222 THE BLOOD 



placing a ligature upon its central end and is then excised after having 

 previously ligated its distal end, the blood so entrapped remains fluid 

 for many days. This preparation, which is known as the ''living test 

 tube," must be kept under proper conditions of moisture and tem- 

 perature, because if its walls are injured, it will act in the same manner 

 as any other foreign body and cause intravascular clotting. If this 

 preparation is suspended for a time, the red corpuscles finally settle by 

 gravity, so that it is possible to obtain the supernatant liquid sepa- 

 rately and to subject it later on to coagulating agents. 



Air was formerly regarded as a necessary factor in coagulation, but 

 as this process also takes place in blood which has been collected in a 

 tube above mercury, this view can no longer be held. Accumulations of 

 air in the form of larger or smaller bubbles act as foreign bodies and 

 hasten the destruction of the thrombocytes and the liberation of 

 thrombokinase. 



Substances Derived from the Tissues. — The observation has been 

 made repeatedly that the blood of certain animals, when prevented 

 from coming in contact with the neighboring tissues, clots less speedily 

 or remains fluid for sometime after its withdrawal. This is true espe- 

 cially of the blood of birds, reptiles and fishes, which clots rather 

 quickly if permitted to flow across the incised tissues, but fails to 

 coagulate for many days if drawn from the cannulized blood-vessel 

 directly into a clean and dust-free beaker. This result indicates that 

 the blood of these animals is devoid of a coagulating agent, although 

 a substance of this character is contained in their tissues. The plasma 

 obtained from this type of blood also remains fluid for a long time, but 

 coagulates within a few minutes if an extract of the tissues is added to 

 it. Under ordinary conditions, however, an animal of this kind is 

 fully protected against serious hemorrhage, because the escaping 

 blood is subjected to the coagulating agent as soon as it leaves the 

 vascular channel. 



Different extracts have been prepared from the tissues of mammals 

 which markedly accelerate the coagulation of the blood. The active 

 principle contained in them has been variously designated as cell- 

 globulin, tissue-fibrinogen, tissue nucleoproteid, coagulin, and zymo- 

 plastic substance. Howell ^ suggests the term of thromboplastic sub- 

 stance, because it permits of a more general application, and refers 

 merely to an agent which accelerates the clotting without indicating 

 the manner in which this acceleration is brought about. A substance 

 of this character has been obtained by Howell from certain tissues with 

 the help of ether or with ether and alcohol. It is known as kephalin, 

 and is held in combination with a protein which is precipitated at a 

 temperature of 60° C. According to Howell, this body possesses the 

 property of neutralizing the antithrombin, while others^ believe that 

 it is identical with fibrin ferment ^nd that the activation of the pro- 



^ Am. Jour, of Physiol., xxxi, 1912, 1. 



* Moravitz, Hofmeister's Beitrage, v, 1904, 133. 



