426 PHYSIOLOGY 



to it. So great, however, is the complexity of this part of the nervous 

 system, and so involved are the genetic relations of the various nerves, 

 that it is difficult or impossible in many cases to state definitely the 

 spinal analogies of the various nerves. 



The cranial nuclei (of origin or termination) may be roughly classed 

 as follows : 



(1) Motor Somatic Nuclei. These consist of an almost continuous 

 column of multipolar cells, lying close to the middle line on each side 



ACUS] 



'ARCUATE 

 NUCLEUS 



XII. 



[HYPOCLOSSAL] 



FIG. 192. Cross-section of medulla showing nuclei of nerves x and xn. 

 (CUNNINGHAM.) 



in the floor of the fourth ventricle, the Sylvian iter, and the back 

 part of the third ventricle. From below upwards these groups of cells 

 give origin to the fibres of : 



(a) The hypoglossal nerve. 



(b) The sixth nerve. 



(c) The fourth nerve. 



(d) The third or oculo-motor nerve. 



(2) -Splanchnic Sensory Nuclei. Immediately outside the column 

 of motor cells is a column of grey matter which receives the termina- 

 tions of the afferent fibres belonging to the ninth, tenth, and eleventh 

 nerves, and is sometimes called the vago-glossopharyngeal-accessory 

 nucleus. This grey matter of course does not give rise to the fibres 

 of these nerves, which, like other sensory nerves, are axons of ganglion - 

 cells lying outside the central nervous system. 



