INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 387 



found that the maximum pressure in the salivary duct when the 

 outflow of saliva from the duct is prevented may, during stimula- 

 tion of the chorda, much exceed the arterial blood-pressure (Ludwig). 

 In one experiment, for example, the pressure in the carotid of a dog 

 was 125 mm., in Wharton's duct 195 mm. of mercury. 



Even in the head of a decapitated animal a certain amount of 

 saliva may be caused to flow by stimulation of the chorda, but too 

 much may easily be made of this. And since the blood is the ultimate 

 source of the secretion, we could not expect a permanent or copious 

 flow in the absence of the circulation, even if the gland-cells could 

 continue to live. In fact, when the circulation is almost stopped by 

 strong stimulation of the sympathetic, the flow of saliva caused by 

 excitation of the chorda is at the same time greatly lessened or 

 arrested, even though the sympathetic itself possesses secretory 

 fibres. So that, while there is no doubt that the chorda tympani 

 contains fibres whose function is to increase the activity of the 

 gland-cells, its vaso-dilator action is, under normal conditions, 

 closely connected with, and, indeed, auxiliary to, its secretory action, 

 although the dilation of the vessels does not directly produce the 

 secretion. This is only a particular case of a physiological law of 

 wide application, that an organ in action in general receives more 

 blood than the same organ in repose, or, in other words, that the 

 tissues are fed according to their needs. The contracting muscle, the 

 secreting gland, is flushed with blood, not because an increased blood- 

 flow can of itself cause contraction or secretion, but because these 

 high efforts require for their continuance a rich supply of what blood 

 brings to an organ, and a ready removal of what it takes away. 



The quantity of blood passing through the parotid of a horse 

 when it is actively secreting during mastication may be quadrupled 

 (Chauveau). The parallel between the muscle and the gland is 

 drawn closer when it is stated that electrical changes accompany 

 secretion (p. 810), and that the rate of production of carbon dioxide 

 and consumption of oxygen (in the submaxillary gland) is three or 

 four times greater during activity than during rest. The temperature 

 of the saliva flowing from the dog's submaxillary during stimulation 

 of the chorda has been found to be as much as 1-5 C. above that 

 of the blood of the carotid, although with the gland at rest no con- 

 stant difference could be detected between the arterial blood and 

 the interior of Wharton's duct. But such measurements are open 

 to many fallacies ; and while there is no doubt that more heat is 

 produced in the active than in the passive gland, it will not be 

 surprising, when the vastly-increased blood-flow is remembered, 

 that no difference of temperature between the incoming and out- 

 going blood has been satisfactorily demonstrated. 



It has already been mentioned that most of the fibres of the chorda 

 tympani proper become connected with ganglion-cells, and lose their 



