104 



COMPARATIVE ZOOLOGY. 



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 arte- 

 ries being elastic, like rubber, and 

 many of the veins being furnished 

 with valves. 68 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 

 microscopic tubes, called capilla- 

 ries (in Man about -^^ of an inch 

 in diameter)/' In these only, so 

 of a Dog. 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 circu- 

 lation. 



There is no true system of blood-vessels below the 

 Star-fish. The simplest provision for the distribution of 

 the products of digestion is shown by the Jelly-fish, whose 

 stomach sends off radiating tubes (Fig. 197). 



