330 MATHEWS. 



diminution in amount of secretion when there is Httle saliva 

 present ; the dependence of the character of the sympathetic 

 saHva upon that present in the gland at the moment of stimu- 

 lation ; the back flow of saliva into the gland on stopping 

 stimulation when the gland is secreting against pressure ; the 

 presence of smooth muscle in the ducts and between the alveoli — 

 these facts point unmistakably in one direction. A stronger 

 chain of circumstantial and direct evidence that this secretion is 

 caused by compression of the ducts and alveoli by contractile 

 tissue would be hard to imagine. If some of these phenomena 

 are susceptible of explanation upon the hypothesis that the 

 secretion is due to gland cell activity, others of them, /. c, the 

 augmented salivary secretion, the back flow of saliva on break- 

 ing stimulation, the paralysis of the nerve when the ducts are 

 empty, and its restoral to power if the ducts be redistended, are 

 explicable, if at all, by that theory, only by means of improba- 

 ble and unproven assumptions. 



The surprisingly ready acceptance of the Ludwig-Heidenhain 

 theory of secretory nerves, acting on gland cells, as an explana- 

 tion of the sympathetic salivary secretion in the face of unmis- 

 takable indications of a muscular mechanism, has been due, 

 largely, I believe, to the generally prevalent belief that there is 

 but one mechanism of secretion. That this belief is erroneous, 

 there has long been, I believe, many indications. For there is 

 direct evidence in many glands, such as the poison glands of 

 snakes, the skin glands of amphibia, many unicellular glands, 

 sebaceous and sweat glands, that many secretions are due to 

 muscular action. And in many other glands the phenomena of 

 secretion have shown as clearly that here the mechanism was 

 some other than muscular. There must evidently be at least 

 two different mechanisms, a muscular and some other one. 

 Once the idea that there is but one mechanism of secretion is 

 abandoned, the salivary secretions will be found, I believe, to 

 lose much of their puzzling character. 



The facts which Heidenhain urges as showing that the sym- 

 pathetic produces secretion by action on the gland cell are 

 readily accounted for if the sympathetic cause compression of 

 the ducts and alveoli and vaso-constriction. 



