1 66 TEXT-BOOK OF PHYSIOLOGY. 



vessels possess that degree of dilatation necessary for nutritive pur- 

 poses. 



With the beginning of mastication the blood-vessels suddenly 

 dilate, the blood-supply is increased, and the gland-cells begin to dis- 

 charge water, inorganic salts, and their organic constituents into the 

 lumen of the acinus. This continues until mastication ceases, when 



dl the structures return to their former condition of relative inactivity, 

 he entire process is reflex in character and takes place through the\ 

 edulla oblongata. ^It requires the usual mechanism necessary for 1 

 ail reflex acts viz., a sentient surface, afferent nerves, emissive 

 cells, efferent nerves, and the responsive organs. 



With the introduction of food into the mouth impressions are 

 made on the terminal branches of the afferent nerves distributed in 

 the mucous membrane. Nerve impulses developed by the mechanic 

 and chemic action of the food are then transmitted to the medulla 

 oblongata and received by emissive cells. These in turn discharge 

 nerve impulses which are transmitted through efferent nerves to the 

 structures, producing the vascular and secretory effects already stated. 

 The nerves and nerve-centers which constitute the reflex mechan- 

 ism for the secretion of saliva are shown in the following table: 



Afferent Nerves. Nerve-centers. Efferent Nerves. 



1. Lingual branch of fifth Medulla oblongata. Chorda tympani for the submax- 



nerve. illary and sublingual glands, 



auriculotemporal branch of the 

 fifth nerve for the parotid 

 gland. 



2. Glossopharyngeal. Sympathetic nerve. 



That the secretion of the saliva is regulated by the above mechan- 

 ism, and that the lingual branches of the fifth nerves and the 

 glossopharyngeals- are the afferent nerves, can be demonstrated by 

 exposing the glands and their nerve connections and subjecting them 

 to experiment. Under such circumstances, if a cannula be placed 

 in the duct of the submaxillary gland, and the lingual nerve stimu- 

 lated by an induced electric current of moderate strength, a copious 

 flow of saliva at once takes place. If now the glossopharyngeal nerve 

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

 same. Division of these two 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 pneumo- 

 gastric, when the mucous membrane of the stomach is stimulated; 

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

 nerve-fibers that originate higher up in the brain and are stimulated 

 by ideas and emotions. 



