DIGESTION 105 



of the intestine forwards. Co-ordinated contraction is observed 

 so long as the local mechanism is intact, even though all 

 external nerves have been cut. The intestines have their own 

 nerve cells and fibres, which, acting as a linked system of reflex 

 centres, provide for the harmonious contraction of their walls. 

 External nerves, sympathetic and splanchnic, convey impulses 

 which either intensify the movements or inhibit them, as need 

 may be. 



In the matter of its nerve-supply, the alimentary canal stands 

 apart from the other organs of the body. It may be supposed 

 that it presents a more primitive condition. Its muscular 

 fibres have the power of contracting spontaneously. The 

 pressure of the contents of the tube acts as a stimulus. When 

 the fibres are stretched, they contract. When the tube is 

 dilated, its muscles endeavour to restore it to its normal calibre. 

 Such direct action would not, however, provide for the forward 

 passage of its contents. To bring about peristalsis, a nervous 

 mechanism is needed, as abundant and complicated as that 

 which ensures the progress of a slug or a worm. To deal satis- 

 factorily with the various contents of the tube liquid, solid, 

 gaseous the mechanism must be capable of complicated ad- 

 justments. The dilated portions of the tube stomach, caecum 

 coli, rectum require special arrangements of muscle and nerve. 

 Nor is the canal altogether independent of the rest of the body. 

 To a large extent its work is carried on without regard to the 

 activities of other organs, yet it is not wholly free from the 

 control of the central nervous system. It is regulated by means 

 of both afferent and efferent nerves of the vagus and sympa- 

 thetic. Even the brain has something to say with regard to 

 the way in which it shall contract. It is a matter of common 

 experience that emotional influences may affect the movements 

 of the stomach and intestines " His bowels yearned." 



Normally, vomiting is due to irritation of the endings of the 

 vagus nerve in the stomach, although the afferent impulses 

 may havQ other sources. Touching the upper surface of the 

 epiglottis with the finger will provoke the reflex. So also will 

 stimulation of the olfactory nerves by a foul smell. In this 

 latter case the emotion of disgust to which the odour gives 

 rise brings about the reflex action. A flow of saliva precedes 

 the act of vomiting. A deep inspiration is then taken, in order 



