26 THE BLOOD 



EXTRACTIVES AND SALTS 



Grouped under the head of extractives, may be included various 

 nitrogenous and non-nitrogenous matters, such as urea, urates, creatin, 

 creatinin, xanthin, hypoxanthin, the lecithins, fats, waxes, sugar and 

 cholesterin. In addition, inorganic salts exist in the proportion of six 

 to eight parts per thousand. The most abundant of the inorganic salts 

 is sodium chloride, the proportion of which is sixty to ninety per cent 

 of the total saline matters. Potassium chloride is found in much 

 smaller quantity and exists chiefly in the red corpuscles. The other 

 saline matters are in the form of sulphates and phosphates. Calcium 

 salts are present and are important in connection with coagulation. 

 Gases are in solution or in a condition of feeble combination. Of the 

 gases, carbon dioxide is important in retaining certain of the salts in 

 solution. 



COAGULATION OF THE BLOOD 



The blood remains liquid so long as it is contained in the vessels 

 and the circulation is not interrupted ; but soon after circulation ceases 

 or the blood is drawn from the body, it coagulates, or " sets " into a 

 jelly-like mass. In a few hours, contraction of the clot will have taken 

 place, and a clear straw-colored serum is then expressed. The serum 

 contains all the constituents of the blood except the corpuscles and the 

 fibrin-factors, which together form the clot. In the human subject, 

 when the blood is received into a moderately deep, smooth vessel, the 

 phenomena of coagulation present themselves in the following order : 



First, a gelatinous pellicle forms on the surface, which occurs in one 

 minute and forty-five seconds to six minutes ; in two to seven minutes, 

 a gelatinous layer has formed next the sides of the vessel ; the whole 

 mass of blood becomes of a jelly-like consistence in seven to sixteen 

 -jniates. Contraction then begins, and drops of clear serum make their 

 appearance on the surface of the clot. This liquid increases in quantity, 

 andjn ten or twelve hours separation is complete. The clot, which is 

 heavier, sinks to the bottom of the vessel, unless it contains bubbles of 

 gas or the surface is very concave. In most warm-blooded animals 

 the blood coagulates more rapidly than in man. Coagulation is pecul- 

 iarly rapid in the blood of birds, and sometimes it takes place almost 

 instantaneously. Coagulation is more rapid in arterial than in venous 

 blood. In the former, the proportion of fibrin formed is notably greater 

 and the characters of the fibrin are somewhat different. A solution of 

 sodium chloride dissolves the fibrin of venous blood but does not dis- 

 solve the fibrin of an arterial clot. 



