CHAPTER XI. 



THE BLOOD. 







The blood is a highly complex nutritive fluid, the presence and 

 proper circulation of which in the living organism are essential to the 

 maintenance and activity of all physiologic processes. The escape 

 of the blood from the vessels, especially in the higher animals, is 

 followed by a loss of the physiologic properties of all the tissues 

 within a short period of time. The immediate dependence of the 

 functional activities of the tissues and organs on the presence of the 

 blood can be demonstrated by the following experiment : If the nozzle 

 of a syringe, adapted to the size of the animal, be introduced through 

 the jugular vein into the right side of the heart and the blood be sud- 

 denly withdrawn, there is an immediate cessation in the activity of 

 all the organs; the return of the blood to the vessels within a limited 

 period of time is promptly followed by a renewal of their activity. 



Though contained within a practically closed system of vessels, 

 the blood is brought into intimate relation with all the tissue elements 

 through the intermediation of the capillaries. As the blood flows 

 through these delicate vessels, portions of its soluble nutritive con- 

 stituents, including oxygen, are given up to the tissues, by which they 

 are utilized for growth, repair, and functional activity. At the same 

 time the tissues yield up to the blood a series of decomposition prod- 

 ucts, resulting from their activity, which vary in quantity and quality 

 according as the blood traverses the muscles, nerves, glands, or 

 other tissues. 



The blood may be regarded, therefore, as a reservoir of^nutritive 

 materials prepared by the digestive apparatus arid 'aBsorbed from 

 thlTfoocTcanal; of oxygen, absorbed from the respiratory surface of 

 the lungs; of decomposition products, produced by and absorbed 

 from the tissues. Though the blood varies in composition in different 

 parts of the body in consequence of the introduction of both nutritive 

 material and decomposition products, it nevertheless presents certain 

 average physical, morphologic, and chemic properties which dis- 

 tinguish it as an individual tissue. 



Constituents of Blood. A microscopic examination of the blood 

 as it flows through the capillary vessels of the web of the frog or the 

 mesentery of the rabbit shows that it is not a homogeneous fluid, but 

 that it consists of two distinct portions: viz., (i) a clear, transparent, 

 slightly yellow fluid, the plasma or liquor sanguinis; (2) small par- 



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