INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 393 



nor is the amount of oxygen consumed by the gland increased. 

 Mere increase of pressure could not in any case of itself account for 

 the secretion, since it has been found that the maximum pressure 

 in the salivary duct when the outflow of saliva from the duct is 

 prevented may, during stimulation 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 dilatation 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. 

 Evidence exists that in the salivary glands, as in muscle (p. 178), 

 metabolic products given off during functional activity contribute 

 to the dilatation of the vessels. This is the simplest explanation of 

 the fact that the dilatation caused by chorda stimulation lasts longer 

 when saliva is being secreted than when the secretion has been 

 abolished by atropine. 



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. 838), 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 



