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CHAPTER XI 



THE CIRCULATORY SYSTEM 



The circulatory apparatus consists of two systems of tubular 

 structures, the blood-vessel system and the lymph-vessel system, 

 which serve, respectively, for the transmission of blood and lymph. 



THE BLOOD-VESSEL SYSTEM 



This consists of (a) a central propelKng organ, the heart; (b) a series 

 of efferent tubules — the arteries — which by branching constantly 

 increase in number and decrease in cahbre, and which serve to carry 

 the blood from the heart to the tissues; (c) minute anastomosing 

 tubules — the capillaries — into which the arteries empty and through 

 the walls of which the interchange of elements between the blood 

 and the other tissues takes place; (d) a system of converging tubules 

 — the veins — which receive the blood from the capillaries, decrease 

 in number and increase in size as they approach the heart, and serve 

 for the return of the blood to that organ. 



The entire system — heart, arteries, veins, capillaries — has a com- 

 mon and continuous hning, which consists of a single layer of endothe- 

 lial cells. Of the capillaries this single layer of cells forms the entire 

 wall. In the heart, arteries, and veins, the endothelium serves 

 simply as the hning for walls of muscle and connective tissue. 



Capillaries 



It is convenient to describe these first on account of their simpUcity 

 of structure. A capillary is a small vessel from 4.5 to i6fjL in diameter. 

 Its waU consists of a single layer of endothelial cells. The cells are 

 somewhat elongated in the long axis of the vessel. The smaller the 

 calibre of the capillary the more elongated are the cells. Two endo- 

 thelial cells suffice to complete the circumference of small capillaries, 

 while larger require three or four. The protoplasm of the cells is 

 clear or finely granular. The nuclei are oval, with their long axes of 

 the vessel. In fixed material the nuclei bulge into the lumen. In 



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