BLOOD SYSTEM. 



379 



'into two branches, one (carotid) going to the right side 

 of the head and brain, the other (right subclavian) 

 going to the right arm. The next in order coming from 

 the arch is the left 

 carotid. It goes to 

 the left side of the 

 head and brain, 

 while the third and 

 last bends to the 

 left as the left sub- 

 clavian and goes to 

 the left arm. In the 

 right heart all that 

 is necessary to point 

 out is the great out- 

 going vessel. The 

 pulmonary artery 

 arches to the left to 

 go to the left lung, 

 but sends back a 



branch of equal size to supply the right lung. The sig- 

 nificance of all of these special arrangements will be 

 seen later. 



The change from the bright blood to the dark blood 

 takes place in the capillaries of the tissues ; the change 

 back again to bright blood in the capillaries of the 

 lungs. Therefore in the systemic circulation the arteries 

 carry bright blood and the veins dark blood, while in 

 the pulmonic circulation the reverse is the case. All the 

 blood going from the lungs to the heart and from the 

 heart to the tissues is bright blood, and all the blood 

 from the tissues to the heart and from the heart to the 

 lungs is dark blood. Or, all the blood that ever visits 

 the left heart is bright and all the blood that ever visits 

 the right heart is dark. It is common to speak of the 



Fig. 266. — The heart and the great vessels, in- 

 coming and outgoing. The right heart and 

 its vessels are shaded. 



