7 4 THE INVOL UNTAR Y NER VO US S YSTEM 



and commence at a marked line of constriction between the 

 cardium and the antrum which is known as the incisura cardiaca. 

 According to his investigations the cardiac end of the stomach, 

 inclusive of the cardiac sphincter, is innervated by both motor 

 and inhibitory fibres in connexion with the vagus nerve alone, 

 while the pyloric end has a sympathetic as well as a vagus supply. 

 The present evidence therefore points to the incisura cardiaca as 

 the upper limit of the sympathetic supply. 



The peculiarities of the innervation of the muscular tissues of 

 that part of the alimentary canal between the stomach and the 

 anus point distinctly to the derivation of its musculature from 

 two distinct sources. The one (Fig. 5) which is endodermal, has 

 arisen from a musculature connected anteriorly with that of a 

 respiratory chamber, to the motor cells of which the vagus nerve 

 supplies the connector fibres, and connected posteriorly with that 

 of a cloacal chamber, to the motor cells of which the pelvic nerve 

 supplies the connector fibres ; the other (Fig. 7) has arisen from 

 a musculature, possibly dermal originally, the motor cells of which 

 are situated in the inferior and superior mesenteric ganglia of the 

 sympathetic, and are connected with the central nervous system 

 by connector fibres in the thoracico-lumbar outflow. Possibly 

 both musculatures originally extended from the stomach to the 

 anus, and with the elongation of the gut the sympathetic mus- 

 culature became more and more confined to those places, where 

 sphincter muscles were a physiological necessity. The innerva- 

 tion of these two musculatures gives the best example of recipro- 

 cal innervation it is possible to conceive, for the motor neurons to 

 the one musculature are always found in close contiguity with 

 the inhibitory neurons of the other, and vice versa. 



Such is the nature of the evidence for the existence of inhibi- 

 tory nerves and nerve cells for the unstriped musculature of the 

 vertebrate apart from that of the vascular system. Further light 

 is thrown upon the question of inhibition and reciprocal innerva- 

 tion by the study of the nervous system of the invertebrate, 

 especially by investigations into the mechanism concerned with 

 the opening and closing of the claw of the crayfish. Richet 

 noticed that stimulation of the nerves in the limb of the 

 crayfish caused opening of the claw if the stimulus was weak, 

 and closing of the claw if the stimulus was strong. Bieder- 

 mann described the two muscles concerned, a short strong ad- 



