28 THE BLOOD. 



years of age, in other respects perfectly healthy, had been subject from early life to per- 

 sistent haemorrhage from slight wounds. 



Circumstances which accelerate coagulation have a tendency to arrest haemorrhage. 

 It is well known that exposure of a bleeding surface to the air has this effect. The way 

 in which the vessel is divided has an important infiuence. A clean cut will bleed more 

 freely than a ragged laceration. In division of large vessels, this difference is sometimes 

 very marked. Cases are on record in which the arm has been torn off at the shoulder- 

 joint, and yet the haemorrhage was, for a time, spontaneously arrested ; while we know 

 that division of an artery of comparatively small size, if it be cut across, would be fatal 

 if left to itself. Under these circumstances, the internal coat is torn in shreds, which 

 retract, their curled ends projecting into the caliber of the vessel and having the same 

 effect on the coagulation of blood as a bundle of twigs. In laceration of such a large 

 vessel as the axillary artery, the arrest cannot be permanent, for, as soon as the system 

 recovers from the shock, the contractions of the heart will force out the coagulated blood 

 which has closed the opening. 



From the foregoing considerations, it is evident that the remarkable phenomenon of 

 coagulation of the blood, which has so much engaged the attention of physiologists, has 

 rather a mechanical than a vital function ; for its chief office is in the arrest of haemor- 

 rhage. Coagulation never takes place in the organism, unless the blood be in an abnormal 

 condition with respect to circulation. Here its operations are mainly conservative ; but, 

 as almost all conservative processes are sometimes perverted, clots in the body may be 

 productive of injury, as in the instances of cerebral apoplexy, clots in the heart occurring 

 before death, the detachment of emboli, etc. 



Cause of the Coagulation of the Blood. If we adopt the views regarding the compo- 

 sition of the blood which involve the production of fibrin as a result of the decomposition 

 of plasmine, we must change in toto our ideas of the cause of the coagulation of the blood. 

 According to our present ideas, fibrin does not exist as a proximate principle, and plas- 

 mine is never decomposed in the body, under perfectly normal conditions; but, if the 

 blood be drawn from the body, effused from the vessels, or if the circulation be arrested 

 for a certain time, plasmine is decomposed, fibrin is formed, and the blood coagulates. 



In another work, written in 1864, we discussed the question of the cause of the co- 

 agulation of the blood quite fully ; but fibrin was then generally regarded as a proxi- 

 mate principle itself, and not as a product of decomposition. The theory that we then 

 adopted was the one proposed by Richardson, in 1856 ; viz., that the blood normally 

 contains a small quantity of ammonia, the presence of which keeps the fibrin in a liquid 

 state ; that ammonia is constantly being taken up by the blood from the tissues and ex- 

 haled by the lungs, and that, when the circulation of the blood is arrested, or when the 

 blood is effused or drawn from the vessels, ammonia is exhaled and coagulation takes 

 place. This theory has been formally abandoned by Richardson, who adheres, however, 

 to the accuracy of his experiments. If these experiments be entirely reliable, they seem 

 to prove the theory ; but it is stated by Robin, that, using chemical processes which will 

 detect T.-fftfl.Tnnr f ammonia, not a trace of this substance is to be found in the blood ; 

 that a small quantity of ammonia added to the blood does not prevent coagulation ; and 

 that the blood secured against evaporation will nevertheless coagulate. The chemical 

 experiments of Richardson were not very delicate, and the objections to them, made by 

 Robin, are probably well-founded. We are justified, therefore, in abandoning the the- 

 ory that coagulation of the blood is due to the evolution of ammonia. 



We may take the same position with regard to the older theories of coagulation, 

 which were nearly all vague and unsatisfactory. The idea that exposure to the air is the 

 cause of coagulation, which was held by Hew son, 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 



