What parts of the heart most irritable.—The inner surface of the heart is considerably more irritable than the outer. In experiments, when the heart has become quiescent, and refuses to obey a stimulus applied to the outer surface, it frequently contracts readily for a short time niter this when air is introduced into its cavi ties, or when any other stimulant is applied to its inner surface. After death the different cavities of the heart generally lose their contrac tility in the following order, the left ventricle, the right ventricle, the left auricle, and last of all the right auricle.* And as the heart is gene rally the part of the body which shews the latest evidences of contractility, the right auricle has long received the name of ultimum moriens. hailer supposed that the greater persistence of contractility in the right side of the heart over the left might depend on the circumstance that the right side of the heart generally contains a greater or less quantity of blood after death, while the left side is generally empty. In this manner the inner surface of the right side of the heart is subject after death to the presence of a stimulant from which the left side is compara tively free. Ile put this opinion to the test by performing repeatedly the following experi ment.* He emptied the right side of the heart by the section of the pulmonary artery and venw caves, having previously retained the blood of the left side by passing a ligature around the aorta. The experiment succeeded many times : the right auricle remained per fectly immoveable, and the only motion which the right side retained arose from the connexion of its fibres with those of the left ventricle. The left auricle retained its movements for a certain time, the ventricles during a longer period, sometimes even for two hours. Ile adds, we thus transfer from the right auricle to the left ventricle the property of being the last living part in the body, in preserving for it during a longer period the irritation produced by the contact of blood. These experiments of Haller certainly chew that the left side of the heart will continue to contract longer than the right where it is subjected to a stimulant of which the other is deprived ; but they do not entitle us to conclude that the persistence of their contractility is the same when placed un der similar circumstances. We have every reason for believing that the right auricle is the part of the heart which last loses its contracti lity. Indeed Haller himself confesses, that if any part of the heart remains longer contractile than another, it is the right auricle. Nysten,t who performed a number of experiments upon the comparative persistence of the irritability in the different contractile parts of the body in the human species, after decapitation by the guillotine, and when the heart was conse quently emptied of its blood, obtained the fol lowing results upon the order in which the dif ferent parts of the heart lose their contractility ; —1st, the left ventricle, the contractility of which is annihilated much more quickly than that of the other organs ; 2d, the right ventri cle, the movements of which generally continue more than an hour after death ; 3d, the two auricles, the right being of all the parts of the heart that which preserves for the longest time its contractile power.
The stimulant used in these experiments was galvanism. The greater persistence of the con tractility in the right auricle over the other parts of the heart has been observed by other experi menters, after it had been cut from the body, and consequently without any contained blood. The particular part of the auricle which last loses its contractility varies in different cases. Sometimes the appendix is frond contracting when the rest of the auricle is quiescent ; at other times, and perhaps more frequently, those parts of the auricle around the entrance of the venw eavx retain their contractility longest.
Harvey and some of the older anatomists ob served the movements of the rem cave to continue in some of the lower animals after the auricles had ceased to move. The apex of the ventricles frequently remains longer contractile than the rest of the ventricle. Haller suggested that this might depend on the remaining blood gravitating to the apex, and there acting as a stimulant.
Duration of contractility after death.—In the cold-blooded animals the heart may be made to contract fourteen, twenty, thirty-four hours, or even longer after death. In warm blooded animals the heart remains contractile for a much shorter period after death than in cold-blooded animals. Haller found the heart contractile in a warm-blooded animal in one case four hours after death, and in another seven hours. He sometimes observed it to cease before the vermicular motion of the intes tines. Wepfer found it irritable in a dog six
hours after death. Nysten, who attended par ticularly to this subject, found in one of his experiments on the human subject, that the ventricles refused to contract upon the applica tion of galvanism one hour after decapitation, while the auricles continued contractile for seven hours five minutes after death.* In ano ther case the right auricle was still contractile eight hours after death ;t and in a subsequent case which he relates, it remained contractile in the neighbourhood of the entrance of the supe rior cava sixteen hours and a half after death.1 In the Mammifera, Nysten found that the left ventricle often refused to contract thirty minutes after death; that the right ventricle retained its contractility two hours, and sometimes longer, while the right auricle was not quiescent upon the application of the galvanism until eight hours after death.
He found it to vary in birds according to the degree of muscular activity which they enjoyed during life. In those of high flight, and which exercise great muscular contractility during life, and have a rapid circulation, as the sparrow hawk, the irritability of the heart and other muscles becomes much more speedily exhaust ed than in those the movements of which are comparatively slow and feeble, as in most domes tic fowls.§ Nysten supposes that the explana tion of the greater persistence of contractility of the right ventricle over the left lies in the cir cumstance that the left acts with greater vigour during life, thus referring it to the important general law which he has established by his experiments upon the comparative excitability of the muscular tissue in the various classes of animals, that the duration of the contractility after death is in the inverse ratio of the muscu lar energy developed during life.II Before we could admit this explanation, it would be ne cessary so show, what we believe it will be found impossible to do, that the left ventricle, apart from its greater quantity of muscular fibre, exerts greater strength or exhibits more ener getic contractions during life than the right ventricle. In young animals, immediately after birth, the contractility of the heart continues longer after death than in the adult animal. We would expect this to be most apparent in those which are born with their eyes shut, as puppies and kittens, and in those birds which are hatched without feathers, since these ani mals at that period of life approach in their physiological conditions to the cold-blooded animals. There is a curious circumstance stated by Mangili, and confirmed by Dr. Mar shall Hall, connected with the hybernation of animals, that if those mammalia which hyber nate are killed while under a state of lethargy, the heart and other muscles remain contractile for a longer period than when they are killed in a state of activity, thus resembling, when under the influence of this lethargy, in this as in many other respects, the physiological con dition of the cold-blooded animals. The con tractions of the heart may frequently be renewed by the application of warmth after they have apparently ceased. I have repeatedly observed the fact which has been stated by Haller and Nysten, that when any of the cavities of the heart become congested with blood, their con tractility becomes arrested, and, in their opi nion, extinguished.* I have also found that unloading the right side of the heart soon after the congestion has taken place, which can be done in many cases by opening the external jugular vein, acts as a valuable adjuvant under certain circumstances in renewing the heart's action. These it would be out of place to dis cuss here; but I may state that it appears to me to be principally useful in certain cases of poisoning, in asphyxia, and after the accidental entrance of air into the veins. Since the intro duction of a considerable quantity of air into the veins produces death by mechanically ar resting the movements of the right side of the heart, we believe that circumstances may occur in which the surgeon may be justified in intro ducing a tube into one of the large veins pass ing into the upper part of the chest, and suck ing the frothy blood from the right side of the heart. It is also necessary to remember this circumstance in experimenting upon the length of time during which the heart remains con tractile after death, as the division or non-divi sion of the large veins at the root of the neck in laying open the thorax may considerably modify the results.* For the probable force exerted by the heart, the share which the heart has in carrying on the circulation, and the probable quantity of blood expelled at each contraction, see the article CI arc LATION.