ARTERIAL- AND VENOUS-BLOOD VESSELS. 147 



they are so closely packed that a pin's point cannot be 

 inserted without wounding some of them. This is illus- 

 trated when the skin is pricked. The blood in it is not 

 lying loose but is flowing in capillaries. We cannot in- 

 sert a needle deeper than the epidermis without wound- 

 ing some of these capillaries and causing bleeding. 



It is while flowing in the capillaries that the blood 

 does its work. Their walls are so thin that nourishing 

 matters easily soak through them to feed the organs; 

 and the waste matters of the organs readily pass 

 through the walls of these tiny vessels into the blood. 



Imagine a piece of the finest net, with all its threads 

 consisting of hollow tubes, and diminished twenty 

 times in size, and you will have some idea of the 

 fine networks formed by the capillaries in the various 

 organs. 



24. Which Vessels contain Arterial and which Venous 

 Blood. As blood flows through the capillaries of the 

 lungs, its red corpuscles take up oxygen from the air 

 (Chap. XV.). The blood thus becomes bright red or ar- 

 terial (p. 134). It flows, keeping this color, through the 

 left auricle and ventricle of the heart, and along the 

 aorta and its branches (the systemic arteries), which con- 

 vey it to the body in general. These arteries pour the 

 blood into the capillaries of all organs except the lungs. 

 As it flows through these systemic capillaries the blood 

 gives up its oxygen to the organs and becomes dark- 

 colored. It is then collected into the systemic veins, 

 and, still of a dark color, is conveyed to the right auri- 

 cle, right ventricle, and thence by the pulmonary artery 



24. In what vessels does the blood become arterial ? Through 

 what part of its course does it keep its bright color ? Where does 

 it lose it? Why? Describe its course until it becomes bright 



