18 PRINCIPLES OF ANIMAL NUTRITION. 
The percentage of dextrose in the blood is small, but remarkably 
constant, the limits of variation being from about 0.11 to about 0.20: 
per cent., and the average about 0.15 per cent. Its amount 
varies but slightly in different regions of the body, and in different 
classes of animals, and is scarcely at all affected by the nature or 
amount of the food. Not only so, but any excess of dextrose in the 
blood is promptly gotten rid of. It is astriking fact that if any con- 
siderable amount of this substance, which forms so large a part of 
the resorbed nutriment, be injected directly into the blood it is. 
treated as an intruder and at once excreted through the kidneys. 
Evidently it is of the greatest importance to the organism that the 
supply of this substance to the tissues shall be constant. 
Under ordinary conditions, however, the influx of sugar from the 
digestive tract is more or less intermittent. After a meal rich in 
easily digestible carbohydrates, an abundant supply of it is taken 
up by the intestinal capillaries, while on a diet poor in carbohydrates 
or in prolonged fasting, the supply sinks to a minimum. This is, of 
course, especially true of animals like man and the carnivora in 
which the process of digestion is comparatively rapid, but even in 
herbivorous animals, with their more complicated digestive appara- 
tus, the rate of resorption of dextrose, and still more its absolute 
amount, must be more or less fluctuating. Evidently there must be 
some regulative apparatus which holds back from the general circu- 
ation any excess of dextrose, on the one hand, and prevents its. 
being excreted unused, and on the other, supplements any lack 
resulting from a deficiency of the food in carbohydrates. This 
regulation is accomplished by the liver. 
Functions of the Liver. 
The functions of the liver in this regard appear to be twofold: 
First, it manufactures dextrose and supplies it to the general circu- 
lation; and second, it serves as a reservoir, or a place of deposit, for 
any excess of carbohydrates supplied by the digestive apparatus. 
Tue Liver as a Source or Drxrrose.—The blood as it 
comes from the intestinal capillaries, bearing the digested carbo- 
hydrates and proteids, enters the liver through the portal vein and 
is distributed by means of the capillary blood-vessels into which this 
vein divides through all parts of that organ, reaching the general 
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