364 A MANUAL OF PHYSIOLOGY 



Even in the head oi 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 tc 

 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 quad- 

 rupled (Chauveau). The parallel between the muscle and the 

 gland is drawn closer when it is stated that electrical changes 

 accompany secretion (p. 734), 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 constant 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 outgoing 

 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 

 medulla inside the submaxillary gland, only a few having already lost 

 it by a similar connection with ganglion-cells in the chordo-lingual 

 triangle. These facts have been made out by means of the nicotine 



