THE NUTRITIVE FLUIDS. 1081 



entangles and encloses the blood-corpuscles. It is supposed that when blood is 

 drawn a nucleo-proteid, termed prothrombin, appears in the plasma, probably as 

 the result of disintegration of some of the white cells and perhaps also the blood- 

 platelets. This substance interacts with soluble lime salts in the blood, and a 

 fresh body, thrombin or fibrin-ferment, is the result. The thrombin then acts on 

 the fibrinogen in solution in the plasma, converting it into insoluble fibrin, while 

 at the same time a very small amount of a new proteid of the globulin type passes 

 into solution. 



Fibrin may be obtained, practically free from corpuscles, by whipping the 

 blood, after it has been withdrawn from the body, with a bundle of twigs, to which 

 the fibrin adheres as it is formed. By various means the clotting of the blood 

 may be retarded, so that the plasma may be obtained free from corpuscles; from 

 this plasma there may be derived fibrin and serum, without the cellular elements. 

 Fibrin thus obtained is a white or buff-colored stringy substance, and when 

 observed in the course of formation, under the microscope, shows a meshwork 

 of fine fibrils. After exposure to the air for some time it becomes hard, dry, 

 brown, and brittle. It is one of the class of coagulated proteids, insoluble in hot 

 or cold water, saline solution, alcohol, or ether. Under the action of dilute hydro- 

 chloric acid it swells up but does not dissolve, but when thus swollen is readily 

 dissolved by a solution of pepsin. 



> 



FIG. 601. Blood-crystals : A. Haemoglobin crystals from human blood. B. Haemin crystals from blood 

 treated with acetic acid. C. Haematoidiii crystals from an old apoplectic clot. 



Serum, with the exception of its proteids, has a composition identical with that 

 of plasma. The fibrinogen characteristic of plasma has disappeared, and the 

 fibrin-ferment or thrombin is found instead, together with the serum albumen 

 and serum globulin which are not involved in the process of coagulation. 



The relation of the various constituents of the blood to each other may be 

 easily understood by a reference to the subjoined plan : 



Corpuscles ^ 



> Clot 



>100d < /'-rvi, i 



j Fibrin ) 

 Plasma < 



( Serum 



Gases of the Blood. When blood is exposed to the vacuum of an air-pump, 

 100 volumes are found to yield about 60 volumes of gas. The gases present are 

 carbon dioxide, oxygen, and nitrogen, and they occur in the following proportions 

 in arterial and venous blood : 



Carbon dioxide. Oxygen. Nitrogen. 



Arterial blood . . 40 vols. 20 vols. 1 to 2 vols. 



Venous blood . . 46 to 50 vols. 10 to 12 vols. 1 to 2 vols. 



The greater quantity of the oxygen is in loose chemical combination with the 

 haemoglobin of the red corpuscles. The carbon dioxide exists in combination for 

 the most part as sodium bicarbonate and carbonate. The nitrogen is in simple 

 solution in the plasma. 



Blood-crystals. Hemoglobin, as already stated, readily crystallizes Avhen 



