CHAPTER \ 11. 

 The Motor System. 



Somatic Efferent Column. 



ALL the somatic muscles (those derived from the myo- 

 tomes of the embryo) controlled by cranial nerves 

 are of a rather highly specialized character and their 

 nerve elements are consequently classified as special somatic 

 efferent, the general somatic efferent component being con- 

 fined to the spinal nerves. These special motor fibres are 

 found in the third, fourth, and sixth nerves, where they 

 control movements of the eyeball, and in the twelfth nerve, 

 where they regulate the movements of the musculature of the 

 tongue. 



The nuclei of these nerves are arranged in a linear series, 

 being parts of a single somatic efferent column which has 

 become broken into separate centres corresponding to the 

 nerves through which its fibres run. The oculomotor and 

 trochlear nuclei lie in the midbrain but the abducent and 

 hypoglossal centres are in the medulla oblongata. 



The position of the column corresponds primitively to 

 that of the ventral gray column in the spinal cord, of which 

 it is to be considered an anterior continuation. The location 

 of the cells, however, has undergone a change, there having 

 been a migration towards the most important source of 

 stimulation. In the case of the hypoglossal nucleus (XII), 

 the majority of stimuli are received, probably, from the 

 visceral afferent nuclei, and the nucleus has mov^ed antero- 

 dorsally and taken up a position near the floor of the ventricle, 

 close to these centres (Pis. VL-VIIL). Here it appears as a 

 slender, elongated mass of gray matter extending from the 



