78 ANATOMY FOR NUESES. [CHAP. V1L 



scanty, except after a meal, or in certain diseased conditions. 

 The extractives, so named because they have to be extracted by 

 special methods from the blood, are very numerous. The most 

 important are perhaps urea, lactic acid, and sugar. 



The saline matters are the chlorides and sodium salts, the 

 phosphates and potassium salts being found chiefly in the 

 corpuscles. 



Of all these substances, albumin probably holds the first place 

 in regard to nutrition, providing, as it does, the greater part of 

 the material necessary for the daily nourishment and renovation 

 of the tissues. In this process, it undergoes a variety of trans- 

 formations by which it is converted into the structural charac- 

 teristics of the tissues which it supplies. 



Para-globulin is closely allied to albumin in its chemical rela- 

 tions, and no doubt also in its physiological action. Both sub- 

 stances are coagulated by heat, and solidified at a temperature 

 of 160 F. 



The fibrinogen of the plasma is the substance which produces 

 the fibrin of coagulated blood. It is very difficult to obtain 

 in the fluid condition, owing to the rapidity with which it 

 solidifies when blood is withdrawn from the circulation. 



Of the mineral salts, the sodium chloride is the most abun- 

 dant, constituting nearly 40 per cent of all the saline ingredi- 

 ents. The mineral salts maintain the alkalescence of the blood, 

 a property which is essential to nutrition, and even to the 

 immediate continuance of life, since it enables the plasma to 

 take up the carbonic acid from the tissues and return it to the 

 lungs for elimination. 



The clotting of blood. Blood when drawn from the blood- 

 vessels of a living body is perfectly fluid. In a short time it 

 becomes viscid, and this viscidity increases rapidly until the 

 whole mass of blood becomes a complete jelly. If the blood in 

 this jelly stage be left untouched in a glass vessel, a few drops 

 of an almost colourless fluid soon make their appearance on the 

 surface of the jelly. Increasing in number and running together, 

 the drops after a while form a superficial layer of pale straw- 

 coloured fluid. Later on, similar layers of the same fluid are 

 seen at the sides, and finally at the bottom of the jelly, which, 

 shrunk to a smaller size and of firmer consistency, now forms a 

 clot or crassamentum, floating in a perfectly fluid liquid. The 



