The air of the atmosphere being a compound body, it was natural to examine the effects of its constituents sepa rately upon the blood, as well as of the other factitious gases. A number of experiments have accordingly been performed on this subject, but, perhaps, without any very decided result. In experiments of this kind, much ma nual dexterity is required ; and there are many circum stances connected with the state of the animal itself—the manner in which the blood flows from the vein, the kind of vessel in which it is received, the temperature and other conditions of the atmosphere, and the manner of ap plying the gases—which may all produce a notable varia tion in the effects. Upon the whole, the most respecta ble authorities would lead us to conclude, that oxygen retards the process of coagulation, but that it is promoted by carbonic acid, and by some other of the unrespirable gases. The coagulation of the blood is much retarded, or entirely prevented, by the addition of certain salts ; and if blood be diluted with twelve times its bulk of water, the clot no longer separates. The process of coagulation takes place more slowly in venous than in arterial blood ; but it is asserted that the coagulation of venous blood is more complete, and the clot firmer.
The cause of the coagulation of the fibrin has never been satisfactorily explained. It is a phenomenon which does not exactly resemble any other with which we are acquainted, and the effect of external agents is not so well marked as to enable us to refer it to any general operation of the physical properties of bodies. What renders the subject peculiarly difficult is, that there are some circum stances which affect the process, and which operate in a way that we are quite unable to explain. Many causes of sudden death prevent the usual coagulation of the blood from taking place; yet. except in this one particular, thcy do not appear to have the least resemblance to each other. Among others we may enumerate injury and electri city, a blow upon the stomach, an njury of the brain, the bites of venomous animals, some acrid vegetable poisons, excessive exercise, and even violent mental emotions. In all these cases there has been found to be a remarkable coincidence, between the want of coagulability in the fibrin and the loss of contractility of the muscles immediately after death. The muscles are found to be relaxed, and arc incapable of being excited by the ordinary stimulants, while, at the same time, the body has been found much disposed to run into the state of putrefaction. These facts have been supposed to identify, to a certain degree, the operation of muscular contraction with the coagulation of the blood ; and this idea has been farther urged, from the consideration, that the chemical composition of fibrin is exactly similar to that of the muscular fibre.
It was probably from facts of this kind that Hunter was led to form his celebrated hypothesis of the life of the blood. He supposed that the blood was not merely the substance which gives life to the animal, by carrying to all parts what is necessary for their support and preserva tion, but that it is itself a living organized body, and even the peculiar seat in which the vitality of the whole system resides. The question concerning the life of the blood cannot be fully examined until we are farther advanced in our view of the animal economy, and especially until we have endeavoured to give a correct definition of the man ner in which the term life ought to be employed. But we may remark, that even were the Hunterian doctrine of the life of the blood to be fully established, it would not afford any real explanation of the cause of its coagulation, for the „same difficulty still remains, in what manner the presence of life operates, so as to produce either the coagulation of the blood, or the contraction of the muscles. Perhaps the most consistent view of the subject is, that the fibrin has a natural disposition to assume the solid form, but that as it is added to the blood, particle by particle, while this fluid is rapidly circulating through the vessels, it has no oppor tunity of concreting; but it exerts its tendency to assume the solid form, as soon as it is at rest, either within or without the vessels.
There is a subject connected with the coagulation of the blood, which is of great importance in the practice of me dicine, that the nature and appearance of the coagulum varies very much, according to the state of the body at the time when the blood is drawn. The most important of these varieties is the formation of what is called the buffy coat, a term which is used to express that state of the coa gulum, when the upper part of it contains no red parti cles, but exhibits a layer of a buff•colou•ed substance, lying upon the top of the red clot. This bully coat is generally formed when the system is labouring under inflammatory o y fever, and when, according to the modern doctrines of pa thology, there is an increased action of the capillary ves sels. The immediate cause of this appearance is obvious; the red particles which give the crassamentum its red co lour, begin to subside before the coagulation is completed, so that the upper part of the clot is left without them. The remote cause of the buffy coat is still undecided, al though many experiments have been made to discover it, especially by 11 ewson, Il unter, and Hey.