CHAPTER IX. 



THE VASCULAR SYSTEM CONTINUED: HEART; ARTERIES; 

 VEINS; CAPILLARIES. 



THE blood, as we have said, is the internal medium on which 

 the tissues live. It is carried through the body by branched 

 tubes named blood-vessels. It is driven along these tubes by 

 the action of the heart, which is a hollow muscular organ placed 

 in the centre of the vascular system. One set of vessels the 

 arteries conducts the blood out from the heart and distributes 

 it to the different parts of the body, whilst other vessels the 

 veins bring it back to the heart again. The blood from the 

 arteries gets into the veins by passing through a network of 

 fine tubes which connect the two, and which are named, on 

 account of their small size, the capillary (i.e. hair-like) vessels. 



All the tissues except the epithelial and cartilaginous tis- 

 sues are traversed by these networks of capillary vessels. 

 It is through the thin walls of the capillaries that the inter- 

 change of material which is continually going on between the 

 blood and the tissues takes place. It is in the capillaries, then, 

 that the chief work of the blood is done ; and the object of the 

 vascular mechanism is to cause the blood to flow through these 

 vessels in the manner best adapted for accomplishing this work. 



The use of the arteries is to carry and regulate the supply of 

 blood from the heart to the capillaries; the use of the veins, to 

 carry the blood from the capillaries back to the heart ; the use 

 of the heart, to drive the blood in a suitable manner through 

 the arteries into the capillaries, and from the capillaries back 

 along the veins to itself again. We shall see that the structure 

 of these several parts is adapted to these several uses. 



The heart. The heart is a hollow muscular organ, divided 

 by a longitudinal partition into a right and a left heart, each of 

 which is subdivided by a transverse constriction into two com- 

 partments, an upper and a lower, which communicate with each 



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