CHAPTER XV. 

 THE BLOOD. 



Introduction. The individual cells forming the most simple 

 types of life are nourished by substanees which they obtain 

 directly from the water in which the animal lives. In exchange 

 for this food, they excrete into the water the waste materials of 

 their metabolism. As the organism becomes more and more 

 complex this direct interchange of materials becomes impossible, 

 and the blood and lymph assume the task of delivering food to 

 the tissues and of removing the waste materials. To accomplish 

 this, these fluids come into close relation with the absorbing, elimi- 

 nating, and general tissue elements of the body, the lymph being 

 in immediate contact with the cells and the blood moving quickly 

 from place to place. Therefore all the elements found in the 

 tissues and all the waste materials produced by the body ;nv 

 present at some time or another in the blood. The blood may in- 

 deed be compared to the wholesaler of commerce, who handles 

 all the materials for the support of life, and the lymph to the 

 retailer, who distributes to the tissue cells the materials which 

 they need. In short, it may be said that the blood replenishes 

 the lymph for the losses which it incurs in supplying the tissues. 



Physical Properties. Ordinary mammalian blood is an 

 opaque, somewhat viscid fluid, varying in color from a bright 

 red in arterial blood to a dark red in venous blood. Contact witli 

 air changes venous blood to arterial blood. Microscopical exami- 

 nation show r s that the blood is not perfectly homogenous, but 

 consists of a clear fluid in which cells called corpuscles are sus- 

 pended. 



The Corpuscles. 



There are three varieties of these: the red corptischs (to which 

 the color of blood is due), the wliite corpuscles and the Mood 

 platelets. 



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