THE VASCULAR SYSTEM 263 



Blood. The human body contains about one gallon of blood, one twen- 

 tieth of the weight of the body. Blood has two constituents, a fluid plasma 

 and blood corpuscles which are cells. Blood, therefore, may be regarded 

 as a tissue composed of cells (corpuscles) and a Hquid intercellular mate- 

 rial. The plasma consists of a liquid serum and a coagulable material, 

 fibrinogen. By stirring blood it is possible to separate these two elements. 

 Blood corpuscles are of two sorts, red erythrocytes and white leucocjrtes. 

 To every cubic milHmeter of blood there are from four and one-half to five 

 million erythrocytes, and from five to seven thousand white corpuscles. 

 Variations from this proportion are of diagnostic value in disease. 



Functions of the Blood. The blood-vessels and blood constitute an 

 organic transportation system. Blood is a common carrier of foods and 

 wastes to and from all parts of the body. Among its numerous functions 

 are equaUzing the temperature of the body, regulating the water content 



Pig, 244. — Diagram of the circulation in an early stage of a small-yolked vertebrate 

 (amphibian), a, anus; ca, cv, caudal artery and vein; da, dorsal aorta; dc, Cuvierian 

 duct; ec, external carotid; h, heart; ha, hypogastric artery; i, intestine; ic, internal 

 carotid; ij, inferior jugular; j, superior jugular; I, liver; m, mouth; oma, omv, omphalo- 

 mesenteric artery and vein; pc, postcardinal vein; si, subintestinal vein; 1-6, aortic 

 arches. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



of the various tissues, distributing hormones and thus assisting in the 

 integration of the body. The red corpuscles carry oxygen, the white act 

 as scavengers of the blood. The color of red corpuscles is due to hemo- 

 globin, a nitrogenous substance with an afhnity for oxygen. Most, if not 

 all, white corpuscles are amoeboid, and, like amoebae, surround and engulf 

 bacteria. Foods and wastes are carried in the plasma and not by the 

 corpuscles. The plasma contains also mineral salts of the same sorts and 

 in nearly the same proportions as those of somewhat dilute sea water. 

 Among other constituents of the blood are enzymes, antibodies, antitoxins, 

 antithrombin, etc. Antibodies are substances produced in the tissues in 

 response to the poisons caused by bacteria. They are regulatory in their 

 action and help to preserve the normal chemical balance of the blood. 

 Antithrombin is a substance, normally present in blood, which prevents the 

 clotting of the blood by preventing the action of thrombin on fibrinogen. 

 Evolution of the Blood-Vessels. Two types of circulation may be dis- 

 tinguished in animals, intracellular and intercellular. The former occurs 

 in all cells alike as foods enter the cell and are distributed to the various 

 cell organs and as the cell wastes are excreted. Intracellular circulation is 



