64 THE IN VOL UNTA R Y NER VO US S YS TEM 



We see that not only is the involuntary nervous system built 

 up on the same plan as the voluntary nervous system, but also it 

 innervates a double arrangement of unstriped musculature of the 

 same character as the double arrangement of striated musculature 

 seen in the somatic and splanchnic segmentations ; for clearly the 

 unstriped muscle under the skin, which forms the sympathetic or 

 epidermal musculature, belongs to the somatic segmentation, 

 and equally clearly the endodermal musculature belongs to the 

 splanchnic segmentation. Naturally, on the theory put forward 

 by me of the origin of vertebrates, the original animal, from 

 which the vertebrates arose, possessed a double segmentation 

 throughout its whole length ; a body segmentation and an ap- 

 pendage segmentation. The muscles of these two segmentations, 

 derived originally in the segmented annelids from unstriped 

 muscle, became in part striated and voluntary and partly re- 

 mained unstriped. The unstriped muscles of the somatic seg- 

 mentation form the sympathetic or epidermal musculature, and 

 those of the appendage segmentation the endodermal musculature, 

 which groups directly with the endodermal striated musculature 

 of the mesosomatic region. The motor neurons of these two 

 musculatures show a corresponding differentiation and agreement 

 with those of the striated muscles. The motor neurons of the 

 unstriped muscles belonging to the somatic segmentation are the 

 motor cells and motor fibres of the so-called sympathetic system, 

 which were originally in close contiguity with the posterior root 

 ganglia of the spinal nerves, belonging to the somatic segmenta- 

 tion ; while the corresponding motor neurons of the splanchnic 

 segmentation are the motor cells and motor fibres of the ' en- 

 teral ' * system, which were originally in close contiguity with 

 the posterior root ganglia of the bulbar and sacral nerves, be- 

 longing to the splanchnic segmentation. Finally, the involuntary 

 muscles of these two segmentations are most markedly differen- 

 tiated from each other by the action of adrenalin on the one hand, 

 which causes contraction of the vasodermal or sympathetic 

 system of muscles, and of acetyl-choline on the other, which 

 causes contraction of the endodermal system of muscles. 



* A well-defined system of nerve cells and nerve fibres is understood universally 

 by the term sympathetic nervous system. As we have seen, the main characteristic 

 of the nerve cells of this system is to send motor fibres to the vascular and dermal 

 muscles. At present there is no term to express that similar nervous system, the 

 main characteristic of which is to send motor fibres to the endodermal muscles. I 

 propose to call it the ' enteral ' nervous system. 



