DIGESTION 157 



submaxillary; moist foods and especially meat causes a large flow from the 

 submaxillary gland, but a slight flow from the parotid. It is also probable 

 that the glands respond by discharging a secretion of special quality in 

 accordance with the properties of the different foods. 



Stimulation of the afferent nerves with induced electric currents also gives 

 rise to a discharge of saliva. This can be demonstrated by exposing the 

 glands and the afferent nerves and subjecting them to experiment. Under 

 such circumstances, if a cannula be placed in the duct of the submaxillary 

 gland, and the lingual nerve stimulated by induced electric currents of 

 moderate strength, a copious flow of saliva at once takes place. If now the 

 glosso-pharyngeal nerve or the central end of the divided chorda tympani 

 nerve be stimulated in a similar manner, the effect on the secretion will be 

 the same. Division of these nerves in an animal, in such a way as to prevent 

 the nerve impulses from reaching the medulla oblongata, is followed by a 

 marked diminution in the amount of saliva secreted. The reflex centers, 

 however, may receive impulses and be excited to activity by impulses coming 

 through other nerves e.g., the pneumogastric, when the mucous membrane 

 of the stomach is stimulated; the sciatic, when after division, its central end 

 is stimulated. 



Resume of the Factors Involved in the Secretion of Saliva. From 

 the foregoing statements it is apparent that the secretion of saliva is a complex 

 act involving the cooperation of several different factors. As the mechanism 

 for the elaboration of this secretion is typical of that for many secretions it 

 will be of advantage to summarize these factors and their specific functions. 

 These are as follows: 



1. Epithelial cells, the physiologic actions of which are the production 



of the specific characteristic constituents of the saliva, e.g., mucin, 

 albumin, the enzyme ptyalin, as well as the absorption and discharge of 

 water and inorganic salts. 



2. Lymph, which contains the nutritive materials necessary for the 



growth, repair, and metabolic activities of the secreting cells. 



3. Capillary blood-vessels, which permit the passage of those constituents of 



the blood that collectively constitute lymph. 



4. Vaso-motor nerves, some of which at the beginning of secretor activity 



dilate the blood-vessels and thus increase the blood-supply and the 

 production of lymph (vaso-dilatator nerves); others of which at the 

 end of secretor activity perhaps actively contract the blood-vessels 

 and thus decrease the blood-supply to the previous condition (vaso- 

 constrictor nerves). 



5. Secretor nerves, which stimulate the epithelial cells to increased activity 



causing them to discharge their specific metabolic constituents along 

 with water and inorganic salt in characteristic proportions into the 

 lumen and thence from the orifices of the gland ducts (secreto- 

 motor nerves). 



The central mechanism is excited to coordinate activity, primarily, by 

 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. 



