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520. THE VASCULAR SYSTEM. 



The nutritious fluid moves in closed flexible tubes, which tra- 

 verse the body in all directions. That division of these tubes, 

 through which the blood moves, has a contractile organ as the 

 central point, from which blood is impelled into the tubes, and to 

 which it returns from the tubes. 



The other division, in which the Chyle and Lymph move, does 

 not possess in itself such a central point, but in the blood sys- 

 tem, to which it carries its contents, without being immediately 

 distributed from its orifice. 



521. The System of Vessels carrying Blood. 



Central organ the heart. From it vessels go off the arteries 

 (pulsating vessels). To it vessels lead back the veins (blood 

 vessels). Very delicate ramifications and transitions between the 

 two kinds of vessels. Capillary vessels. 



522. Course. The vessels, vasci, are cylindrical tubes, the 

 largest trunks of which lie in the neighbourhood of the central 

 organs, and the branches of which, going off at an acute angle, 

 diminish in circumference towards the periphery. The effect of 

 this continued division however is, that the calibre of the branches 

 taken together is wider than that of the trunk from which they 

 came off". 



Many branches divide from one trunk, and pass to another, 

 without diminishing in circumference, that is, they anastomose. 

 Their course is curved or in the form of an acute angle. If 

 they are numerous, and the interspace between them only small 

 in proportion to the organ, they are called Plexuses, which is 

 particularly remarkable in the venous trunks. When larger 

 branches suddenly divide into a number of smaller vessels united 

 together, without previous arborescent separation, the so-called 

 rete mirabile is formed, for example, in the lungs. The capil- 

 lary vessels also form a network, but which by gradual, always 

 more delicate, division, arises from large vessels. The more nar- 

 row and complicated the course through which the blood has to 

 circulate, so much the longer it is delayed therein. 



Structure of the vessels. The parietes of the vessels consist of six mem- 

 branous layers, which are not, however, always present at the same time in 

 every vessel. 



a. The innermost layer, which lies next to the cavity of the vessel, is a thin, 



