142 INTRODUCTION TO GENERAL PHYSIOLOGY 



blood are at first entangled. Liquid slowly exudes, owing to the 

 contraction of the network. This liquid, of course, is not the same 

 as the plasma, since fibrin has been separated from it, and other 

 products of the reaction which results in the deposition of this 

 solid are left in solution. The liquid is called "serum." The 

 importance of the phenomenon is in the spontaneous arrest of 

 bleeding from an injured blood vessel. If the rate of the outflow 

 is not great, the issuing blood clots when it comes into contact 

 with the tissues, and the familiar effect of accelerating the process 

 by the application of cobweb or such-like is merely due to the 

 provision of a large area of foreign surface. 



The precise explanation of the changes that take place in this 

 coagulation process has led to various theories which it would not 

 be profitable to discuss here. It is to be feared that much of what 

 has been written on the question amounts to little more than 

 inventing names. There is one important fact, however, namely, 

 that the blood remains liquid if calcium be removed from it, as by 

 the addition of the appropriate amount of an oxalate, which forms 

 the insoluble calcium oxalate. 



The Salts of Blood. We saw in our second chapter (p. 60) 

 that the cells of the present land animals have become adjusted to 

 the presence of certain salts, probably owing to their presence in 

 the ocean at the time when their ancestors left it. In order, there- 

 fore, that a saline solution may serve as a perfusion fluid for isolated 

 organs, it has been found that particular salts must be present, and 

 in a " balanced," relative concentration. These salts are those of 

 sodium, potassium, and calcium, generally used as chlorides (E., p. 

 223). The function of the sodium chloride is chiefly to afford a 

 sufficient osmotic pressure to balance that of the cell contents. 

 Calcium seems to be necessary to maintain the properties of the 

 cell membrane, but it has doubtless other functions as well. Potas- 

 sium neutralises certain deleterious effects of calcium, and is also 

 said to be of importance on account of its radio-activity, since it 

 can be replaced by salts of other radio-active metals in equivalent 

 radio-active concentrations. The electrical properties of the anions 

 and cations naturally also come into play in the balance of electro- 

 lytes, but the problem is not completely solved. 



Since the cell mechanisms are very sensitive to changes in the 

 concentration of hydrogen ions, while acids are produced in the 

 tissues under active conditions,a means of maintaining the hydrogen- 

 ion concentration of the blood at a constant value is a necessity. 

 The normal reaction is just about the neutrality of distilled water, 

 very slightly on the alkaline side. Although the proteins of the 

 plasma are able to combine with acids and alkalies, this capacity is 

 limited and scarcely comes into play within the region of the most 



