348 A MANUAL OF PHYSIOLOGY 



The only real cholagogues at present positively known appear to be 

 the salts of the bile acids, which, given by themselves or in the bile, 

 cause not only an increase in the volume of the biliary secretion, but 

 also an increase in its solids. Certain compounds of salicylic acid, 

 as salol (phenyl salicylate) and sodium salicylate, also appear to 

 slightly increase the flow, while usually diminishing the concentration 

 of the bile. The injection of haemoglobin into the blood-stream, or 

 its liberation there by substances, such as toluylene-diamin and 

 arseniuretted hydrogen, which cause solution of the corpuscles, leads 

 to an increased secretion of bile-pigment as well as a more rapid flow 

 of bile. 



Summary. Here let us sum up the most important points 

 relating to the secretion of the digestive juices. They are 

 all formed by the activity of gland- cells originally derived from the 

 epithelial lining of the alimentary canal. The organic constituents 

 or their precursors (including the mother -substances of the ferments) 

 are prepared in the intervals of rest absolute in some glands, 

 relative in others and stored up in the form of granules, which 

 during activity are moved towards the lumen of the gland tubules, 

 and there discharged. 



The nerves of the salivary glands are, as regards their origin, 

 (a) cerebral, (b) sympathetic; the former group is vaso-dilator, 

 the latter vaso-constrictor both are secretory. Secretion of saliva 

 depends strictly on the nervous system. That nerves influence the 

 gastric and pancreatic secretions is also made out. As regards 

 the intestinal glands and the liver, it has not been proved that 

 their secretive activity is at all under the control of the nervous 

 system, except in so far as the latter may indirectly govern it 

 through the blood-supply, although various circumstances suggest 

 the probability of a more direct action. In all the glands the 

 blood- flow is increased during activity in some (salivary glands) 

 this is known to be caused through vasomotor nerves. In the 

 salivary glands electromotive changes accompany the active state, 

 while more heat is produced, more carbon dioxide given off, and 

 more oxygen used up, during secretion than during rest. In the 

 other glands we may assume that the same occurs. 



IV. Digestion as a Whole. 



Having discussed in detail the separate action of the 

 digestive secretions, it is now time to consider the act 

 of digestion as a whole, the various stages in which are 



