46 Anatomy of the Nervous System 



level of the decussation of the pyramids to a point some little 

 distance forward under the floor of the fourth ventricle. 

 Several different groups of cells, which are related to special 

 muscles or sets of muscles, are distinguishable in the nucleus. 

 The hypoglossal root fibres are arranged in small bundles 

 which pass directly ventrally and a little laterally to emerge 

 at the outer edge of the pyramid (pp. 12, 53). 



The nucleus ahdttcens (VI) lies considerably farther 

 forward, having migrated from a position between the levels 

 of the nuclei VII and IX to one in front of the nucleus VII. 

 This change probably took place largely under the influence 

 of fibres from the anterior parts of the medial and lateral 

 vestibular nuclei running chiefly in the medial longitudinal 

 bundle. Having reached this more anterior position, it has 

 farther moved dorso-laterally in many mammals, so as to lie 

 still nearer these centres. This is the condition seen in man, 

 where the nucleus VI lies near the floor of the fourth ventricle 

 and lateral to the genu of the VII nerve root (p. 48), as it 

 does also in the rabbit. In the rat, however, the nucleus has 

 not undergone the lateral movement, but appears as a small, 

 rather scattered group of cells wedged in between the genu 

 of the VII root and the medial longitudinal fasciculus and 

 extending a short distance ventrally along the side of the 

 latter (PI. X.). Its root fibres pass out directly in a course 

 exactly parallel to that of the XII root fibres farther back. 



Visceral Efferent Nuclei. 



The visceral efferent centres, like the somatic, may be 

 considered to be an anterior continuation of the corresponding 

 column in the spinal cord. They have become divided into 

 separate general and special columns, however, and the 

 changes of location undergone by the individual nuclei are 

 more diverse than are those of the somatic centres. 



General visceral efi^erent fibres occur in the roots of 

 nerves III, MI, IX, X, and XI, whence they run into 



