DIGESTION. 155 



nerve impulses descending from the cerebrum as a result of psychic states 

 developed by the sight and odor of food, and secondarily, by nerve impulses, 

 transmitted by the nerves of gustation and general sensibility and developed 

 by the contact of food on their peripheral terminations during the act 

 of mastication. 



Modifications of the Nerve Mechanism of Insalivation due to the 

 Physiologic Action of Drugs. The functions of different portions of 

 the nerve mechanism of insalivation may be made apparent by an analysis of 

 the effects that follow the administration of physiologic or slightly toxic 

 doses of the alkaloids of various drugs. The effects can be shown to be due 

 to a depression or stimulation of the normal activity of one or more portions 

 of the mechanism. As a result the secretion may be decreased or increased 

 in volume. The following examples will illustrate the action of alkaloids 

 in general. 



Nicotin. When nicotin in sufficiently large doses is given to an animal 

 hypodermatically, the secretion of saliva after a variable period of time ceases 

 and the mouth becomes dry. If the chorda tympani nerve, i.e., the pre- 

 ganglionic portion, be then stimulated with induced electric currents the 

 usual phenomenon, viz., a free flow of saliva, fails to occur. If, however, 

 the nerve branches emerging from the submaxillary ganglion, i.e., the post- 

 ganglionic portion, be stimulated with electric currents, the saliva will be 

 discharged as usual. The inference is that the conductivity of the peripheral 

 terminations of the preganglionic chorda fibers is depressed so that the nerve 

 impulses discharged by the central mechanism fail to reach, and therefore to 

 stimulate, the submaxillary ganglion cells. The inference as to the seat of 

 action of nicotin is supported by the fact that painting the surface of the 

 superior cervical sympathetic ganglion with nicotin will impair the conductiv- 

 ity of the terminal branches of the preganglionic fibers emerging from the 

 cord so that stimulation of these fibers fails to produce beyond the ganglion 

 the usual secretor effects. It is probable that nicotin has a similar action 

 on the peripheral terminations of Jacobson's nerve which arborize around 

 the nerve cells of the otic ganglion. 



Atropin. Atropin in doses of i milligram also causes a complete cessa- 

 tion in the flow of saliva and consequently an extreme dryness of the mouth. 

 After the occurrence of this condition neither stimulation of the preganglionic 

 chorda tympani fibers nor of the postganglionic fibers, will cause the glands to 

 secrete. But as stimulation of the sympathetic nerve in the cervical region 

 will excite a secretion the inference is that the atropin exerts a depressing 

 effect on the conductivity of the nerve endings in contact with the gland cells 

 thus interfering with the transmission of nerve impulses, rather than on the 

 gland cells themselves. The same holds true for the nerve terminations in the 

 postganglionic fibers distributed to the parotid gland. The action of atropin 

 is not limited, however, to the nerve terminations in connection with salivary 

 glands but extends to the nerve terminations in connection with many other 

 glands in the 'alimentary canal and skin. Even though the dose of atropin 

 be large, 10 to 15 milligrams for a dog, its action is confined to the terminal 

 nerve fibers in connection with the gland cells, for when the chorda tympani 

 is stimulated the blood-vessels around the gland dilate as usual, a fact which 

 indicates that the submaxillary ganglion gives off fibers of a vaso-dilatator as 



