SECT. XXIV. 2. 3-4- OF THE SALIVA AND TEARS. 31 j 



themfelves are brought into increafed aftion at the 

 fame time by afibciation, and feparate a greater 

 quantity of their juices from the blood ; in the fame 

 manner as tears are produced in greater abundance 

 during the ftimulus of the vapour of onions, or of 

 any other acrid material in the eye. 



The faliva is thus naturally poured into the mouth 

 only during the ftimulus of our food in maftication ; 

 for when there is too great an exhalation of the mu- 

 cilaginous fecretion from the membranes, which 

 line the mouth, or too great an abforption of it, the 

 mouth becomes dry, though there is no deficiency in 

 the quantity of faliva ; as in thofe who fleep with 

 their mouths open, and in fome fevers. 



2. Though during the maftication of our natural 

 food thefalival glands are excited into aclion by the 

 ftimulus on their excretory duds, and a due quantity 

 of faliva is feparated from the blood, and poured 

 into the mouth; yet as this maftication of our food 

 is always attended with a degree of pleafure ; and 

 that pleafurable fenfation is alfo connected with our 

 ideas of certain kinds of aliment : it follows, that 

 when thefe ideas are reproduced, the pleafurable fen- 

 fation arifes along with them, and the falival glands 

 are excited into action, and fill the mouth with faliva 

 from this fenfitive alTociation, as is frequently feen in 

 dogs, who Haver at the fight of food. 



3. We have alfo a voluntary power over the ac- 

 tion of thefe falival glands, for we can at any time 

 produce a flow of faliva into our mouth, and fpit out, 

 or fwallow it at will. 



4. If any very acrid material be held in the mouth, 

 as the root of pyrethrum, or the leaves of tobacco, 

 the falival glands areftimulated into ftronger action 

 than is natural, and thence fecrete a much larger 

 quantity of faliva ; which is at the fame time more 

 vifcid than in its natural ftate ; becaufe the lympha- 

 tics, that open their mouths into the duels of the 



falival 



