THE ESSENTIAL PHENOMENA OF CIRCULATION. 



21 



carries it to other organs; the venous system, which 

 brings the blood from the organs back to the heart, and 

 the capillary system, which joins together the other two, 

 forming in fact the termination of each. 



The arteries start from the heart; they are distin- 

 guishable by the structure of their walls, which are com- 



FTG. 9. 



DIAGRAMS EXHIBITING THE ARRANGEMENT 

 OF THE VALVES OF VEINS. A, vein laid 

 open, showing the valves in pairs. B, 

 longitudinal section of a vein, indicating 

 the mode in which the valves, by ap- 

 position of their free edges, close its 

 calibre. The dilated condition of the 

 walls behind the valves is also seen. 

 C, vein distended, showing how the 

 sinuses behind the valves become di- 

 lated. 



CAPILLARIES. a, the ar- 

 tery; b, the vein; c, the 

 intervening capilla- 

 ries. 



posed of three distinct layers ; the external and internal 

 layers are soft and flexible, but the middle layer is elastic 

 and resisting. The veins run to the heart ; their walls 

 have two layers, corresponding to the internal and the 

 external coats of the arteries, but the middle one is 

 missing. 



