BLOOD. 359 



nucleo-albumins of different kinds, etc. According to the theory of coagu- 

 lation adopted above, injections of these latter substances ought to cause coagu- 

 lation very readily, since the blood already contains fibrinogen, and needs only 

 the presence of ferment to set up coagulation. As a matter of fact, however, 

 iutravascular clotting is produced with some difficulty by these methods, show- 

 ing that the body can protect itself within certain limits from an excess of 

 ferment in the circulating blood. Just how this is done is not known, but 

 possibly it is due to some defensive activity of the living endothelial cells lining 

 the interior of the blood-vessels. Moreover, injection of leucocytes, nucleo- 

 albumins, etc. sometimes diminishes instead of increasing the coagulability of 

 blood, making the so-called "negative phase" of the injection. In the case 

 of leucocytes it is probable that this result is accounted for by the fact that 

 the nucleohiston liberated by their disintegration may undergo decomposition 

 in the blood with the formation of histon, which is known to prevent coagu- 

 lation (see p. 356). 



Why Blood does not Clot within the Blood-vessels. The reasons 

 why blood remains fluid while in the living blood-vessels, but clots quickly 

 after being shed or after being brought into contact with a foreign substance in 

 any way, have already been stated in describing the theories of coagulation, 

 but they will be restated here in more categorical form. Briefly, then, blood 

 does not clot within the blood-vessels because nucleo-proteids are not present 

 in sufficient quantities at any one time. Leucocytes and blood-plates probably 

 disintegrate here and there within the circulation, but the small amount of fer- 

 ment thus formed is insufficient to act upon the blood, and probably the ferment 

 is quickly destroyed or changed. When blood is shed, however, the formed 

 elements break down in mass, as it were, liberating a relatively large amount 

 of nucleo-proteids, which, in combination with the calcium salts, produce fibrin 

 from the fibrinogen. In shed blood the restraining action of the endothelial 

 cells of the blood-vessels, a more or less unknown factor, is also eliminated. 



Means of Hastening or of Retarding- Coagulation. Blood coagulates 

 normally within a few minutes, but the process may be hastened by increasing 

 the extent of foreign surface with which it comes in contact. Thus, moving 

 the liquid when in quantity, or the application of a sponge or a handkerchief to 

 a wound, will hasten the onset of clotting. This is easily understood when it is 

 remembered that nucleo-proteids arise from the breaking down of leucocytes 

 and blood-plates, and that these corpuscles go to pieces more rapidly when in 

 contact with a dead surface. It has been proposed also to hasten clotting in 

 case of hemorrhage by the use of ferment solutions. Hot sponges or cloths 

 applied to a wound will hasten clotting, probably by accelerating the formation 

 of ferment and the chemical changes of clotting. Coagulation may be retarded 

 or be prevented altogether by a variety of means, of which the following are 

 the most important : 



1. By Cooling. This method succeeds well only in blood which clots 

 slowly for example, the blood of the horse or the terrapin. Blood from 

 these animals received into narrow vessels surrounded by crushed ice may be 



