234 TEXT-BOOK OF PHYSIOLOGY. 



a specific gravity of 1.026 to 1.029. It is composed largely of water 

 holding in solution proteids, sugarjatt^ ma^^arg^mc^^ur^Q. t 

 cholesterin, lecithin, etc. In composition it is quite complex, con- 

 taining 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 expressed 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 for- 

 mation of fibrin. The average composition of plasma is shown in 

 the following table : 



COMPOSITION OF PLASMA. 



Water, - 9- 



( Serum-albumin, 4-5 



(Fibrinogen, o-3 



Fatty matters, - 2 5 



Sugar, - 10 



Extractives, - 6 



Inorganic salts, -&5 



Serum-albumin. Of the proteid constituents of the blood, 

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

 4 to 5 per cent. From its similarity to egg-albumin it is regarded as 

 holding an important position as a nutritive agent, for it is out of this 

 common proteid that in all probability each individual tissue elabor- 

 ates the special proteid characteristic of it, since during starvation 

 the albumin steadily diminishes in amount. As it passes through 

 the walls of the capillary vessels it is found in the lymph, pericardial 

 fluid, and similar secretions in various parts of the body, as well as in 

 various pathologic transudates. It is also present in serum. While 

 circulating in the lymph-spaces the serum-albumin is utilized in 

 replacing the proteids which have undergone disintegration during 

 tissue metabolism, jj Its supply in the blood is maintained by the 

 absorption of peptones which are formed from the proteids of the food 

 and which during the time of absorption are changed in some unknown 

 way into serum-albumin.Jf It is readily obtained from plasma or 

 serum by saturating either of these fluids with magnesium sulphate, 

 when all the proteids except serum-albumin are precipitated. After 

 their removal the remaining fluid is subjected to a temperature of 

 from 70 to 75 C., when the serum-albumin is precipitated in a 

 coagulable form, after which it can be removed and its chemic 

 features determined. 



&. This proteid, though present in plasma, is best 



