GENERAL CHARACTERISTICS OF THE BLOOD 159 



to it. Outside the capillary wall, the lymph serves as the medium with 

 which the tissue cells interchange their material. This process having 

 been completed, it then moves onward through the special channels 

 constituting the lymphatic system, until it again reaches the venous 

 collecting tubes of the main circulatory system. The lymph, so to 

 speak, plays the part of a middle man between the blood and the cells. 

 The lymph is comparable to the general body-fluid of the lower 

 animals, while the blood forms a much more specialized carrier. Func- 

 tionally, however, these media must be regarded as fulfilling the same 

 purpose, because they : 



(a) Equip the cells of the tissues with the material necessary for their existence, 

 and remove from them the substances that are of no further use to them ; 



(6) provide the tissues with oxygen in a readily assimilable form, and relieve 

 them of carbon dioxid, one of the products of their metabolism; 



(c) help in the equalization and regulation of the body-temperature; 



(d) protect the organism against microbic infection and toxic influences of differ- 

 ent kinds, and 



(e) disseminate the products of the ductless glands, known as atacoids. 



The blood is a thick and viscous fluid, containing different bodies 

 and substances in solution and suspension. It is composed of a fluid 

 part, commonly designated as plasma, and a relatively large amount 

 of solids. The latter embrace nutritive particles of all kinds, as well 

 as formed elements, or corpuscles, which in turn are made up of red 

 corpuscles or erythrocytes, white corpuscles or leukocytes, and blood 

 platelets or thrombocytes. 



Blood 



Water Plasma 



Nutritive particles. 

 Blood dust (hemoconia) 



Solids 



f red (erythrocytes) 

 Corpuscles j white (leukocytes) 



[ platelets (thrombocytes) 



Relative Amount of Plasma and Corpuscles. — Two methods have 

 been devised for the determination of the amount of the corpuscular 

 material. The direct method possesses the advantage of being easily 

 executed. The sample of blood to be examined is mixed with a definite 

 quantity of potassium bichromate and is centrifugahzed ^ until the 

 corpuscular elements have been forced to the bottom of the receptacle. 

 Since the glass-tubes used in this test are calibrated, the amounts of 

 plasma and corpuscles may be read off directly. This entire procedure 

 requires no special aptitude nor complicated apparatus and can be 

 completed before coagulation has set in. It is also possible to ascer- 

 tain the corpuscular content by measuring the electrical conductivity 

 of the serum and corpuscles. This method depends upon the fact 

 that the latter place a considerable resistance in the path of an elec- 

 trical current which is directly proportional to the thickness of the 



^ For clinical purposes a small centrifuge, called a hematocrit, is commonly 

 employed (Blix and Hedin). 



