236 TEXT-BOOK OF PHYSIOLOGY 



gravity of 1.026 to 1.029. It is composed largely of water holding in solution 

 proteins, sugar, fat, inorganic salts, urea, cholesterin, lecithin, etc. In 

 composition it is quite complex, containing as it does not only the nutritive 

 materials derived from the digestion of the food, but also the substances 

 resulting from the disintegration of the tissues consequent on their functional 

 activity. 



Serum. The serum is the clear, transparent, slightly yellow fluid ex- 

 pressed from the coagulated blood during the contraction of the fibrin. It 

 consists practically of the ingredients of the plasma, with the exception of 

 those substances which entered into the formation of fibrin. 



The average composition of plasma is shown in the following table 



CHEMIC COMPOSITION OF PLASMA 



Water 90 .00 



f Plasma-albumin 4 . 50 



Proteins . . . \ Paraglobulin 3 .40 



[ Fibrinogen o .30 



Fatty matters 0.25 



Sugar o . 10 



Extractives. . . , o .60 



Inorganic salts o .85 



Plasma-albumin. Of the protein constituents of the blood, plasma- or 

 serum-albumin is the most abundant, existing to the extent of from 4 to 5 

 per cent. It is readily obtained from plasma or serum by saturating either 

 of these fluids with magnesium sulphate, when all the proteins except 

 serum-albumin are precipitated. After their removal the remaining fluid 

 is subjected to a temperature of from 70 to 75C., when the serum-albumin 

 is precipitated in a coagulable form, after which it can be removed and its 

 chemic features determined. Recent investigations indicate that plasma- 

 albumin consists of, at least, more than one substance, since it can be separated 

 into a crystallizable and non-crystallizable or an amorphous portion both 

 of which however respond to the usual tests for albumin. 



The origin and function of plasma-albumin are both obscure. Until 

 quite recently it has been regarded, owing to its similarity to egg-albumin, 

 and to its presence in the blood, as holding an important position as a 

 nutritive agent. It was assumed that it originated in a synthesis of the 

 products of protein digestion peptones and amino-acids in the epithelial 

 cells covering the villi and by them discharged into the blood of the portal, 

 and finally into the blood of the general circulation. On reaching the capil- 

 laries it was supposed to cross the endothelial wall, to become a constituent 

 of the lymph and then to be utilized by the tissue cells for growth and repair. 



As was stated in the chapter on absorption, this view has been made 

 untenable by reason of the apparent fact that the products of protein 

 digestion the amino-acids are absorbed and enter the blood as such and 

 carried direct to the tissue cells by which they are directly synthesized 

 to the protein by which they are characterized. Plasma-albumin must 

 therefore have, if this view prevails, some other origin and function. 

 Future experimentation may disclose both. 



Paraglobulin. This protein, though present in plasma, is best obtained 

 from serum when this fluid is saturated with magnesium sulphate. As the 

 line of saturation is approached the fluid becomes turbid, and after a few 



