256 DIGESTION 



SECOND SECTION 



SECRETION OF THE DIGESTIVE FLUIDS 



1. GENERAL SURVEY 



The secretory process presents many points of similarity in all digestive 

 glands. For this reason, it is desirable to consider the process in broad out- 

 line before taking up in detail the peculiarities of the individual glands. 



In the year 1851 Ludwig showed that section of the cerebral nerve supply 

 to the salivary glands was followed by complete cessation of the flow of saliva 

 (submaxillary, parotid). For hours there was not the least trace of fluid 

 in the cannula which had been inserted into the duct. As soon however as 

 the cerebral nerve was stimulated, saliva gushed out of the duct. In a thor- 

 oughgoing investigation, which is to be reported more fully under 2, Ludwig 

 demonstrated that this secretion is not a filtrate from the blood, but is pro- 

 duced by the specific activity of the gland cells under the influence of the 

 nerves. 



These discoveries stood quite alone for several decades. It is true that 

 some observations were collecting, from which it appeared with a certain 

 degree of probability that the secretion of the gastric glands and of the pan- 

 creas were influenced considerably by secretory nerves (Heidenhain, Richet 

 et al.). But the existence of such nerves was conclusively proved by Pawlow 

 only a few years ago. We do not know definitely even yet whether the other 

 digestive glands, those of Lieberkiihn, of Brunner, and the liver, are under the 

 influence of secretory nerves in the same way as those already mentioned. 



It would be a matter of the greatest interest to know exactly the anatomical 

 connection between the secretory nerves and the gland cells. The many efforts 

 of histologists in this direction have not been entirely successful as yet, although 

 it has been stated recently that the nerves penetrate the membrana propria of 

 the acini and terminate in end organs lying in direct contact with the secreting 

 cells. The end organs are said to have either the form of mulberrylike clumps 

 or of small twigs beset with nodules. 



Under normal circumstances the secretion of those glands which are 

 plainly under the control of the central nervous system is evoked by reflex 

 action, set up in many cases by perfectly definite chemical substances (Paw- 

 low) (cf. page 264). These reflexes as a rule stand in a very close relation- 

 ship with the ingestion of food, and in general it may be said that in the 

 intervals of digestion when there is no desire for food, the digestive fluids are 

 secreted only in very small quantities. 



Bile forms an exception to this rule, since even in the fasting condition it 

 is produced and is given off by the liver. Possibly this is due to the fact that 

 bile is not only a digestive fluid, but contains also various substances which, so 

 far as our knowledge at present goes, have no significance whatever in diges- 

 tion, and must be looked upon as real waste products. As such they would 

 naturally be produced continuously, just as in the case of urea and the decom- 



