CONNECTOR NEURONS OF THE NERVOUS SYSTEM 135 



posterior horn cells belonging to the somatic segmentation re- 

 main closely contiguous to the fibres of the ascending root of 

 the trigeminal. 



I have already in Chapter I sketched out the peculiarities of 

 the segmentation in this region as far as the voluntary nervous 

 system is concerned, and have come to the conclusion that the 

 cells of the connector neurons belonging to the splanchnic seg- 

 mentation are situated in the dorsal nucleus of the vagus, and 

 are continued into the spinal cord as a column of cells accom- 

 panying the fasciculus solitarius, the fibres of which terminate in 

 these cells (Fig. i). 



The cells in the dorsal nucleus of the vagus are not all of the 

 same kind, they form two distinct groups, a large-celled and a 

 small-celled group. The smaller cells were discovered by Sta- 

 derini and form a group of cells extending over the whole of 

 the dorsal nucleus, which have been called the nucleus intercalatus 

 of Staderini. We should naturally suppose that the small-celled 

 group gave origin to the small medullated fibres of the vagus 

 nerve, which form the connector fibres to the motor cells of the 

 involuntary muscles belonging to the vagus system. The evi- 

 dence available appears to support this view. The literature on 

 the subject is given by Kidd and Molhant ; the experimental 

 work consists mainly of degenerative sections of the vagus nerve 

 or spinal branches of it and the observation of signs of chroma- 

 tolysis in the bulbar cell groups belonging to the vagus nerve. 

 All observers are agreed that cells in the dorsal nucleus of the 

 vagus give origin to centrifugal fibres, which form connexions 

 with the motor cells of the enteral system, and more recent 

 work, as pointed out by Kidd, places these cells in that small- 

 celled part of the dorsal nucleus known as the nucleus inter- 

 calatus of Staderini. This nucleus then consists of the connector 

 cells of this region belonging to the involuntary nervous system. 

 It might therefore be considered, as Kidd holds, to represent 

 in the medulla the intermedio-lateral cell column so characteristic 

 of the thoracico-lumbar region of the cord. 



The arrangement for these direct or segmental reflexes in 

 the involuntary nervous system of the medullary region are of 

 the same kind as in the thoracic region of the cord. There 

 is however this difference ; in the medulla oblongata we are 

 dealing exclusively with the enteral muscular system as far as 



