388 COMPARATIVE ZOOLOGY OF CHORDATES 



sympathetic outflows is to contract the ordinary muscles 

 round the gut, but to slacken the sphincters, to slacken the 

 muscles of the heart and of the blood-vessels near the urethra 

 (causing erection of the penis), to tighten the ciliary muscle 

 and the sphincter of the iris, to slacken the radial muscles of 

 the iris (which allows the pupil to be contracted, and to secrete 

 saliva and tears). 



The antagonism between the effects of the sympathetic and 

 parasympathetic systems is remarkable. It may be expressed 

 in the form of a table. 



Ordinary muscles Sphincters Radial muscles Sphinctex 



of the gut. of the gut. Heart. of the iris. of the iris. 



Sympathetic. Slackens. Tightens. Tightens. Tightens. Slackens. 



Parasympathetic. Tightens. Slackens. Slackens. Slackens. Tightens. 



It is also interesting to note that the action of the sym- 

 pathetic system can partly be simulated by the injection of 

 adrenalin, and that of the parasympathetic by injection of 

 acetyl-cholin. The similar effects of adrenalin and the sym- 

 pathetic are less surprising when it is remembered that the 

 supra-renals and the medulla of the adrenal bodies are derived 

 from cells similar to sympathetic neurons, and which like 

 them have migrated out from the spinal cord. 



The case of the gut is particularly interesting, because the 

 ordinary muscles of its coat are antagonistic in their effects 

 to those of the sphincters. It stands to reason, that if the 

 ordinary gut-musculature contracts and propels the contents 

 of the gut along, contraction of the sphincters would prevent 

 this movement of the contents. Now the parasympathetic 

 system tightens the ordinary musculature and slackens the 

 sphincters, and the sympathetic system contracts the sphincters 

 and slackens the ordinary musculature. Further, the cell- 

 bodies of the neurons which tighten the sphincters and slacken 

 the ordinary muscles are in the same ganglion (anterior or 

 posterior mesenteric ganglion, according to the region of the 

 gut). It is possible that it is one and the same neuron which 

 produces two axon fibres, one tightening the sphincters and 

 the other slackening the ordinary muscles. This provides an 

 explanation of how the co-ordination between antagonistic sets 

 of muscles may be brought about. 



