480 VETERINARY PHYSIOLOGY 



parent, but, after a fatty diet, they become milky. They 

 have a specific gravity of about 1025, and contain about 

 90 per cent, of water and 10 per cent, of solids. The chief 

 solids are the native proteins — serum albumin and serum 

 globulin (with, in the plasma, the addition of fibrinogen). 

 The proportion of the two former proteins to each other 

 varies considerably in different animals, but the variations 

 are small in the same animal at diiTerent times. The 

 globulin probably consists of at least two bodies — euglobulin 

 precipitated by weak acid, and pseudoglobulin not so pre- 

 cipitated. The amount of albumin is generally greater 

 when the body is well nourished. In man they together 

 form about 7 per cent, of the serum. In virtue of the 

 presence of these proteins the plasma is colloidal, and it has 

 little tendency to transude through the walls of the vessels. 

 These proteins further seem to have a small osmotic 

 pressure (p. 574). 



The other constituents of the serum are in much smaller 

 amounts, and may be divided into — 



1 . Substances to be used by the tissues. 



Glucose is the most important of these. It occurs only 

 in small amounts — about O'l to O'lo per cent. Part of it 

 is free, but part is probably in combination. It is present 

 in larger amount in blood going to muscles than in blood 

 coming from them, and this difterence seems to be more 

 marked when the muscles are active. 



Fats occur in very varying amounts, depending upon the 

 amount taken in the food, but in addition to these true fats 

 there is also a small amount of other lipoids. 



2. Substances given off by the tissues. 



The chief of these is urea, which occurs constantly in 

 very small amounts in the serum — about '05 per cent. It 

 will afterwards be shown that it is derived from the liver, 

 and that it is excreted in the urine by the kidneys (p. 5 59). 



Creatin and uric acid, and some allied bodies, appear to 

 be normally present in traces, and their amount may be 

 increased in diseased conditions, especially of the kidneys. 



3. Inorganic constituents. — The most abundant is chloride of 



