and the colour of the skin, which is partly dependent upon these differences, thus becomes an index of the state of individual health or disease. In the morbus cæruleus, cyanopathy or blue disease, the whole surface is coloured blue, especially in those parts where the skin is delicate as on the lips,—owing to a communication existing between the right and left sides of the heart, so that the blood can pass from one to the other, without proceeding through the lungs; and the appearance of the jaundiced is familiar to all.

The formation of the clot, and its separation from the serum, is manifestly dependent upon the fibrine; which, by assuming the solid state, gives rise to the coagulation of the blood;—a phenomenon, which has occasioned much fruitless speculation and experiment.

The process of coagulation is influenced by exposure to the air. Hewson affirmed, that it is promoted by such exposure, but HUNTER was of an opposite opinion. If the atmospheric air be excluded, by filling a bottle completely with recently drawn blood, and closing the orifice with a good stopper,—coagulation is retarded. Yet Sir C. Scudamore mentions the singular fact, that if blood be confined within the exhausted receiver of an air pump, the coagulation is accelerated. The presence of air is certainly not essential to the process.

Experiments have also been made on the effect produced by different gases on the process of coagulation; but the results have not been such as to afford much information. It is asserted, for example, by some, that it is promoted by carbonic acid, and certain other of the irrespirable gases, and retarded by oxygen; by others, the reverse is affirmed; whilst Sir Humphry Davy informs us, that he could not perceive any difference, in the period of the coagulation of venous blood, when it was exposed to azote, nitrous gas, oxygen, nitrous oxide, carbonic acid, hydrocarbon, or atmospheric air.

The time, necessary for coagulation, is also affected by temperature. It is promoted by warmth; retarded, but not prevented, by cold. Hewson froze blood, newly drawn from a vein and afterwards thawed it; when it first became fluid, and then coagulated as usual. Hunter made a similar experiment with the like result. It is obviously, therefore, not from simple refrigeration that the blood coagulates. Sir C. Scudamore found, that blood, which begins to coagulate in four minutes and a half in a temperature of 53° Fahr., undergoes the same change in two minutes and a half at 98°; and that, which coagulates in four minutes at 98° Fahr., becomes solid in one minute at 120°. On the contrary, blood, which coagulates firmly in five minutes at 60° Fahr., will remain quite fluid for twenty minutes, at the temperature of 40° Fahr., and requires upwards of an hour for complete coagulation.

During the coagulation of the blood, a quantity of caloric is dis

engaged. Fourcroy relates an experiment, in which the thermometer rose no less than 11° during the process; but as certain experiments of Hunter, appeared to show, that no elevation of temperature occurred, the observation of Fourcroy was disregarded. It has, however, been confirmed by some experiments of the late Dr.Gordon, of Edinburgh, in which the evolution of caloric, during coagulation, was rendered more manifest, by moving the thermometer during the formation of the clot, first into the coagulated, and afterwards into the fluid, part of the blood, when he found, that, by this means, he could detect a difference of 6°; which continued to be manifested for twenty minutes after the process had commenced. In repeating the experiment on blood, taken from a person labouring under inflammatory fever, the thermometer was found to rise 12°. Sir C. Scudamore affirms, that the rate at which blood cools is distinctly slower than it would be, were no caloric evolved; and that he observed the thermometer to rise one degree at the commencement of coagulation. On the other hand, Dr. John Davy accords with Hunter in the belief, that the increase of temperature, from this cause, is very slight or null. Again we have to deplore the discordance amongst observers; and it will perhaps have struck the reader more than once, that such discordance applies as much to topics of direct observation as to those of a theoretical character. The discrepance, regarding anatomical and physical facts, is even more glaring than that which prevails amongst physiologists in accounting for the corporeal phenomena; a circumstance which tends to confirm the notion promulgated by one of the most distinguished teachers of his day, that "there are more false facts in medicine, (and the remark might be extended to the collateral or accessary sciences,) than false theories."

