CHAPTER XXXIV 

 THE BLOOD 



208. Blood. In all animals above the corals and sea- 

 anemones, and certain kinds of worms, there is present a 

 circulating mass of liquid which is commonly called blood, 

 although not all kinds of blood are alike. In the clams the 

 blood contains a bluish substance, called hemocyanin, which 

 easily combines with oxygen, and thus carries oxygen obtained 

 from the surrounding water by diffusion into the capillaries of 

 the skin and gills. In the earthworm there is a reddish sub- 

 stance, called hemoglobin, dissolved in the blood, which be- 

 haves in much the same way as the hemocyanin of the clams. 



The blood of back-boned animals has a rather complex 

 structure, and is associated with an elaborate system of vessels 

 and a pumping organ called the heart. 



209. Composition of human blood. When examined with 

 the microscope, human blood is seen to consist of a colorless 

 liquid, called the plasma, and a number of small bodies float- 

 ing in it. The more numerous particles are the so-called red 

 corpuscles. These are very small J and have the shape of a 

 disc or coin, with rounded edges and compressed towards the 

 middle (Fig. 67). In addition to the red corpuscles, there are 

 also white, or colorless, corpuscles, some barely larger than the 

 red ones, others many times as large. 



The fluid portion of the blood, the plasma, consists chiefly 

 of water. In this are dissolved various salts, organic substances 

 derived from the digested food, some oxygen, some carbon 

 dioxid, certain ferments, and other organic substances derived 



1 On the average, about of an inch in diameter. 



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