156 THE BLOOD. 



Cause of the Coagulation of the Blood. Though the phe- 

 nomena of coagulation, and the circumstances which modify 

 it, especially as occurring in the organism, are of more prac- 

 tical importance than any thing else, the study of these 

 phenomena naturally leads us to inquire into the reason why 

 fibrin thus changes its form. When we say that this prin- 

 ciple is endowed with the property of spontaneous coagula- 

 bility, we do not express what is strictly the fact. It remains 

 fluid until it is placed in abnormal conditions, when, without 

 the application of heat, or any chemical reagents, it coag- 

 ulates ; but so long as it remains in the circulating blood, 

 lymph, or chyle, coagulation does not take place. This 

 property, which has been so long recognized, has been the 

 subject of many speculations as to its cause, and some experi- 

 ments ; but until the last few years the experiments have done 

 nothing but familiarize us with the actual phenomena which 

 take place, and left the cause, as before, entirely a matter of 

 speculation. Under these circumstances it will not be found 

 very profitable to discuss the old theories on the subject. 

 Our object in the historical review of physiological questions 

 is to show the gradual development of truth, as facts have 

 been accumulated by different observers, which those last in 

 the field have been able to coordinate, rather than to exhume 

 hypotheses which have fallen before actual observation. On 

 no subject have hypotheses been more vague and unsatis- 

 factory, and more readily disproved by experiment, than 

 with regard to the cause of coagulation of the fibrin. The 

 idea that exposure to the air is the cause of coagulation, 

 which was held by Hewson, is disproved by the simple fact 

 that coagulation takes place in a vacuum. The vital theory 

 of Hunter, which was adopted by most physiologists of his 

 time, is too indefinite for discussion at the present day, and 

 really expresses utter want of knowledge on the subject. 

 The theory that motion is the cause of the fluidity of fibrin 

 in the body, is disproved by the fact that violent agitation of 

 the blood out of the body does not prevent coagulation. 



