THE DIGESTIVE SECRETIONS 915 



this experiment may be repeated for many hours without any apparent 

 decrease in the intensity of the reaction. Contrary to this result, the 

 excitation of the cervical sympathetic nerve yields only a few drops of 

 saliva which is characterized by its turbidity and richness in total 

 sohds (6 per cent.). On the one hand, therefore, we obtain a vaso- 

 dilatation and copious flow of a very watery saliva and, on the other, 

 a vaso-constriction and a scanty flow of a very viscous saliva. Very 

 similar changes may be evoked in the parotid gland; in this case, 

 however, the excitation of the sympathetic nerve does not yield an 

 appreciable quantity of secretion, although it produces marked his- 

 tological changes in the secretory cells. 



Regarding the intracellular changes little can be said unless we 

 confine this discussion to the structural alterations during rest and 

 activity. Secretion is essentially a transudation of water from the 

 blood-vessels into the excretory ducts, controlled, of course, by the 

 constituents of the cell. It is conceivable that the agents most 

 actively concerned in this process are those granules of the cytoplasm 

 which take up water, swell and discharge their contents into the ducts. 

 How this osmotic play may be influenced by impulses brought to 

 these cells by way of the secretory fibers, is largely a matter of specula- 

 tion into which we cannot enter at this time. It is certain, however, 

 that the cell is not merely a pumping mechanism for the flow of water, 

 but also serves as a generator of organic material which is later on 

 transferred into the watery medium.^ Obviously, the waves of excita- 

 tion derived from the chorda tympani, must increase this transudation 

 as well as rupture the granules, but it is also conceivable that the 

 aforesaid secretory nerves may innervate different elements of the 

 gland. While it has been shown that they terminate around the vari- 

 ous alveoli, forming here delicate arborizations below tl^e basement 

 membrane, certain evidence has also been presented to prove that the 

 cerebral autonomic fibers are apportioned to the chief cells and the 

 sympathetic fibers to the cells of the crescents of Gianuzzi. At least, 

 this arrangement seems to prevail in the submaxillary gland of the dog. 

 Upon the basis of Mtiller's Law of the specific nerve energy we might 

 then assume that these mechanisms react differently to different kinds 

 of stimulations enacted by the food. In one case, we would obtain a 

 typical chorda-saliva, and in another, a typical sympathetic-saliva, 

 or even a mixture of the two. 



Barcroft and Piper^ have sought to obtain a measure of the energy 

 evolved by a secreting gland by ascertaining its respiratory exchange 

 when resting and when active. Thus, it has been found that the sub- 

 maxillary gland of the dog consumes 0.25 c.c. of oxygen in a minute and 

 liberates 0.17 c.c. of carbon dioxid. During this time 6 grams of 

 gland-tissue furnish about 1.1 calorie of heat. The active organ, on 



1 MacuUum introduces the factor of differences in surface-tension, Ergebn. der 

 Physiol., xi, 1911. 



2 Jour, of Physiol., xliv, 1912, 359. 



