CHAPTER XII. 

 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 mechanisms. The escape of the blood from 

 the vessels, especially in the higher animals, is followed by cessation of the 

 physiologic activities 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 experi- 

 ment: If the nozzle of a syringe, adapted to the size of the animal, be intro- 

 duced through the jugular vein into the right side of the heart and the blood 

 be suddenly withdrawn, there is an immediate cessation in the activity of 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 constituents, 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 or katabolic products, 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 and absorbed from the intestinal canal ; 

 of oxygen, absorbed from the respiratory surface of the lungs; of katabolic 

 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 katabolic products, it neverthe- 

 less presents certain average physical, morphologic, and chemic properties 

 which distinguish 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 particles termed corpuscles floating in 

 it, of which there are two varieties, the red or the erythrocytes and the white 

 or the leukocytes. By appropriate methods it can be shown that a third cor- 

 puscle, colorless in appearance and smaller in size than the ordinary white 

 corpuscle, is present in the blood-stream and known as the blood-platelet 

 or plaque. The different constituents can be roughly separated by appropriate 

 means when the blood is withdrawn from the body. If the blood of the 

 horse is allowed to flow directly into a tall cylindric glass vessel, surrounded 



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