190 BIOLOGY 



system. The arteries, which take blood away from the heart, 

 are thick-walled and elastic; while the veins, which bring it 

 back again, are thin-walled. The distribution of the chief 

 blood vessels is shown in Figure 92. The bulbus arteriosus 

 soon divides into two branches that turn backward and finally 

 unite with each other in the abdomen beneath the stomach. 

 These two branches, the aortae, ao, give off in their course 

 many vessels, the chief of which are the lingual to the tongue, 

 I, the carotid to the head, co, the brachial to the arms, 6, 

 and the coeliac axis to the organs of the abdomen, ca. After 

 dividing many times into smaller and smaller branches, the 

 arteries finally break up into an immense number of minute 

 thin-walled vessels called capillaries (Lat. capillus = hair). 

 These are microscopic, but of great importance, since all of 

 the interchanges between the blood and the tissues of the body 

 take place through them; Fig. 101. The blood, after passing 

 through the capillaries, enters again into a series of vessels of 

 constantly increasing diameter and finds its way back to the 

 heart. These larger returning vessels are veins, and they 

 unite with others, until finally a few large veins are formed 

 which empty into the sinus; Fig. 92 B. The blood vessels 

 thus form a closed system, and the blood that leaves the heart 

 returns without leaving the vessels. The blood that goes to 

 the intestine by the coeliac axis, ca, passes through two series 

 of capillaries before again entering the heart. It first passes 

 into capillaries in the intestine, where it receives nutriment 

 absorbed from the food; then it is collected into a large vein, 

 the portal vein, pv, which enters the liver, and breaks up into 

 another system of capillaries; then, by the way of the hepatic 

 vein (Gr. hepar = liver), hv, it enters into the large posterior 

 vena cava (Figs. A and B, ptc), which leads to the venus sinus. 

 This system of veins and capillaries in the liver is called the 

 portal system. Part of the blood that goes to the legs also 

 has a double system. It first enters the capillaries in the 

 muscles of the legs, and on its way back a part of it passes 



