THE MECHANISMS OF DIGESTION 467 



page 796) endowed with the power of directing nervous impulses so as to 

 bring about relaxation of the gut in front of the stimulus and contrac- 

 tion over it. 



NERVOUS CONTROL OP MOVEMENTS 



The influence of the central nervous system on the intestinal movements 

 has been studied by the usual methods of cutting and stimulating the 

 extrinsic nerve supply. Through the splanchnic nerves tonic inhibitory 

 impulses are conveyed to the intestine (except the ileocolic sphincter), 

 for after these nerves' are severed the movements become more distinct. 

 Indeed, in many animals after opening the abdomen no intestinal move- 

 ment can be observed until these nerves have been cut. Stimulation of the 

 peripheral end of the nerve also inhibits any movement which may mean- 

 while be in progress. The impulses through the vagus nerve are of an 



Fig. 163. The effect of excitation of both splanchnic nerves on the intestinal contractions. (From 



Starling.) 



opposite character. Section of these nerves has little effect, but stimula- 

 tion causes contraction. (Figs. 163 and 164.) 



By observing the rhythmic contractions of an isolated strip of the small 

 intestine suspended in a bath of oxygenated saline solution at body tem- 

 perature, it can readily be shown that the presence of even a minute trace 

 of epinephrine is sufficient to produce complete inhibition of the movement. 

 The parallelism between the effects of splanchnic stimulation and those of 

 epinephrine injection is very significant, for in this way the marked inhi- 

 bition of intestinal movement which occurs during fright may possibly 

 be explained (see page 736). 



The circular muscular coat of the last two or three centimenters of 

 the ileum before it joins the cecum is definitely thicker than the rest of 

 this coat, indicating that it has a sphincter-like action. This ileocolic 

 sphincter, as it is called, opens when food is pressed against it from the 

 ileum, but remains closed when food is pressed against it from the cecum. 



