HISTORY OF THE INVOLUNTARY NERVOUS SYSTEM 5 



corresponding motor neurons of the somatic segmentation and 

 supply in this region only dorso-ventral muscles. It is a great 

 mistake to call this bundle of motor fibres the descending root 

 of the trigeminal without adding the word motor ; the right dis- 

 tinction between the two sets of trigeminal fibres called at 

 present the " descending" and "ascending" roots is "descending 

 motor" and "descending sensory" roots; for the latter root 

 behaves with respect to the somatic segmentation in exactly 

 the same manner as the fasciculus solitarius, which is called 

 the descending root of the vagus, with respect to the splanchnic 

 segmentation. 



Similarly, in the mesosomatic region (Fig. i) the groups of 

 motor cells, known as the facial nucleus and the nucleus ambiguus, 

 represent the motor neurons of the splanchnic segmentation, and 

 represent therefore the motor neurons of the muscles of the meso- 

 somatic appendages ; they are quite separate from the nucleus of 

 the abducens, which supplies motor fibres to the only remaining 

 dorso-ventral muscles, a pair of which originally existed in each 

 segment, and from the hypoglossal nucleus containing the motor 

 cells of the longitudinal somatic muscles. These mesosomatic 

 groups also extend down the cord ; the splanchnic group being 

 represented by the nucleus accessorius or the lateral horn of the 

 cervical region, which is formed from the lateral cell groups of 

 the anterior horn ; the somatic group by the cells of the anterior 

 horn, supplying motor fibres to the longitudinal trunk muscles. 



Thus the cells of the motor neurons of the voluntary system 

 form two well-defined groups in accordance with the double 

 segmentation of the striated musculature in the cranial region. 



On the sensory side (Fig. i) there are also two distinct sets of 

 sensory fibres represented in the double segmentation, belonging 

 respectively to the somatic and splanchnic segments. In the pro- 

 somatic region the sensory neurons for both segmentations are 

 found in the Gasserian ganglion ; but in the mesosomatic region 

 the somatic sensory neurons belong mainly to the trigeminal and 

 are also found in the Gasserian ganglion, while the splanchnic 

 sensory neurons are found in the sensory ganglia on the facial, 

 glosso-pharyngeal, and vagus nerves. So also there must be 

 corresponding connector neurons for these two sets of segments, 

 in order to carry out the primary or segmental reflexes, similar to 

 those in the trunk region. These primary connector neurons are 



