ACTIVITY DURING DIGESTION. 23 



is to say, that even the variations which occur from time, to time in 

 secretory work are determined by fixed conditions. 



The work of the glands, viz., the secretion of juice, follows therefore 

 a definite periodic law. The fluid is not poured out at the same rate 

 from the beginning to the end of digestion, nor even in regularly 

 diminishing quantities, after having attained an initial maximum. The 

 curve is by no means a straight line gradually approximating the 

 abscissa. It is a special curve which slowly or rapidly ascends, or pre- 

 serves for a time a uniform height, or gently or suddenly falls, as the 

 case may be. Examples of these will be given later. Since these 

 curves repeat themselves under the same conditions with stereotyped 

 exactitude, we must admit that this or that rate of secretion is not 

 determined by mere blind chance, but in all cases follows a necessary 



Hours 1 



Via. ?>. Curve of secretion of ir:i>trie juice, after n men I 

 of flesh. (Two experiments.) 



law, requisite for the due elaboration of the food and therefore beneficial 

 to the organism. The curves, however, are not easy of interpretation in 

 all their separate fe?,tures; indeed, for the present, this is almost impossible. 

 The line of descent with its fluctuations can more or less satisfactorily 

 be explained on the principle of corresponding variations in the quantity 

 of ingesta at any particular part of the digestive canal. But the meaning 

 of the complex line of ascent remains in many cases obscure and inex- 

 plicable. How, for example, can the late appearance of the maximum be 

 explained, which we see during the third hour after a meal, in the curve of 

 pancreatic juice ? A scientific exposition of the curves, that is, one which 

 fully and accurately corresponds with the actual facts of the case, can 

 only be furnished by physiology when, as mentioned in our first lecture, 



