206 VETERINARY PHYSIOLOGY 



means of it wounds in blood vessels are sealed and haemor- 

 rhage stopped. 



Although an important and very prominent change in the 

 blood, clotting is really produced by change in one constituent 

 of the plasma, which is present in very small quantities. 



II. Plasma and Serum 



These may be considered together, since serum is merely 

 plasma minus fibrinogen. As serum is so much easier to 

 procure, it is generally employed for analysis. 



Both are straw-coloured fluids, the colour being due to a 

 yellow lipochrome. Sometimes they are clear and transparent, 

 but after a fatty diet they become milky. They are alkaline 

 in reaction, and have a specific gravity of about 1025. They 

 contain about 90 per cent, of water and 10 per cent, of solids. 

 The chief solids are the proteins serum albumin and serum 

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

 The proportion of the two former proteins to one another 

 varies considerably in different animals, but in the same animal 

 at different times the variations are small. The globulin 

 probably consists of at least two bodies euglobulin precipitated 

 by weak acid, and pseudoglobulin not so precipitated. The 

 amount of albumin is generally greater when the body is well 

 nourished. In most animals, they together form about 7 per 

 cent, of the serum. 



The other organic constituents of the serum are in much 

 smaller amounts, and may be divided into 



1. Substances to be tised by the tissues. 



Glucose is the most important of these. It occurs only in 

 small amounts about 1 to 2 per mille. Part of it is free, but 

 part is probably combined in organic combinations such as 

 jecorin. It is probably in larger amount in blood going to 

 muscles than in blood coming from muscles, and this difference 

 seems to be specially well marked when the muscles are active, 



Fats occur in very varying amounts, depending upon the 

 amount taken in the food. 



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. "We shall 