There are certain substances, again, which, when added to the blood, prevent or retard its coagulation. Hewson found, that the sulphate and muriate of soda, and the nitrate of potassa were amongst the most powerful salts in this respect. The muriate of ammonia and a solution of potassa have the same effect. On the contrary, the coagulation is promoted by alum, and by the sulphates of zinc and copper. How these salts act on the fibrine, so as to prevent its particles from coming together, it is not easy to explain. But these are not the only inscrutable circumstances that affect the coagulation of the blood. Many causes of sudden death have been considered to have this effect:—lightning and electricity; a blow upon the stomach; injury of the brain; the bites of venomous animals; certain narcotico-acrid vegetable poisons; also excessive exercise and violent mental emotions, when they suddenly destroy, &c. Many of these affirmations doubtless rest on insufficient proof. Sir C. Scudamore, for example, asserts that lightning has not this effect. Blood, through which electric discharges were transmitted, coagulated as quickly as that which was not electrified; and, in

animals, killed by the discharge of a powerful galvanic battery, the blood in the veins was always found in a solid state.

We shall find, hereafter, that these affirmations have been considered evidence that the blood may be killed; and, consequently, that it is possessed of life. The whole phenomena, indeed, of coagulation, inexplicable in the present state of our knowledge, have been invoked to prove this position. The preservation of the fluid state, whilst circulating in the vessels, although agitation, when it is out the body, does not prevent its coagulation, has been regarded, of itself, sufficient evidence in favour of the doctrine. Dr. Bostock, indeed, asserts, that perhaps the most obvious and consistent view of the subject is, that fibrine has a natural disposition to assume the solid form, when no circumstance prevents it from exercising this inherent tendency. As it is gradually added

to the blood, particle by particle, whilst that fluid is in a state of agitation in the vessels, it has no opportunity, he conceives, of concreting; but when it is suffered to lie at rest, either within or without the vessels, it is then liable to exercise its natural tendency. It is not our intention, at present, to enter into the subject of the vitality of the blood. The general question will be considered in a subsequent part of this work. We may merely observe, that, by the generality of physiologists, the blood is presumed, either to be endowed with a principle of vitality, or to receive from the organs, with which it comes in contact, a vital impression or influence, which, together with the constant motion, counteracts its tendency to coagulation. Even Magendie,—who is unusually and properly chary in having recourse to this method of explaining the notum per ignotius,—affirms, that instead of referring the coagulation of the blood to any physical influence, it should be considered as an essentially vital process; or, in other words, as affording a demonstrative proof, that the blood is endowed with life.

Lastly; within a few years, Vauquelin has discovered in the blood a considerable quantity of a fatty matter; of a soft consistence; and which he, at first, regarded as fat. Chevreul, however, after careful investigation, has declared it to be identical with the matter of the brain and nerves, and to form the singular compound of an azoted fat. Prevost and Dumas have likewise demonstrated the existence of urea in the blood of animals, from which the kidneys had been removed. Chemical analysis is, consequently, adding daily to our stock of information on this matter; and is exhibiting to us, that many of the substances, which compose the tissues, exist in the very state in the blood, in which we meet with them in the tissues.

Other facts connected with the vital fluid; its quantity, &c. will be considered, after we have inquired into the changes produced on the venous blood in the lungs, through the agency of respiration.

Physiology of Venous Absorption.

Whilst the opinion prevailed universally, that the lymphatics are the sole agents of absorption; the fluid, circulating in the veins, was considered to consist entirely of the residue of the arterial blood, after it had passed through the capillary system, and been subjected to the different nutritive processes there effected. We have already seen, however, that the drinks are absorbed by the mesenteric veins; and we shall hereafter find, that various other substances enter the venous system by absorption. It is obvious, therefore, that the venous blood cannot be simply the residue of arterial blood; and we can thus account for the greater capacity of the venous system than of the arterial.

