810 PHYSIOLOGY 



injection of secretin always causes a secretion of intestinal juice. In 

 the upper part of the gut therefore the simultaneous presence of 

 the three juices necessary for complete duodenal digestion is 

 ensured by one and the same mechanism, namely, by the simul- 

 taneous activity of the secretin, produced in the intestinal cells by 

 the action of the acid chyme, on pancreas, liver, and intestinal 

 glands. Recently a further chemical mechanism for the arousing of 

 intestinal secretion has been described by Frouin. According to 

 this observer, the flow of juice can be excited by intravenous injection 

 of intestinal juice itself. Since this juice is alkaline, and does not 

 produce any effect on the pancreas, it must be free from pancreatic 

 secretin. It would seem therefore that the flow of juice in the 

 upper part of the gut, excited by the pancreatic secretin, causes 

 also a production of a different secretin or hormone, which can be 

 absorbed from the lumen of the gut, travel by the blood stream to 

 other segments of the small intestine, and there exciie a secretion 

 in preparation for the on-coming food. Further experiments are, 

 however, necessary on this point. 



The glands of the small intestine can also be excited by direct 

 mechanical stimulation . of the mucous membrane. The easiest 

 method of exciting a flow of intestinal juice from a permanent fistula 

 is to introduce into the intestine a rubber tube. The presence of 

 the solid object in the gut causes a secretion, and within a few minutes 

 drops of juice can be obtained from the free end of the tube. The 

 object of such a sensibility to mechanical stimuli is obvious ; it is 

 of the highest importance that the onward passage of any solid object, 

 especially if it be indigestible, shall be aided by the further secretion 

 of juice in the portions of gut which are immediately stimulated. 

 This mechanical stimulation probably acts on the tubular glands 

 of the intestine through the intermediation of the local nervous 

 system, the plexus of Meissner. It is stated by Pawlow that a juice 

 obtained by mechanical stimulation differs from that produced by 

 the introduction of pancreatic juice into the loop in containing little 

 or no enterokinase, so that the pancreatic juice excites the secretion 

 of the substance which is necessary for its own activation. 



CHARACTERS OF INTESTINAL JUICE 



The intestinal juice obtained from a permanent fistula has a 

 specific gravity of about 1010. It is generally turbid from the presence 

 in it of migrated leucocytes and disintegrated epithelial cells. It 

 contains about 1-5 per cent, total solids, of which 0-8 per cent, are 

 inorganic and consist chiefly of sodium carbonate and sodium chloride. 

 It is markedly alkaline in reaction, but less so than the pancreatic 

 juice. The organic matter, besides a small amount of serum albumin 



