CHAPTER XIII. 



THE CONSTITUTION OF THE BLOOD AND BLOOD PLASMA. 



In all animals, except those which form the lowest class (Pro- 

 tozoa), the distribution of the nutritious materials to the various 

 parts of the body, as well as the collection of the effete matters 

 prior to their expulsion, is carried on by the medium of a fluid 

 which circulates through the different parts of the body. This 

 fluid is the blood. 



In vertebrate animals the blood passes through a closed system 

 of elastic pipes, and it is kept in constant motion by the action of 

 a muscular pump. It is first forced through strong-walled, branch- 

 ing canals called arteries, whose walls gradually become thinner 

 as the branches get smaller, and these end in a network of delicate 

 channels (capillaries), through which it slowly trickles into the 

 wide, soft- walled veins, by means of which it flows gently back 

 again to the heart. In its course it receives the nutritive materials 

 from the stomach and intestines after digestion, the specially elab- 

 orated substances from the liver, spleen and lymph glands, and 

 the oxygen absorbed from the air in the lungs. In short, it con- 

 tains and bears to their destination all the materials required for 

 the chemical changes of the various tissues. While passing 

 through the capillary networks of the various tissues, it takes up 

 the waste materials resulting from the tissue changes and bears 

 them to their proper point of exit from the body ; at the same 

 time the nutriment is allowed to ooze through the delicate vessel 

 walls and be diffused in the tissues. 



GENERAL CHARACTERISTICS OF THE BLOOD. 

 The blood of vertebrate animals is of a bright scarlet color 

 when exposed to the ox} r gen of the air, but when not in contact 

 with oxygen it is a dark, purplish red. 



The blood is remarkably opaque, as may be seen by placing a 

 thin layer on a piece of glass over the page of a book. This 

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