NATURE OF THE SALIVA IN HORSE AND DOG. 
167 
ceases in an abrupt manner, although the triturating process 
is still going on. The reason of this singular phenomenon 
long remained uncertain ; we are now aware that the gland 
which lies on that side of the mouth where mastication is 
going on almost entirely suffices for the insalivation of 
aliments; when the animal (as in the present case) has had 
both ducts laid open, and a tube passed into them, it becomes 
evident that during mastication the two parotids are 
alternately called into action; while the one secretes, the 
other is at rest. The horse we have operated upon is at 
present satisfying its hunger, and you perceive that the saliva, 
however abundant, only flows on one side at a time. 
In the normal state the chemical composition of saliva, as 
secreted by each gland in particular, remains invariably the 
same; it may, however, accidentally contain foreign sub¬ 
stances. We gladly seize this opportunity of exhibiting, in 
a strong light, the elective action of glands; the secretion we 
are investigating at the present moment offers a remarkable 
instance of this singular property. Among the various 
bodies introduced into the blood we find some whkh almost 
instantaneously pass into the saliva; iodine and its various 
compounds enjoy this property. Other substances cannot 
penetrate without the greatest difficulty into this secretion; 
the salts of iron belong for the most part to this latter class. 
A direct experiment will enable you to judge for yourselves. 
The crural vein of a dog being opened, the extremity of a 
small syringe is introduced into the vessel; a determinate 
quantity* of a solution containing 1-lOOth part of the yellow 
prussiate of potash is then injected into the torrent of circula¬ 
tion, together with an equal amount of a similar solution of 
iodide of potassium ; this latter substance will almost instan¬ 
taneously be met with in the animaFs saliva, while none of 
the usual tests exhibit the slightest vestige of the yellow 
ferro-cyanuret of potash. In the animal’s urine we shall, on 
the contrary, discover it in considerable quantities, a fact 
which amply demonstrates the penetration of this substance 
into the system, although the salivary glands refuse to 
eliminate it. 
(These various experiments are all tried on the dog, and 
meet with perfect success. It is necessary, before employing 
the usual tests, to neutralize, by adding a few drops of acetic 
acid, the alkaline reaction which saliva naturally presents; 
the chemical actions might otherwise be impeded.) 
You therefore see, gentlemen, that iodine and its com¬ 
pounds will readily pass into the saliva, while the principal 
* Five dubic centimetres. 
