212 SERUM—USES OF BLOOD. 
matter of the serum bears to the whole mass of blood, in 
health, is about 53 parts in 1000; and of these about 40 
parts are albumen, 8 parts saline matter, and 5 parts fat, with 
certain ill-defined substances, of which some appear to be 
organic compounds that are undergoing metamorphosis into 
solid tissues, whilst others are the products of the decay of 
the tissues, which are being progressively withdrawn and 
eliminated by the excretory organs. 
239. The influence of the Blood as a whole upon the 
animal as well as on the nutritive functions, is easily proved. 
When an animal is bled largely, it is gradually weakened as 
the flow proceeds, and at last it seems to lose all consciousness 
and power of movement. If allowed to remain in this con- 
dition, it seldom or never recovers of itself. But if we inject 
into its veins, by small quantities at a time, blood similar to 
that which it has lost, the apparent corpse becomes as it were 
reanimated, and all its functions are completely re-established. 
The importance of the red particles is manifestly seen in the 
effect of this remarkable operation, which is called the trans- 
fusion of blood ; for if, instead of blood freshly obtained from 
another living animal, we inject serum without these particles, 
the effect is but little greater than if so much water were 
introduced, and the animal dies of the hemorrhage. By this 
operation, practised on the Human subject, many valuable 
lives have been saved, that would otherwise have been de- 
stroyed by loss of blood. Again, if, by mechanical means, as 
by tying the principal blood-vessel going to any organ, we 
cause a permanent diminution to any considerable extent, in 
the quantity of blood with which it is supplied, a decrease in 
its size is soon apparent, and it may even shrink almost to 
nothing. On the other hand, we observe that, the more active 
the function of a part, the larger is the quantity of blood with 
which it is supplied. Thus, when the antlers of the Stag, 
which fall off every year, are being renewed, the arteries that | 
supply the parts of the skull from which they spring, are 
greatly increased in size; but they shrink again, as soon as 
the growth of the horns is completed for that” year. A similar 
increase takes place among animals that suckle their young, 
in the size of the arteries that supply the mammary glands, 
by which the milk is formed ; and these also shrink, when 
this liquid is no longer required. 
