234 HAND-BOOK OF PHYSIOLOGY. 



chorda tympani, is not merely the result of a filtration of fluid from the 

 blood-vessels, in consequence of the largely increased circulation through 

 them. This is proved by the fact that, when the main duct is obstructed, 

 the pressure within may considerably exceed the blood-pressure in the 

 arteries, and also that when into the veins of the animal experimented 

 upon some atropin has been previously injected, stimulation of the 

 peripheral end of the divided chorda produces all the vascular effects as 

 before, without any secretion of saliva accompanying them. Again, if 

 an animal's head be cut off, and the chorda be rapidly exposed and stimu- 

 lated with an interrupted current, a secretion of saliva ensues for a short 

 time, although the blood supply is necessarily absent. These experiments 

 serve to prove that the chorda contains two sets of nerve fibres, one set 

 (vaso-dilator) which, when stimulated, act upon a local vaso-motor centre 

 for regulating the blood supply, inhibiting its action, and causing the 

 vessels to dilate, and so producing an increased supply of blood to the 

 gland; while another set, which are paralyzed by injection of atropin, 

 directly stimulate the cells themselves to activity, whereby they secrete 

 and discharge the constituents of the saliva which they produce. These 

 latter fibres very possibly terminate in the salivary cells themselves. If, 

 on the other hand, the sympathetic fibres be divided, stimulation of the 

 tongue by sapid substances, or of the trunk of the lingual, or of the glosso- 

 pharyngeal, continues to produce a flow of saliva. From these experi- 

 ments it is evident that the chorda tympani nerve is the principal nerve 

 through which efferent impulses proceed from the centre to excite the 

 secretion of this gland. 



The sympathetic fibres appear to act principally as a vaso-constrictor 

 nerve, and to exalt the action of the local vaso-motor centres. The 

 sympathetic is more powerful in this direction than the chorda. There 

 is not sufficient evidence in favor of the belief that the submaxillary gan- 

 glion is ever the nerve centre which controls the secretion of the sub- 

 maxillary gland. 



B. On the Parotid Gland. The nerves which influence secretion in 

 the parotid gland are branches of the facial (lesser superficial petrosal) and 

 of the sympathetic. The former nerve, after passing through the otic 

 ganglion, joins the auriculo-temporal branch of the fifth cerebral nerve, 

 and, with it, is distributed to the gland. The nerves by which the stimu- 

 lus ordinarily exciting secretion is conveyed to the medulla oblongata, are, 

 as in the case of the submaxillary gland, the fifth, and the glossopharyn- 

 geal. The pneumogastric nerves convey a further stimulus to the secre- 

 tion of saliva, when food has entered the stomach; the nerve centre is the 

 same as in the case of the submaxillary gland. 



Changes in the Gland Cells. The method by which the salivary 

 cells produce the secretion of saliva appears to be divided into two stages, 

 which differ somewhat according to the class to which the gland belongs, 



