THE CLOTTING OF BLOOD. 59 



Whatever the part played by the blood-vessels in reference 

 to coagulation it is only exhibited when their inner surfaces 

 are healthy and uninjured. If their lining be ruptured or 

 diseased the blood clots. Accordingly, after death, when 

 post-mortem changes have affected 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, because 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 gravity 

 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 its fibrinogen and gained fibrin-ferment and probably 

 some additional paraglobulin; from an analysis 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 para- 

 globulin. Serum-albumin agrees with egg-albumin in coagu- 

 lating when heated : for this reason 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 coagu- 

 lated by ether; and in the fact that although present in such 

 large quantities in the blood, it is not excreted by the kid- 

 neys, as egg-albumin is, if injected into a blood-vessel. The 

 paraglobulin is also precipitated by heat, but may be pre- 

 cipitated alone by saturation of the serum with magnesium 

 sulphate. Fats are present in the serum in small quantity 

 except after a meal at which fatty substances have been 

 eaten; serum obtained from the blood of an animal soon 

 after such a meal is often milky in appearance from the large 



