282 LIFE : OUTLINES OF GENERAL BIOLOGY 



the various functions, and making the body more of a unity. 



(5) The blood has also a protective r61e, for among the white blood 

 corpuscles there are some called "phagocytes" which are able to 

 engulf and digest invading microbes. These phagocytes may leave 

 the blood-vessels altogether and serve as a mobile bodyguard, and 

 they also help in processes of wound-healing and the like. More- 

 over, some of the white blood corpuscles are able to form anti- 

 bodies in the fluid or serum of the blood which counteract poisons. 

 It is plain, then, that the blood is a very important fluid-medium, 

 from which every part of the body takes and to which every part 

 likewise gives. It is indispensable that it should be kept in circu- 

 lation and that with rapidity. 



What Harvey showed was that the blood moves quickly round 

 in a circle, from the heart to various parts of the body and back 

 again to the heart. But this needs further clearness. 



There are two pumps side by side in the heart of any of the 

 highest Vertebrates — the birds and the mammals. The right pump 

 (or ventricle) drives impure blood to the lungs, whence there is a 

 return of pure blood to the left receiving-chamber (or auricle) of 

 the heart. 



From the left auricle the purified blood, i.e. relatively rich in 

 oxygen and with little carbon dioxide, passes into the left ventricle, 

 which drives it to the body by the arteries. The arteries end in fine 

 capillaries which penetrate everywhere, bringing the tissues what 

 they need. From the arterial capillaries the blood passes into venous 

 capillaries, as Malpighi first discerned, and thence into the veins, 

 which bring it back to the right auricle of the heart. 



The left pump of the heart drives pure blood (a) to the tissue 

 of the heart itself, for the engine itself must be kept effective; 



(6) to the stomach and intestine; (c) to the kidneys; and (d) to the 

 head, tnink, limbs, and body generally. 



The impure blood from the head region is brought back to the 

 right auricle of the heart by superior veins (superior venae cavae), 

 and from the posterior body by a large vein (inferior vena cava). 

 Into this posterior vein there also passes (i) by the renal vein, the 

 blood which has been filtered in the kidney as regards its nitro- 

 genous waste-matter, (2) by the hepatic vein the blood from the 

 liver, which mediates between the general circulation and the 

 portal system, bringing in the digested proteins and carbohydrates 

 from the stomach and intestine, and (3) the blood from the tissue 

 of the heart itself. 



The vessels which bring back blood to the heart are the veins, 

 and all of them carry venous or impure blood except the pulmonary 

 veins from the lungs, which bring the oxygenated blood into the 

 left auricle. The vessels which carry blood from the heart are the 

 arteries, and all of them carry pure blood except the pulmonary 



