CHAPTER IX 



THE CIRCULATION OF THE BLOOD 

 A. BLOOD AND LYMPH 



1. The blood as a common carrier. In previous chapters 

 some of the more general features of the circulation have 

 already been touched upon. In studying the parts of the 

 body the student has become somewhat acquainted with the 

 heart, the arteries, and the veins; in considering the typical 

 structure of the organs (Chap. Ill) he has seen how the 

 arteries are connected with the veins by a system of com- 

 municating tubes, the capillaries, through the thin walls of 

 which interchange takes place between the lymph and the 

 blood ; and in studying the interdependence and cooperation 

 of the cells and organs (Chap. VI) he has learned how the 

 blood leaving each organ returns to the heart, there to be 

 mixed with that coming from all other organs and thence 

 pumped first to the lungs and then to the rest of the body. 

 The need of a circulation is obvious, for the food received 

 from the alimentary canal and the oxygen received from the 

 lungs must somehow be carried to the muscle fibers, the 

 nerve cells, the gland cells ; the cellular wastes must be 

 taken away to the organs of excretion ; and the internal 

 secretions of the body must be transported from the organs 

 in which they are made to those in which they are to be 

 used. In other words, it is a necessary corollary to the fact 

 that no cell or organ " liveth unto itself " that there should 

 be some common carrier of matter and of energy from one 

 organ to another. Such a common carrier is the blood. The 

 analogy of the blood system of the body with the railway 



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