Thus far we have only spoken of the blood in a general manner, and without respect to the part of the system in which this fluid is examined ; it is, however, very far from being identical in every part, and there are wide differences between the physical and physio logical properties of arterial and of venous blood.
The blood which is tending to the several parts of the body is in the first place of a bright vermilion red colour (arterial blood); whilst that which has already passed through the different tissues, and is on its way back from them, is of a dusky or blackish red of various degrees of intensity (venous blood). Arterial blood also coagulates more quickly than venous blood, and, from the researches of Dr. John Davy, appears to have rather a less capacity for caloric,* and a somewhat in ferior specific gravity (1,049: 1,051) ; we are, however, led to think that in the normal state the contrary of the latter proposition will be found to obtain, for Messrs. Prevost and Du mas have shown that in this case arterial blood contains a larger proportion of globules than venous blood.t When the physiological action of arterial and of venous blood is investigated, still more striking differences are discovered ; the first maintains vital excitation in the economy, and the second is insufficient to support life. Physiologists have even gone so far as to regard the influence of the venous blood upon the brain as deleterious ; t but more recent experiments show that though inadequate to keep up life, it is far from being a poison ; on the contrary, it rather tends to prolong existence, for frogs whose vascular system is filled with this liquid die less speedily than those placed under similar circumstances, but which have lost almost the whole of their blood by hmmorrhage.§ The blood thus modified by the influence of the organs it permeates, is still susceptible of resuming its primary colour, and of ac quiring at the same time its vivifying pro perties : it is enough to expose it to the con tact of oxygen, to give it back all its peculiar qualities. We find, in fact, that if venous blood be agitated with atmospheric air, or better still with oxygen gas, it speedily assumes the vermilion tint that characterizes arterial blood, and if the air thus employed be afterwards analysed, a certain quantity of oxygen will be found to have disappeared, and its place to be occupied with a corresponding measure of carbonic acid. Now that which happens here under the influence of mere chemical affinity, also takes place in the ani mal economy, and it is even thus that venous blood in being exposed to the contact of atmospheric air in the respiratory apparatus, whatever its nature, changes into arterial blood and again becomes fit to minister to life. (See RESPIRATION.) On the other hand, if ver milion-coloured blood be subjected to the action of carbonic acid, it speedily acquires a deep or blackish hue, and then resembles venous blood in its appearance and pro perties.
It now became a question of the very highest importance in the theory of respiration to ascertain whether the oxygen acting upon the blood in the manner specified, produced the carbonic acid disengaged, by combining directly with carbon supplied by the colouring matter or some other element of the blood, or whether the oxygen was simply dis solved by the blood and in dissolving ex pelled the carbonic acid which existed in it ready formed.
Various experiments satisfy us that venous blood contains carbonic acid already formed. My brother, Dr. W. F. Edwards, has shown that those animals which possess the greatest powers of resisting asphyxia continue for a long time to disengage carbonic acid when kept in vessels filled with pure azote or hy drogen, circumstances under which it is im possible that the carbonic acid evolved can proceed from the direct combination of in spired oxygen with the carbon of the blood.
By placing venous blood under the receiver of an air-pump, several inquirers had indeed already found that bubbles of carbonic acid gas were disengaged from it, when the pres sure of the atmosphere was withdrawn. This fact, first observed by Vogel, has been verified by Messrs. Braude, Bauer,t and others. The quantity of carbonic acid disengaged in this way, however, is very small, and altogether inadequate to explain the phenomena accom panying respiration ; but if, after having freed a quantity of blood as completely as possible from its carbonic acid by means of the air pump, it be agitated with hydrogen or any other gas, this will be absorbed, and a fresh and corresponding disengagement of carbonic acid will be determined.$ On the other hand there is an experiment of Girtanner, mentioned by Hassenfratz,§ which goes to prove that arterial blood contains a portion of free oxygen in its constitution ; but this conclusion appears to require confirmation.
The bright vermilion or dusky red colour of the blood, however, does not depend solely on the nature of the gas it holds in solution, or with which its colouring matter is in com bination. The recent experiments of Dr. Hoffmann skew that the presence of the saline matters it contains is necessary to the phe nomena in question. Blood freed from these saline ingredients is black, and cannot be brought to the vermilion red tint as usual by the action of oxygen. The same physiologist also ascertained that the presence of an over dose of saline matter in blood charged with carbonic acid, equally prevented the ordinary action of oxygen in changing its colour.
The blood does not invariably exhibit the properties and the mode of composition which we have just ascribed to it in the normal state. There was a time when physicians ascribed the greater number of internal maladies to alterations of this fluid ; the general errone ousness of this opinion, however, was at length detected, and at the present day patho logists have probably fallen into the opposite extreme, namely, that of neglecting the study of the changes which the blood does actually undergo, although these are sufficiently striking in many cases, and undoubtedly exert an im mense influence upon the animal economy. A careful examination of their kinds and effects were undoubtedly fraught with results of equal importance in a medical as in a physiological point of view.
(H. Milne Edwards.)