THE CIRCULATION 149 



coagulation were impossible, the slightest injury in drawing 

 blood would prove fatal. Whereas now, in many cases, bleed- 

 ing ceases spontaneously, because the blood, as it coagulates, 

 stops the mouths of the injured blood-vessels. In another class 

 of cases, where larger vessels are cut or torn, it is ordinarily 

 sufficient to close them by a temporary pressure ; for in a few 

 minutes the clot will form and seal them up. In still more 

 serious cases, where the blood-vessel is of large size, the sur- 

 geon is obliged to tie a ligature about it, thus preventing the 

 force of the blood-current from washing away the clots, which, 

 forming within and around the vessel, close it effectually. 



11. It is worthy of remark that this peculiarity is early 

 implanted in the blood, even before birth, and in advance of 

 any existing necessity for it thus anticipating and guarding 

 against danger. But this is not all. Of most of the inferior 

 animals, which, as compared with man, are quite helpless, the 

 blood coagulates more rapidly, and in the case of the birds, 

 almost instantly. The relative composition of fluid and coagu- 

 lated blood may be thus represented : 



Fluid Blood Coagulated Blood 



Plasma^ Serum Serum 



Corpuscles Corpuscles- 



{Read Note 2.) 



12. The Uses of the Blood. The blood is the great pro- 

 vider and purifier of the body. It both carries new materials 

 to all the tissues, and removes the worn-out particles of matter. 



2. Experiment. Coagulation of Blood. The coagulation of blood 

 can be shown to a class with but little trouble. Obtain from a butcher or 

 slaughter-house about a pint of fresh blood, have it drawn into a tin can 

 or pail, and put it immediately into a freezing mixture of ice and salt. If 

 it be during the winter and freezing, this is not necessary, as freezing 

 suspends coagulation. 



11. What is worthy of remark ? Coagulation of the blood of inferior animals ? Of the 

 blood of birds ? 



- 12. The blood, as a provider and purifier ? What uses does the blood subserve ? Ex- 

 periments ? Transfusion ? 



