COMPOSITION OF THE BLOOD. 57 



tin or the ferment, while foreign bodies in contact with 

 the blood do excite these changes and so lead to coagula- 

 tion. 



Whatever the part which the blood-vessels play, it is only 

 exhibited when their inner surfaces are healthy and unin- 

 jured. If this lining be ruptured or diseased the blood 

 clots. Accordingly, after death, when post-mortem changes 

 have aifected the blood-vessels, the blood clots in them; 

 but often very slowly, since the vessels only gradually alter. 

 If the Body be left in one position after death, the clots 

 formed in the heart have often a marked buffy coat, be- 

 cause the corpuscles have had a long time to sink in the 

 plasma before coagulation occurred. In medico legal cases 

 it is thus sometimes possible to say what was the position 

 of a corpse for some hours after death, although it has been 

 subsequently moved. 



Lymph clots like the blood, but not so firmly. The 

 clot formed is colorless. 



Composition of the Blood. The average specific grav- 

 ity of human blood is 1055. It has an alkaline reaction, 

 which becomes less marked as coagulation occurs. About 

 one half of its mass consists of moist corpuscles and the 

 remainder of plasma. Exposed in a vacuum, 100 volumes 

 of blood yield about 60 of gas consisting of a mixture of 

 oxygen, carbon dioxide, and nitrogen. 



Chemistry of Serum. Blood serum is plasma, which 

 has lost most of its fibrinogen and gained fibrin ferment 

 and probably some additional paraglobulin; from an analy- 

 sis of it we can draw conclusions as to the plasma. In 100 

 parts of serum there are about 90 parts of water, 8.5 of 

 proteids, and 1.5 of fats, salts, and other less-known 

 solid bodies. Of the proteids present the most abundant 

 are serum albumin and paraglobulin. Serum albumin 

 agrees with egg albumin in coagulating when heated: serum 

 when boiled sets into an opaque white mass, just as the 

 white of an egg does. Serum albumin differs from egg 

 albumin in not being coagulated by ether; and in the fact 

 that although present in such large quantities in the blood, 



