144 FUNCTIONS OF NUTRITION. 



The parotid saliva may be obtained, from the human subject, in a 

 state of purity, by introducing into the orifice of Steno's duct, through 

 the mouth, a silver canula, about one millimetre in diameter. The 

 other extremity of the canula projects between the lips, and the saliva 

 is collected from its orifice. 



The result of many observations, conducted in this manner, is that 

 human parotid saliva is colorless, watery, and alkaline in reaction. It 

 differs from the mixed saliva of the mouth, in being perfectly clear, 

 without turbidity or opalescence. Its flow is scanty while the jaws 

 remain at rest ; but if the movements of mastication are excited by 

 the introduction of food, it runs in much greater abundance. We have 

 collected, in this way, from the parotid ducts of one side only, in a 

 healthy man, 31.1 grammes of saliva in twenty minutes; and in seven 

 successive observations, made on different days, comprising in all 

 three hours and nine minutes, we have collected a little over 194 

 grammes. 



Parotid saliva may be obtained from the dog by exposing Steno's 

 duct where it crosses the masseter muscle, and introducing into it, 

 through an artificial opening, a silver canula. The secretion then runs 

 from the external orifice of the canula, without being mixed with the 

 other salivary fluids. It is clear, limpid, and watery, and without 

 perceptible viscidity, resembling in these respects the parotid saliva of 

 man. The submaxillary saliva of the dog is obtained in a similar 

 manner, by inserting a canula into Wharton's duct. It differs from the 

 parotid secretion, as regards its physical properties, chiefly in possessing 

 a well marked viscidity. The sublingual saliva is also colorless and 

 transparent, and is more viscid than that from the submaxillary. The 

 secretion of the muciparous glandules has been obtained by placing a 

 ligature simultaneously on Wharton's and Steno's ducts, and on that 

 of the sublingual gland, so as to shut out from the mouth all their secre- 

 tions, and then collecting the fluid supplied by the mucous membrane. 

 This fluid is very scanty, and so much more viscid than the other secre- 

 tions that it adheres strongly to the surface of a glass vessel. All the 

 salivary secretions of the dog are alkaline in reaction. They differ from 

 those of man chiefly in the absence of ptyaline, and in their consequent 

 want of action on starchy substances. 



Mode of Secretion of the Saliva. The salivary glands differ 

 from each other in the abundance of their secretion and in the influ- 

 ences which excite their activity. The parotid saliva is most abun- 

 dantly poured out under any stimulus which excites the movement of 

 the jaws, such as the mastication of dry substances, or continuous 

 speaking. According to Bernard, the submaxillary secretion is espe- 

 cially increased by the introduction of substances which excite the 

 taste ; while that of the sublingual glands in the dog is exuded at the 

 moment of deglutition, and aids, with that of the muciparous glandules, 

 in lubricating the mouth and fauces, and facilitating the passage of the 



