DIGESTION. 51 



but as the peptone-making advances the pyloric opening permits 

 the escape of chyme, and this is aided by strong peristaltic efforts 

 on the part of the stomach at its pyloric end. Toward the end of 

 digestion the pylorus permits the escape of undigested as well as 

 of digested matter. The circulation of the stomach contents is cir- 

 cumferentially toward the pylorus, but centrally toward the cardiac 

 opening. 



What is the capacity of the stomach ? 



About a quart in the adult, but its muscular walls enable it to 

 contract so as to fit its contents if much or little. 



What is the nervous mechanism of stomach digestion ? 



The pneumogastric and sympathetic (splanchnic from solar 

 plexus) are the nerves which supply the stomach, and besides these 

 there are numerous ganglia in the stomach-walls. The ordinary 

 motion-stimulus of the organ lies in the intrinsic ganglia. Irri- 

 tation of the pneumogastric nerve causes contraction ; its division, 

 cessation of peristalsis. But, further than this, the vagus has con- 

 trol to considerable degree over secretion in the stomach. 



Describe vomiting. 



The regurgitation of food from the stomach through the cardiac 

 orifice, and thence through the mouth, may occur when the cardiac 

 opening is free and the pylorus is closed. This is usually a reflex 

 act, and is performed by the contraction of the stomach, aided by 

 the pressure of the abdominal muscles. opposing the fixed diaphragm. 

 It may be described as a reversed peristalsis. The stimuli which 

 excite the reflex may be either local in the stomach or peripheral. 

 Violent irritation of the gastric mucous membrane will excite it ; 

 also mental impulses, from ocular, auditory, or olfactory sources ; 

 injury or irritation of the testis, ovary, kidney, etc.; unusual 

 motion, as swinging ; certain diseases ; and effort of will in some 

 is sufficient. 



How is this reflex controlled? 



By a centre in the medulla oblongata acting through the pneu- 

 mogastric nerve. 



What is meant by rumination in man? 



Some few persons have the ability to force half-digested food 

 back from the stomach into the mouth, remasticate it, and again 

 swallow it. 



