312 THE BLOOD. 



in thin layers. If the haemoglobin is removed from the stroma and 

 dissolved by the blood liquid by any of the above-mentioned means 

 (see page 273), the blood becomes transparent and has then a " lake 

 color." l Less light is now reflected from its interior, and this laky 

 blood is therefore darker in thicker layers. On the addition of salt 

 solutions to the blood-corpuscles they shrink, more light is reflected, 

 and the color appears lighter. A great abundance of red corpuscles 

 makes the blood darker, while by diluting with serum or by a greater 

 abundance of white corpuscles the blood becomes lighter in appearance. 

 The different colors of arterial and of venous blood depend on the vary- 

 ing quantities of gas contained in these two varieties of blood, or, bet- 

 ter, on the different amounts of oxyhsemoglobin and haemoglobin they 

 contain. 



The most striking property of blood consists in its coagulating within 

 a shorter or longer time, but as a rule very shortly after leaving the veins. 

 Different kinds of blood coagulate with varying rapidity; in human 

 blood the first marked sign of coagulation is seen in two to three minutes, 

 and within seven to eight minutes the blood is thoroughly converted into 

 a gelatinous mass. If the blood is allowed to coagulate slowly, the red 

 corpuscles have time to settle more or less before the coagulation, and 

 the blood-clot then shows an upper yellowish-gray or reddish-gray layer 

 consisting of fibrin enclosing chiefly colorless corpuscles. This layer 

 has been called crusta inflammatoria or phlogistica, because it has been 

 especially observed in inflammatory processes and is considered one 

 of the characteristics of them. This crusta, or " buffy coat," is not 

 characteristic of any special disease, and it occurs chiefly when the blood 

 coagulates slowly or when the blood-corpuscles settle more quickly 

 than usual. A buffy coat is often observed in the slowly coagulating 

 equine blood. The blood from the capillaries is not supposed to have the 

 power of coagulating. 



Coagulation is retarded by cooling, by diminishing the oxygen, and by 

 increasing the amount of carbon dioxide, which is the reason that venous 

 blood and to a much higher degree blood after asphyxiation coagulates 

 more slowly than arterial blood. The coagulation may be retarded or 

 prevented by the addition of acids, alkalies, or ammonia, even in small 

 quantities; by concentrated solutions of neutral alkali salts and alkaline 

 earths, alkali oxalates and fluorides; also by egg-albumin, solutions of 

 sugar or gum, glycerin, or much water; also by receiving the blood in 

 oil. Coagulation may be prevented by the injection of a proteose solu- 

 tion or of an infusion of the leech into the circulating blood, but this 



1 R. Du Bois-Reymond presents objections to the general use of the above terms 

 in Centralbl. f. Physiol., 19, p. 65. 