The facts that were referred to, when considering the absorption of fluids from the intestinal canal, may have been sufficient to show, that the veins are capable of absorbing; as the odorous and colouring properties of substances were distinctly found in the mesenteric veins. A question arises, whether any vital elaboration is concerned, as in the case of the chyle, or whether the fluid, when it attains the interior of the vessel, is not the same as without? Adelon,—who, with many of the German physiologists, believes in both venous and lymphatic absorption, and venous and chyliferous absorption,—conceives, that a vital action takes place at the very mouths of the venous radicles, precisely similar to that which is presumed to be exerted at the mouths of the lymphatic and chyliferous radicles. In his view, consequently, an action of elaboration is exerted upon the fluid, which becomes, in all cases, converted into venous blood, at the very moment of absorption, as chyle and lymph are elaborated under similar circumstances.

On the other hand, Magendie, Fodera and others maintain, that the substance soaks through the vessel, when possessed of the necessary tenuity; that this act of imbibition is purely physical, and consists in the introduction of the absorbed materials through the pores of the veins by capillary attraction. In his view, therefore, the fluid within the vessel should be the same as that without.

In favour of the vital action of the veins we have none of that evidence, which strikes us in regard to the chyliferous and lymphatic vessels. In these last we invariably find fluids, identical,—in all essential respects,—in sensible and chymical characters; and never containing extraneous matter, if we make abstraction of certain salts, which have been occasionally met with in the thoracic duct. In the veins, on the other hand, the sensible properties of odorous and colouring substances have been apparent. But, it may be remarked, the fluid, flowing in the veins, is as identical in composition as the chyle or the lymph; this is true. It must, however, be recollected, that the greater part of it is the residue of the

arterial blood; and that its hue and other sensible properties are such as to disguise any absorbed fluid, not itself possessing strong characteristics. The fact then, now indisputable, that various substances, placed outside the veins, have been detected in the blood within, is not only a proof, that the veins absorb; but that no action. of elaboration has been exerted on the absorbed fluid. Of this we have the most manifest proof, in some experiments by Magendie, in the last edition of his Précis Elémentaire de Physiologie. In exhibiting to his class the mode, in which medicines act upon the system, he showed, on a living animal, the effects of introducing a quantity of water, of the temperature of 104° Fah., into the veins. In performing this experiment, it occurred to him to notice what would be the effect, produced by artificial plethora on the phenomena of absorption. Having injected nearly a quart of water into the veins of a dog of middle size, he placed in the cavity of the pleura a small dose of a substance with the effects of which he was familiar, and was struck with the fact, that these did not exhibit themselves for several minutes after the ordinary period. He immediately repeated the experiment, and with a like result. In several other experiments, the effects appeared at the ordinary time, but were manifestly feebler than they ought to have been from the dose of the substance employed, and were kept up much longer than usual.

In another experiment, having introduced as much water as the animal could bear without perishing,—which was about two quarts, -the effects did not occur at all. After having waited nearly half an hour for their development, which generally required only about two minutes, he inferred, that if the distention of the bloodvessels was the cause of the defect of absorption; provided the distention were removed, absorption ought to take place. He immediately bled the animal largely in the jugular; and, to his great satisfaction, found the effects manifesting themselves as the blood flowed.

He next tried, whether, if the quantity of blood were diminished at the commencement of the experiment, absorption would be more rapid; and the result was as he anticipated. An animal was bled to the extent of about half a pound; and the effects, which did not ordinarily occur until after the second minute, appeared before the thirtieth second. As the results of these experiments seemed to show, that absorption is evidently in an inverse ratio to the degree of vascular distention, Magendie inferred, that it is effected physically; is dependent upon capillary attraction; and that it ought to take place as well after death as during life. To prove this he instituted the following experiments:—

He took a portion of the external jugular vein of a dog, about an inch long and devoid of branches. Removing carefully the surrounding cellular tissue, he attached to each of its extremities a glass tube, by means of which he kept up a current of warm water