CHAPTER VIII. 

 THE CIRCULATORY SYSTEM. 



The circulatory system is made up of organs which serve to pro- 

 pel and convey to the various parts of the body the fluids through 

 the medium of which those parts make the exchanges of material 

 incident to their nutrition and functional activities. 



For some of these exchanges it appears necessary for the circu- 

 latins: fluids to come into the most intimate contact with the tissue- 

 elements ; to penetrate the interstices of the tissues and bathe their 

 structures. For mechanical reasons these fluids must circulate 

 slowly and consume a considerable time in traversing a relatively 

 short distance. Such a sluggish current could not avail for the 

 transportation of oxygen from the lungs to the tissues, and we 

 find that the circulatory system is divided into two closely related 

 portions : the haematic circulation and the lymphatic circulation. 

 The former is rapid, and the circulating fluid is the blood, the red 

 corpuscles of which serve as carriers of oxygen. The latter is slow, 

 and the circulating fluid, called " lymph," is derived from the liquid 

 portion of the blood (" the plasma "). The blood is confined within a 

 system of closed tubes, the bloodvessels ; but the lymph, when first 

 produced by transudation through the walls of the bloodvessels, is not 

 enclosed within vessels, but permeates the tissues or enters minute 

 interstices between the tissue-elements surrounding the bloodvessels. 

 Thence it gradually makes its way into larger spaces — lymph-spaces 

 — which open into the thin-walled vessels constituting the radicles 

 of the lymphatic vascular system. These smaller lymphatic vessels 

 join each other to form larger tubes, which finally open into the 

 venous portion of the haematic circulation, thus returning to the 

 blood the lymph which has made its way through the tissues. 



The circulating fluids are kept in motion chiefly by the pumping 

 action of the heart, which forces blood into the arteries, whence it 

 passes through the capillaries into the veins, and thence back to the 

 heart. During its passage through the smaller arteries, the capil- 



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