SECRETION PHYSIOLOGY. 299 



those secretions may be different in different cases. However 

 probable it may seem, a prion, that there is everywhere one 

 fundamental mechanism underlying all these secretions, a de- 

 cent regard for truth forbids one accepting so far reachmg a con- 

 clusion, unless it be supported by very strong evidence. 



In the present paper, therefore, I wish to reopen the question 

 whether all secretions are due to the activity of the gland cells, 

 and to re-examine the evidence of the existence of nerves act- 

 ing on those cells. The great theoretical and practical im- 

 portance of Ludwig's conception is a sufficient excuse for a 

 critical and experimental review, in the light of the physiology 

 of the present day, of the evidence upon which that theory rests. 

 Since the publication of Ludwig's and Heidenhain's work on se- 

 cretion knowledge has been acquired of vaso-motor changes, 

 osmosis, lymph formation as well as secretion proper, which 

 might, possibly, cause even Heidenhain or Ludwig, if consider- 

 ing the subject at this time, to adopt a somewhat different 

 interpretation of much of this evidence from that heretofore pro- 

 posed. Such a review seems the more necessary for the reason 

 that special applications of the theory have been, from time to 

 time, questioned, and because, as will be apparent in the course 

 of the following discussion, some of Heidenhain's inferences are 

 unsound, owing to his having neglected to consider possibilities 

 now known to be of importance. His recent extension of the 

 theory to lymph formation, for example, has been seriously dis- 

 puted by Starling,^^ Cohnheim and others. Starling especially 

 has shown the uselessness of assuming any such secretory 

 mechanism in certain special cases, and has thus thrown doubt 

 upon the truth of the theory as a whole. Langley^^ has ques- 

 tioned the necessity for assuming distinct -trophic" fibres to 

 explain salivary secretion, and for the kidney secretion special 

 inferences of Heidenhain have been challenged by Senator, 

 Adami^ and v. Sobiranski.^^ The difference in pressure be- 

 tween blood and secretion observed by Ludwig may be readily 

 accounted for on the basis of osmosis quite apart from any cell- 

 activity.'' The difference in temperature between saliva and 

 blood has been denied by Bayliss and Hill,^ working with bet- 



