[ CHAPTER XII 



THE BLOOD 



The blood may be defined as the nutritive fluid of the body since it con- 

 tains all those materials that are necessary to the maintenance of the nutri- 

 tion. The presence and proper circulation of the blood in the living or- 

 ganism are essential for the maintenance of tissue irritability and for the 

 manifestation of the activities 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. 

 Thejrritability, however, persists for a variable length of time though it too 

 gradually declines and finally disappears. 



Though contained within a practically closed system of vessels, the blood 

 is brought into intimate relation by reason of its relation to the lymph, 

 with all the tissue elements, through the intermediation of the capillaries. 

 As the blood flows through these delicate vessels, portions of its soluble 

 nutritive constitutents, including oxygen, are given up to the lymph 

 from which they are absorbed by the tissues and utilized for growth, 

 repair, and the liberation of heat. 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. 



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 the heart and the blood be suddenly withdrawn, there is an immediate 

 cessation in the activity of all the organs and the animal falls to the floor 

 as though dead; the return of the blood to the vessels within a limited 

 period of time is promptly followed by a renewal of their activities, 

 the animal recovers consciousness and exhibits its usual modes of activity. 



The Physical Constitution 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 corpuscle, colorless in appearance and smaller 

 in size than the ordinary white corpuscle, is present in the blood-stream 



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