168 
NATURE OF THE SALIVA IN HORSE AND DOG. 
salts of iron are not to be found in that fluid, under ordinary 
circumstances. We possess, however, various means of over¬ 
coming the resistance (so to speak) of the salivary glands. 
If, in the first place, we combine one of the refractory sub¬ 
stances with another body that enjoys the property of passing 
into the saliva, the difficulty is overcome ; iodide of iron, for 
instance, passes readily into the saliva, on account of the 
iodine it contains. 
In the second place, if a direct injection be had recourse 
to, the salivary glands may be compelled, as it were, to 
eliminate the obnoxious substances. The largest doses 
of prussiate of potash may, for instance, be poured into the 
animahs veins, without giving the slightest indication of its 
presence in the saliva, even when recourse is had to the most 
sensible tests; but if, by means of an injection into the 
common carotid artery, we create, in a manner, a local 
plethora in the atmosphere of the salivary glands, we succeed 
in obtaining the desired result, which, under any other cir¬ 
cumstances, we should invariably have failed in producing. 
It therefore remains an established fact, that the so-called 
affinity of glands for certain bodies only expresses the greater 
facility with which they gain admittance into the fluid 
secreted; all substances are capable of passing into the 
secretions, provided that a sufficiently large quantity of them 
be conveyed into the blood that furnishes the elements of 
these various fluids. 
The affinity of the salivary glands in particular produces, 
however, a great many singular phenomena. If, for instance, 
iodide of potassium be administered to a dog, vestiges of that 
substance will be discovered in the saliva for several weeks 
together. A sort of circuhis is, in fact, established ; iodine 
passes into the saliva through the elective affinity of the 
glands ; but the animal, which keeps on swallowing its own 
saliva, impregnated as it is with iodine, absorbs new quantities 
of the substance, and the process might last indefinitely ; but 
if the animal be strongly purged, the iodine is at once evacu¬ 
ated by the intestinal discharge, and appears no longer in the 
saliva. 
It now remains for us to perform the inverse experiment. 
The internal surface of glands is endowed with a considerable 
power of absor[)tion, provided the substances injected into 
their cavity belong to that class of bodies for which the gland 
exhibits a certain elective affinity. But when the secreting 
process is in full activity, absorption is almost entirely 
suspended ; thus (as we have already informed you), if a 
strong solution of strychnia is injected into a dog^s parotid 
