THE CIRCULATION OF THE BLOOD 



309 



In nearly all animals, the nutritive fluid is conveyed 

 to the various parts of the body by a system of tubes, 

 called blood vessels. The higher forms have two sets 

 arteries and veins, in which the blood moves in opposite 

 directions, the former carrying blood from a central 

 reservoir or heart, the latter taking it to the heart. In 

 the vertebrates the walls of these 

 tubes are made of three coats, 

 or layers, of tissue, the arteries 

 being elastic, like rubber, and 

 many of . the veins being fur- 

 nished with valves. 112 The great 

 artery coming out of the heart 

 is called aorta, and the grand 

 venous trunk, entering the heart 

 on the opposite side, is called 

 vena cava. Both sets divide and 

 subdivide until their branches 

 are finer than hairs; and joining 

 these finest arteries and finest 

 veins are intermediate micro- 

 scopic tubes, called capillaries 

 (in man about 3^0 of an inch in 

 diameter). 113 In these only, so 

 thin and delicate are their walls, 

 does the blood come in contact 

 with the tissues or the air. 



In those vertebrates which have lungs there are two 

 sets of capillaries, since there are two circulations the 

 systemic, from the heart around the system to the heart 

 again, and the pulmonary, from the heart through the 

 respiratory organ back to the heart. This double 

 course may be illustrated by the figure 8. In gill- 

 bearing animals there are capillaries in the gills, but 

 not a double circulation. 



FIG. 265. Relation of artery, a, 

 vein, 6, and capillaries, c, as seen 

 in the muscles of a Dog. 



