In this difficulty it is important to remember, that we have many facts to indicate the exist ence of powers which move the blood and other organized fluids in living animals, inde pendently of any contractions of moving solids. It would appear that the power by which any texture is nourished, or secretion or excretion is formed from the blood, in any part of the circu lation, is, to a certain degree, a cause of move ment of the blood towards that part,and that any stimulus given to such act of nutrition or secre tion, although applied at the extremity of the capillaries, produces an effect on the circulation which, as Sir C. Bell expresses it, is retrograde along the branches of the arteries. Thus, the flow of blood to the mucous membrane of the stomach and bowels during digestion, to the uterus during gestation, to the mammze during lactation, to any part of the body during inflammation, sup puration, or the growth of a tumour, is excited by causes acting at the extremities of the arte ries of these parts ; although there is the same difficulty in all these cases, as in the case of the lungs, in understanding how a cause acting there, and exciting the only vital power which arteries can be shewn to possess, should in crease the flow of blood through them.
It is always to be remembered, that pre cisely analogous phenomena are observed from the application of heat, or other stimuli, to single branches, or roots, of vegetables, where there is no evidence of the existence, either of a structure or of a contractile power, in the vessels or cells through which the fluids pass, capable of giving them a determinate direction towards the parts, which are thus stimulated ; and where the movement of fluids that can be seen, (in the case of those plants that have milky juices,) is not only unattended with any visible contraction of solids, but is of a kind, (as the recent observations of Schultze, Amid, Raspail, and others indicate,) which no contractions of solids appear capa ble of producing.
It is farther to be observed, that when venous blood becomes arterial, it acquires an increase of fibrin,* and that its tendency to coagulation is decidedly increased,t which implies such an increase of an attraction of aggregation in the particles of the fibrin, as may be held to be strictly vital. And on the other hand, when arterial blood becomes venous, according to the microscopical observations of Kalten brunner, its globules seem to separate some what from one another, and its whole bulk ap pears somewhat increased.X Lastly, it is to be remembered, that when a vessel is opened in a living animal, and the blood exposed to the air, the consequence is, a movement of derivation of the blood, in all directions, towards the aperture ; which is cer tainly altogether independent of the heart's action, and which the elaborate investigations of Haller led him (and apparently with good reason) to think inexplicable likewise by any contraction of vessels.§ The consideration of all these facts may lead us strongly to suspect, that the stimulus to the circulation which is given by the arte rialization of the blood, and which we have found to act chiefly in the capillaries of the lungs, is of the nature of an attraction of the venous blood towards the part where it is to undergo this change, and towards the arterial blood in advance of it in the vessels ; not of the nature of an increased contraction of the vessels themsleves ; and that it is in conse quence of the failure of this auxiliary power in the circulation, that the stagnation of the blood in the lungs in asphyxia, and the extinc tion of the organic life, are effected.
What has been said of the manner in which death is produced in asphyxia, enables us to understand in what circumstances it can hap pen, that life may be retained, even by a' warm-blooded animal, for an unusual length of time, without respiration. As the stop to the circulation is the immediate cause of death, it is obvious that an animal which can exist for a time, in a lowered state of vitality, with little or no circulation, will during that time require no exposure of its blood to air, to maintain that grade of vitality ; and farther that in such an animal, as the brain will not suffer from the afflux of venous blood, and as the lungs will not be hurtfully congested, these organs will retain a condition much better adapted for the recovery of their functions, than they will in those cases where asphyxia is produced at a time when the circulation is vigorous.
Hence we can easily understand, that per sons who are in a state of syncope, (from a temporary cause,) in whom the circulation is nearly at a stand before the access of air to their lungs is obstructed, may survive a longer suspension of the acts of respiration than per sons in health. This has been stated, by Des Granges and Fodere, as the explanation of some cases in which it appears certain, that recovery has taken place after fifteen minutes or more of submersion in water.* The case of hybernating animals was, until lately, considered to be of this nature, i. e. it was supposed that circulation is gradually sus pended in those animals, simultaneously with respiration, and therefore that such animals, although consuming little or no air, did not suffer the noxious influence of venous blood on their solids, and remained susceptible even of sensation. But the experiments of Dr. Marshall Hall l- appear to have established that in warm-blooded hybernating animals in the complete state of torpor, when respiration is quite at a stand for many hours, circulation, although slow and feeble, still goes on regu larly; so that we must suppose the essential peculiarity of these animals, during the state of lowered vitality, to which they are reduced by cold, to be this, that the venous blood has little of the noxious effect, in any part of the system, which it has, on them as on other animals, during the state of activity ; it has neither the same difficulty of making its way through the lungs, nor the same destructive influence on the brain.X The nearest approach to this mode of vita lity in the human body, is in the case of the new-born child, which has never felt the in fluence of perfectly arterial blood, and which has been known to live, although its natural respiration was not established for nearly an hour after birth.