8 THE INVOL UNTAR Y NER VO US S YSTEM 



situated in the spinal region in the posterior horns (Fig. 2, A). We 

 must look for the corresponding cells in the cranial region in two 

 situations corresponding respectively to the posterior horns be- 

 longing to the somatic and splanchnic segmentations. The pos- 

 terior horn cells of the cord are characterized by the presence of 

 the substantia gelatinosa Rolandi close to them, and the character- 

 istic of the descending sensory root of the trigeminal (Fig. 3, A) is 

 the presence of the substantia gelatinosa Rolandi along its whole 

 length. In this substance are found cells with which the fibres 

 of this root continuously make connexion, called by Edinger the 

 end nucleus of the " ascending " root. Such cells clearly corre- 

 spond to a series of connector nuclei of the same kind as those 

 belonging to the voluntary nervous system in the segments of the 

 spinal cord, and form in my opinion the primary connector neurons 

 of the somatic segmentation. I imagine therefore that, as far as 

 the somatic segmentation is concerned, the primary or segmental 

 reflexes, which must take place in each cranial segment as well as 

 in each spinal one, are effected through these connector neurons, 

 as represented diagrammatically in Fig. 3, A. With respect to the 

 splanchnic segmentation (Fig. 3, B) in which the motor neurons 

 are found in the nucleus of the facial, nucleus ambiguus and the 

 accessory nucleus, and the sensory neurons in the ganglia on 

 the roots of the corresponding nerves, we must look for the 

 connector neurons in that part of the grey matter of the medulla 

 oblongata which continues into the spinal cord as the posterior 

 horn. 



The posterior horn cells belonging to the vagus segments in 

 the medulla oblongata have become part of the mass of cells in 

 the floor of the fourth ventricle, known as the dorsal nucleus of 

 the vagus, and according to Edinger the sensory roots of the 

 vagus terminate in many of these cells and in their continuation 

 as a cell column close along the "descending" root of the vagus 

 (the fasciculus solitarius). In fact this group of cells forms the 

 connector neurons belonging to the splanchnic segmentation in 

 exactly the same manner as the corresponding group of cells in 

 connexion with the sensory trigeminal fibres form the connector 

 neurons belonging to the somatic segmentation. 



I imagine therefore that, so far as the splanchnic segmentation 

 is concerned, the primary or segmental reflexes, which must take 

 place in each cranial segment as well as in each spinal one, are 



