SECT. XXIV. j. 3. AND TEARS. 213 



cretory dufts, 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 repro- 

 duced, the pleafurable fenfation arifes along with them, and the 

 falival glands are excited into adion, and fill the mouth with fa- 

 liva from this fenfitive aflbciation, as is frequently feen in dogs, 

 who flaver at the fight of food. 



3. We have alfo a voluntary power over the aUon 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 acid material be held in the mouth, as the 

 root of pyrethrum, or the leaves of tobacco, the falival glands are 

 ftimulated into flronger action than is natural, and thence fe- 

 crete a much larger quantity of faliva ; which is at the fame 

 time more vifcid than in its natural Rate ; becaufe the lymphat- 

 ics, that open their mouths into the dufts of the falival glands, 

 and on the membranes, which line the mouth, are likewife ftim- 

 ulated into ftronger adlion, and abforb the more liquid parts of 

 the faliva with greater avidity ; and the remainder is left both in 

 greater quantity and more vifcid. 



The increafed abforption in the mouth by fome ftimulanng 

 fubftances, which are called aftringents, as crab-juice, is evident 

 from the inftant drynefs produced in the mouth by a fmall 

 quantity of them. 



As the extremities of the glands are of exquifite tenuity, as 

 appears by their difficulty of injedlion, it was neceflary foi them 

 to fecrete their fluids in a very dilute (late ; and, probably lor the 

 purpofe of ftimulating them into adion, a quantity of neutral 

 fait is likewife fecreted or formed by the gland. This aqueous 

 and faline part of all fecreted fluids is again reabforbed into the 

 habit. More than half of fome fecreted fluids is thus imbibed 

 from the refervoirs, into which they are poured ; as in the urin- 

 ary bladder much more than half of what is fecreted by the kid- 

 neys becomes reabforbed by the lymphatics, which are thickly 

 difperfed around the neck of the bladder. This feems to be the 

 purpofe of the urinary bladders of fifh, as otherwife fuch a re- 

 ceptacle for the urine could have been of no ufe to an animal 

 immerfed in water. 



5. The idea of fubftances difagreeably acrid will alfo produce 

 a quantity of faliva in the mouth ; as when we fmell very putrid 

 vapours, we are induced to fpit out our faliva, as if fomething 

 difagreeable was a&ually upon our palates. 



6. When difagreeable food in the ftomach produces naufea, 



a flow 



