DIGESTION OF FOOD. 361 
relieved by the natural secretions. If, however, fluid be intro- 
duced into the blood either directly or through the alimentary 
canal, the thirst is also relieved speedily. The fact that we 
know when to stop drinking water shows of itself that there 
must be local sensations that guide us, for it is not possible to 
believe that the whole of the fluid taken can at once have en- 
tered the blood. 
Again, in the case of hunger, the introduction of innutritious 
matters, as earth or sawdust, will somewhat relieve the urgent 
sensations in extreme cases; as will also the use of tobacco by 
smokers, or much mental occupation, though the latter is 
rather illustrative of the lessening of the consciousness of the 
ingoing impulses by diverting the attention from them. But 
hunger, like thirst, may be mitigated by injections into the 
intestines or the blood. It is, therefore, clear that, while in the 
case of hunger and thirst there is a local expression of a need, 
a peculiar sensation, more pronounced in certain parts (the 
fauces in the case of thirst, the stomach in that of hunger), 
yet these may be appeased from within through the medium 
of the blood, as well as from without by the contact of food or 
water, as the case may be. 
Up to the present we have assumed that the changes 
wrought in the food in the alimentary tract were identical 
with those produced by the digestive ferments as obtained by 
extracts of the organs naturally producing them. But for 
many reasons it seems probable that artificial digestion can not 
be regarded as parallel with the natural processes except in a 
very general way. When we take into account the absence of 
muscular movements, regulated according to no rigid prin- 
ciples, but varying with innumerable circumstances in all 
probability ; the absence of the influence of the nervous sys- 
tem determining the variations in the quantity and compo- 
sition of the outflow of the secretions; the changes in the rate 
of so-called absorption, which doubtless influences also the act 
of the secretion of the juices—by these and a host of other con- 
siderations we are lead to hesitate before we commit ourselves 
too unreservedly to the belief that the processes of natural 
digestion can be exactly imitated in the laboratory. 
What is it which enables one man to digest habitually what 
may be almost a poison to another? How is it that each one 
can dispose readily of a food at one time that at another is quite 
indigestible? To reply that, in the one case, the digestive 
fluids are poured out and in the other not, is to go little below 
