 and enter the cerebellum external to the inferior peduncle. 



Group 3. In the cord the gray matter between the two 

 horns is much broken by fibres, and is called the format 'io 

 reticularis. It is a reflex centre. In the brain-stem it is 

 greatly developed. It occupies the dorsal half of each region. 

 It contains countless cells and fibres. The fibres are either 

 scattered or in more or less definite bundles. One short patli 

 in the brain-stem is very distinct, the posterior longitudinal 

 bundle. It receives fibres from the ventral column of the cord 

 and lies just ventral to the central canal throughout the brain- 

 stem. It receives descending fibres from the midbrain. 



Group 4- The nuclei of the motor cerebral nerves are de- 

 rived from the ventral horn ; of the sensory, in part from the 

 dorsal horn. Of the motor nuclei (Fig. 217), four lie near the 

 raphe just ventral to the central canal. They are the hypoglossal 

 in the medulla, the abducens in the pons, and the trochlear and 

 oculomotor in the midbrain. The fibres of all these nerves, 

 except the troclilear, pass ventralward and emerge near the 

 median line. The fibres of the trochlear nerve pass dorsal- 

 ward, decussate in the velum, and pass out near the median 

 line. The other four motor nerves, the spinal accessory, the 

 glossopharyngeal and vagus together, in the medulla, the 

 facial and the trigeminal in the pons, lie farther lateral and 

 ventral. The fibres of all these nerves, except the trigeminal, 

 pass inward toward the central canal, there turn outward and 

 ventralward to emerge on the ventral surface in a lateral line. 

 in 



