THE DIGESTIVE SYSTEM. 295 



if the distal portion of the nerve be stimulated, a profuse flow of saliva 

 follows within two or three seconds, while at the same time the blood- 

 vessels of the gland are dilated, and the output of carbonic acid and the 

 intake of oxygen are both increased. Section of the sympathetic fibres 

 is likewise followed by a negative result, while stimulation of the distal 

 portion in the dog leads to the production of a very small flow of viscid 

 saliva, accompanied by constriction of the blood-vessels. In view of 

 these experiments it might appear that the primary effect of the 

 stimulation of the chorda tympani was the dilatation of the vessels, and 

 that the flow of saliva was due to filtration. 



vn N , 



S 



FIG. 115. Scheme of nerve supply of submaxillary gland. 



VII N, facial nerve ; C.T., chorda tympani ; L, lingual nerve ; G, Langley's ganglion ; 

 S, sympathetic fibres to the gland. 



Filtration is a purely physical process in which fluids pass through 

 a permeable membrane under the influence of pressure. In the process 

 of secretion, on the other hand, work is done by cells ; these take up 

 material from the lymph which bathes them and is derived from the 

 blood, effect chemical changes in that material, and discharge the re- 

 sulting products in the form of a "secretion." In the case of the 

 salivary glands, several facts indicate that the process is a secretory 

 one. (1) The cells of the salivary gland, as will be described more 

 fully later, accumulate granules during the period when saliva is not 

 being poured out, and discharge them during the period of activity. 

 (2) The consumption of oxygen by a salivary gland is increased during 

 the production of saliva, indicating that work is being done. (3) Two 

 of the constituents of saliva, mucin and ptyalin, do not exist in the 



