DIGESTION. 153 



Stimulation of the sympathetic fibers passing to the parotid gland is 

 followed by contraction of the vessels and an abolition of the secretion; but 

 at the same time there is an increased activity of the secretor cells, for sub- 

 sequent stimulation of the auriculo-temporal nerve not only causes an 

 increase in the amount of water and inorganic salts, but an increase also in 

 the amount of organic matter far beyond that produced when the auriculo- 

 temporal alone has been stimulated. Histologic examination shows that 

 the small ducts of the gland are filled with thick organic matter after stimula- 

 tion of the cervical sympathetic. 



The foregoing facts led Heidenhain to the conclusion that there are two 

 physiologically distinct efferent nerve-fibers distributed to the glands, viz., 

 trophic nerves, derived from the sympathetic which stimulate the cells to the 

 production of organic constituents; and secretor nerves, derived from the 

 cranial nerves, chorda tympani and glosso-pharyngeal, which stimulate 

 the cells to the production of water and inorganic salts. This view has 

 however, been controverted by Langley, who regards the secretor fibers to the 

 glands as essentially the same, and considers the differences in the character 

 of the secretion to be dependent on differences in the quantity of the blood- 

 supply induced by the simultaneous stimulation of the vaso-motor nerves. 



The Central Mechanism. The central mechanism that excites the 

 glands and blood vessels to activity through efferent nerves originating in its 

 cells maybe aroused to action (i) by nerve impulses descending from the cere- 

 brum in consequence of psychic states induced by the sight or the odor of 

 foods especially after long abstinence; (2) by nerve impulses transmitted 

 through afferent nerves from the mouth, developed by the contact of the food 

 with the peripheral terminations of the gustatory or general sensor nerves. 



That psychic states, ideas and feelings aroused by the sight, odor, and 

 contemplation of food can give rise to a stimulation of the cells of the central 

 mechanism in the manner just stated is shown by the flow of saliva which is 

 familiarly known as watering of the mouth. This fact has been experiment- 

 ally demonstrated by Pawlow on dogs. This investigator cause the ducts 

 of the glands to be brought to the surface in such a manner that they healed 

 into the edges of the skin wounds. By means of suitable receivers applied 

 over the orifices of the ducts the saliva could be readily collected. When the 

 dog under such circumstances was tempted by the sight of foods there was 

 at once a free discharge of saliva, the quantity and quality of which varied 

 with the character of the foods. 



That the central mechanism can be excited to activity by nerve impulses 

 reflected from the periphery can be demonstrated by the introduction of food 

 into the mouth, as well as by stimulation of the branches of the afferent 

 nerves distributed to the mouth which constitute the afferent part of this 

 mechanism. 



The Afferent Nerves. The afferent nerves that transmit nerve impulses 

 from the mouth to the central mechanism, are the taste fibers in the chorda 

 tympani, the taste and sensor fibers of the glosso-pharyngeal, and the sensor 

 fibers of the lingual and buccal branches of the trigeminal nerve. This of 

 shown by the fact that if they are transversely divided there is a cessation of 

 the discharge of saliva when the peripheral nerve endings in the mouth are 

 stimulated by the presence of food. With these nerves intact the introduc- 



