746 PHYSIOLOGY 



sphincter. The exact parts played in this mechanism by the local 

 system and by the central nervous system respectively have not yet been 

 thoroughly made out, though there is no doubt that these movements may 

 proceed independently of any connection with the central nervous system. 



Stimulation of the peripheral end of the vagus nerves may exercise vary- 

 ing effects on the gastric wall as well as on its sphincters. In the normal 

 animal stimulation of the peripheral end of the vagus as a rule causes strong 

 contractions of the oesophagus as well as of the cardiac sphincter. After 

 the administration of atropine, stimulation of the same nerve will occasion 

 dilatation of the cardiac sphincter. On both cardiac and pyloric portions of 

 the stomach the vagus exercises inhibitory as well as augmentor effects. So 

 far as concerns the musculature of the fundus or body of the stomach, the 

 most usual result is an inhibition during stimulation of the vagus succeeded 

 by an augmented tonus immediately the stimulus is removed. If the vagus 

 be excited a number of times, the tonus of the muscular wall augments with 

 each stimulus. On the pyloric portion stimulation of the vagus also causes 

 inhibition, followed by contraction. The inhibition may however be very 

 short and in rare cases altogether absent, so that during the excitation this 

 inhibition is followed by a series of large rhythmic contractions. The pre- 

 vailing motor effect of the vagus therefore is in the fundus increased tonus, 

 in the pyloric portion augmented peristaltic waves. On the pylorus itself 

 we may obtain from vagal stimulation either increased or diminished con- 

 traction. The conditions under which each of these may be evoked have not 

 yet been definitely ascertained. Whether the splanchnic nerve, i. e. the 

 sympathetic system, has a direct influence on the movements of the stomach 

 has been disputed. According to Page May any effect prodaced by stimula- 

 tion of this nerve, generally consisting in diminished motor activity, is 

 probably due to the simultaneous influence on the vascular supply to the 

 organ ; the blood vessels being constricted, an artificial anaemia- is produced 

 which in itself is sufficient to account for diminished activity. Other 

 observers regard the splanchnic as having an influence on the stomach similar 

 to its action on the intestine, and regard it as the chief inhibitory nerve to 

 this organ. It is possible that the extent to which the stomach is brought 

 under the control of the sympathetic system may vary in different species of 

 animals. 



Cannon has shown that the * pangs of hunger * are associated with and probably 

 due to rhythmic contractions of the stomach wall, which come on about meal time, 

 especially if this be delayed. 



VOMITING 



Expulsion of the stomach contents may occur as a result of over- 

 distension of this organ, of the presence of irritating material in its contents, 

 or from abnormal conditions of the brain. It is generally preceded by a 

 feeling of nausea, which is associated with salivation. The large quantities 

 of saliva swallowed still further distend the stomach and assist the opening 

 of the cardiac orifice. In^the act of vomiting itself the first event is a deep 

 inspiration. The glottis is then closed, and this is followed by a strong 



