THE REGULATION OF SALIVARY SECRETION. 181 



It has been found that for a short time at least, a secretion 

 of saliva can be obtained even from the head of a decapi- 

 tated animal, in other words, in the entire absence of a 

 circulation. That such a secretion continues no longer 

 than it does is not strange, for in the end all the constituents 

 of a secretion, not the least important of which is the water 

 itself, must be obtained from the blood. The increased blood- 

 pressure, which has so often and so long been considered the 

 determining factor in increasing the amount of the salivary as 

 well as the amount of other secretions, can by itself be of 

 little moment, as is shown by LUDWIG'S classical observa- 

 tion. When the outflow of saliva from the salivary duct 

 is prevented, the pressure in this duct may, when the chorda 

 tympani is stimulated, rise far above that found in even 

 the larger arteries, such as the carotid. Finally, it is pos- 

 sible to bring about a vascular dilation in a salivary gland and 

 yet get no secretion. This happens when, after a dose of 

 atropin, the chorda tympani or the auriculo-temporal nerve 

 is stimulated. The converse of this experiment can be 

 performed with pilocarpin. When this alkaloid is injected, 

 a free flow of saliva is brought about, even when no change 

 has occurred in the calibre of the blood-vessels. 



Besides the change which occurs in the calibre of the 

 blood-vessels supplying a salivary gland, there is also a 

 change in temperature. The active gland becomes warmer. 

 The amount of thermal change is not determined simply 

 by the arterial blood coursing through the gland. As 

 LUDWIG and SPIESS have shown, when one thermometer is 

 inserted into the secretory duct of the submaxillary of a 

 dog, a second into the efferent vein, and a third into the 

 carotid artery, then, when the gland is excited to secretion 

 by stimulation of the chorda tympani, the thermometers 

 in the duct and in the vein register a higher temperature 

 than the thermometer in the carotid. This observation is 

 of fundamental importance, as it gives us some clue to the 

 character of the changes which occur in salivary and 



