132 THE INVOL UNTAR Y NER VO US S YSTEM 



segmentations, somatic and splanchnic, apply to the unstriated 

 as well as to the striated muscular groups. Just as the somatic 

 segmentation is manifest throughout the length of the ani- 

 mal by the segmental arrangement of the striated longitudinal 

 muscles of the trunk region, so also it is manifested in the same 

 region by the segmentally innervated unstriped pilo-motor 

 muscles. Since new appendicular segments were not formed 

 after the vertebrate stage had been reached, not only the striated 

 muscles of the gut receive their motor supply from the bulbo- 

 sacral nerves, forming the enteral nervous system, but also the 

 motor cells which send motor nerves to the corresponding un- 

 striped endodermal muscles have travelled out from the bulbo- 

 sacral regions of the cord. 



So far as connexions to the peripheral motor neurons are con- 

 cerned, the thoracico-lumbar outflow of connector nerves termin- 

 ates around the motor cells of the vascular and dermal muscles, 

 while the bulbo-sacral outflow terminates round the motor cells 

 of the endodermal muscles. What evidence is there for the 

 position of the cells in the central nervous system which respec- 

 tively give origin to these connector fibres ? 



THE CONNECTOR NEURONS OF THE THORACICO-LUMBAR 



REGION. 



It has long been known that the thoracic region of the cord is 

 distinguished from the regions above and below by the presence 

 of two distinct groups of nerve cells, of which the one forms a 

 marked lateral horn, with small cells, and the other forms the 

 cells of Clarke's column, with large cells. I pointed out in 1885 

 that the distribution of these two groups of nerve cells corre- 

 sponded closely with the distribution of the connector fibres to 

 the sympathetic nerve cells, and I suggested that the cells of the 

 lateral horn gave rise to these connector fibres. The truth of 

 this suggestion was confirmed by the experiments of Anderson, 

 who cut the cervical sympathetic nerve in young kittens, and 

 allowed the animals to grow up ; he then found a marked differ- 

 ence in the number of the cells of the lateral horn column in the 

 upper thoracic region on the two sides. The side on which the 

 cervical sympathetic nerve had been cut showed many fewer cells 

 than the intact side. Many observers since then have come to 



