90 STUDIES IN PHYSIOLOGY 



the secretion of gastric juice, supplies the building materials 

 required for the growth and repair of the muscles, and, as 

 we shall see later, carries away from this organ whatever 

 digested food it can absorb. 



Muscles of the Stomach. The chief function of the human 

 stomach is to secrete the gastric juice and to mix thoroughly 

 this juice with the food. The muscular walls are admirably 

 adapted to this purpose. On stripping off the outer cover- 

 ing of connective tissue, one finds layers of longitudinal, 

 circular, and oblique muscles that constitute the larger por- 

 tion of the thickness of the stomach wall (Fig. 32). Cir- 

 cular fibers form the strong ring of muscle (pyloric sphincter) 

 that closes the pyloric end of the stomach from the small 

 intestine (Fig. 31, d). 



When the food reaches the stomach, the gastric juice 

 oozes out upon it, and the mixture is pushed back and forth 

 and up and down by the successive action of the different 

 sets of muscles. The return of the food to the mouth cavity 

 is prevented by the contraction of the circular muscles at 

 the cardiac orifice, except in case of nausea, when they 

 relax and allow the stomach to rid itself of its contents. 

 The pyloric sphincter relaxes from time to time, and the 

 food that has been sufficiently digested is pushed on into the 

 intestine. Fortunately for the well-being of the body, all 

 these processes are entirely automatic, that is, they are 

 carried on without our conscious direction. The muscles 

 of the alimentary canal for this reason are called in-voVun- 

 ta-ry (Latin in = without -f voluntas = will). Their activ- 

 ity is largely controlled by the syni-pa-thet'ic nervous system, 

 which will be described later. 



Digestion in the Stomach. 1 The gastric juice has no effect 

 whatever on the nutrients starch and fats. Sugars and 

 soluble salts (that is salts that dissolve in water), if not 

 dissolved in the mouth, are readily liquefied by the water of 

 the gastric juice. 



i See "Laboratory Exercises," No. 20. 



