LECTURE XXIII. 



THE BLOOD. 

 COAGULATION. COMPOSITION. 



BLOOD is the intermediary in the general metabolism. It carries in part 

 directly from the intestine, and in part indirectly by the aid of the lymphatics, 

 the proper nourishment for each individual cell of the body. The oxygen, 

 which is so indispensable for the work of the cell, is also carried to it by 

 the blood. On the other hand, the cells give up the products of their 

 activity, whether as residues from the various combustion processes, or 

 whether as secretion products which are yet to play an important part 

 in the total metabolism, to the blood. From all this it is obvious what 

 a dominating position the blood holds in the animal organism. In contrast 

 to the other tissues, it is a liquid which is kept in constant circulation by 

 the action of the heart. The blood always contains numerous cells, 

 especially the red and the white blood-corpuscles. We have already dis- 

 cussed the important part that the former play in external and internal 

 respiration. Besides these form-elements there are blood-plates, the sig- 

 nificance of which has never been explained satisfactorily. The cell 

 elements of the blood lie suspended in a liquid rich in albumin, the plasma. 

 They may be removed from the latter by a centrifugal machine. The 

 clear yellowish plasma is then obtained above the deposited blood-cor- 

 puscles. This separation into these form-elements and the plasma can be 

 effected, however, only under quite definite conditions. If we take blood 

 from any blood-vessel (usually the carotid is chosen), and simply allow it 

 to stand, it soon undergoes a peculiar transformation. There settles over 

 the bottom of the dish containing it a firm coagulum which incloses the 

 corpuscles. Above this so-called blood-clot there is a clear liquid which is 

 very similar to the plasma, but is, notwithstanding, an entirely distinct 

 substance, as we shall soon see. This liquid produced by the contraction 

 of the blood-corpuscles, and whose volume subsequently increases con- 

 siderably, is called the serum. If, instead of merely allowing the blood to 

 stand, it is stirred vigorously with a wooden, or glass, stirring rod, imme- 

 diately after taking it from the animal, a different coagulum is soon formed 

 which is known as fibrin. In this case the blood-corpuscles remain for 

 the most part suspended in the serum. This mixture of serum and blood- 

 corpuscles is spoken of as defibrinated blood. The difference between the 



535 



