306 FOOD AND DIGESTION 



that the flow of saliva may be increased or diminished by mental states, it 

 is evident that impressions from the cerebrum also are capable of stimulating 

 the center to action or of inhibiting its action. 



Influence of Nerves on the Submaxillary Gland. The sub maxillary 

 gland has been the gland chiefly employed for the purpose of experimentally 

 demonstrating the influence of the nervous system upon the secretion of saliva, 

 because of the comparative facility with which the gland, with its blood- 

 vessels and nerves, can be exposed to view in the dog, rabbit, and other 

 animals. 



The chief nerves supplied to the gland are: (i) the chorda tympani, a 

 branch given off from the facial in the canal through which it passes in the 

 temporal bone; and (2) branches of the sympathetic nerve from the plexus 

 around the facial artery and its branches to the gland. The chorda, figure 

 250, ch. t, passes downward and forward, under cover of the external ptery- 

 goid muscle, and joins the lingual or gustatory nerve, proceeds with it for a 

 short distance, and then passes along the submaxillary- gland duct, sm. d, 

 giving branches to the submaxillary ganglion, sm. gl, and sending others to 

 terminate in the superficial muscles of the tongue. It consists of fine medul- 

 lated fibers which lose their medulla in the gland. If this nerve be exposed 

 and divided anywhere in its course from its exit from the skull to the gland no 

 immediate result will follow, nor will stimulation either of the lingual or of 

 the glosso-pharyngeal produce a flow of saliva. But if the peripheral end 

 of the divided nerve be stimulated, an abundant secretion of saliva ensues, 

 and the blood supply is enormously increased by dilatation of the arteries. The 

 veins may even pulsate, and the blood contained within them is more arterial 

 than venous in character. 



When, on the other hand, the stimulus is applied to the sympathetic fila- 

 ments (mere division producing no apparent effect), the arteries contract, 

 and the blood stream is in consequence much diminished; and only a sluggish 

 stream of dark blood escapes from the veins. The saliva, instead of being 

 abundant and watery, becomes scanty and tenacious. If both chorda tym- 

 pani and sympathetic branches be divided, the gland, released from nervous 

 control, may secrete continuously and abundantly (paralytic secretion). 



The abundant secretion of saliva which follows stimulation of the 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 atropine has been previ- 

 ously injected, stimulation of the peripheral end of the divided chorda pro- 

 duces all the vascular effects as before, without any secretion of saliva accom- 

 panying them. Again, if an animal's head be cut off, and the chorda be 

 rapidly exposed and stimulated with an interrupted current, a secretion of 



