HEART. 



611 



two sides of the heart, and the different results 

 obtained when the one side of the heart was 

 emptied of blood, and when it was retained in 

 the other, are sufficient to shew the effect which 

 the presence of blood in the cavities of this 

 organ has upon the continuance of its action 

 after the circulation has ceased. If the heart 

 has been removed from the body and emptied 

 of its blood, it must naturally follow that its 

 different cavities will be tilled with atmospheric 

 air; and it has been well ascertained that this 

 acts as a very powerful stimulant upon the 

 inner surface of the heart.* Every circumstance 

 connected with these experiments is in exact 

 conformity with the opinion that the movements 

 of the heart are only called into action by the 

 application of a stimulant. Thus, when the 

 irritability of the heart becomes more languid, 

 and when the blood or the atmospheric 

 air in its cavities becomes insufficient to 

 raise it to contraction, strong and energetic 

 movements may still generally be excited by 

 having recourse to a more powerful stimulant, 

 such as the prick of a scalpel or the application 

 of galvanism. Since the heart is highly en- 

 dowed with irritability, various other mild fluids 

 besides the blood are capable of exciting it to 

 contraction. As every organ, however, has its 

 irritability adapted for the function which it is 

 destined to perform, so we find that the heart, 

 the central organ of the circulation, is most 

 fitly called into action by the blood, its appro- 

 priate and natural stimulant. 



In examining the nature of the irritability of 

 the heart, and contrasting it with that of the 

 voluntary muscles, we must not compare its 

 contractions with those excited by volition in 

 the muscles of voluntary motion, for these last 

 are evidently modified by the nervous influence 

 for an obvious purpose ; but let us observe both 

 when placed under similar circumstances, and 

 irritated by the application of the same stimu- 

 lant applied to the muscles themselves, and we 

 will find that they only differ in this, that in 

 the voluntary muscles each successive appli- 

 cation of the stimulant is generally followed by 

 a single contraction, while in the heart it is 

 followed, except when the contractility is 

 much impaired, by several consecutive con- 

 tractions alternated with relaxations. This ten- 

 dency to successive contractions is also observed, 

 though not to the same extent, in the muscular 

 coat of the intestines. 



We must admit, however, that the contrac- 

 tions of the heart proceed under circumstances 

 where it is difficult to point out the presence 

 of any sufficient stimulus, and where, to account 

 for their continuance, we are almost obliged to 

 have recourse to the supposition, that there is 

 some innate moving power in the heart itself. 

 It has been stated, for example, that the move- 



* Peyer, Brunner, and Haller have seen the con- 

 tractions of the heart renewed by blowing air into 

 the cava ascendens. Wepfer and Steno produced 

 the same effect by inflation of the thoracic duct. 

 Enman states that he once observed the renewal 

 of the heart's action in the human subject by 

 blowing air into the thoracic duct. Vide Senac. torn, 

 i. p. 326. 



ments of the heart will proceed under the ex- 

 hausted receiver of an air-pump. I have 

 repeatedly placed under the bell-glass of an 

 air-pump the heart of a frog when removed from 

 the body and emptied of its blood, and I could 

 never satisfy myself that the frequency or 

 strength of its contractions was at all affected 

 by the withdrawal or renewal of the air ; and 

 though it might be urged that the air is only 

 rarefied, not entirely removed, in the best ex- 

 hausted receiver of an air-pump, and that con- 

 sequently in such experiments a stimulant still 

 existed in the presence of the rarefied air, yet 

 I would not consider this explanation of the 

 continuance of its contractions by any means 

 satisfactory. In these experiments there is ano- 

 ther source of stimulation present which ought 

 to be taken into account, for, as I shall after- 

 wards shew, the slightest movement of the 

 heart, such as that caused by its contraction, 

 upon the surface upon which it is placed when 

 removed from the body, is sufficient, from the 

 great irritability of the organ, to act as a stimu- 

 lant upon it. If these external stimuli appear 

 to be insufficient to account for the persistence 

 of the contractions of the heart under the cir- 

 cumstances we have mentioned, we may have 

 recourse to another explanation drawn from the 

 mechanical structure of the organ; for it is 

 possible, as has been suggested by Dr. Alison, 

 that from the peculiarly convoluted arrangement 

 of the fibres, the outer may, during the con- 

 traction of the organ, pinch or stimulate the 

 inner, and so cause this tendency to repeated 

 contractions from one application of a stimu- 

 lant. We do not, however, consider that we 

 have succeeded perfectly in accounting for the 

 continuance of the contractions of the heart 

 under all circumstances, but we are unwilling 

 to admit the existence of any peculiar innate 

 and unknown agency in the production of any 

 phenomenon, until it is satisfactorily established 

 that it cannot be accounted for on the known 

 laws which regulate similar phenomena in the 

 same texture in other parts of the body. And 

 it must also be remembered that these move- 

 ments of the heart have only been observed 

 when its contractility was still comparatively 

 vigorous, and where sources of stimulation 

 were still present. We ought, besides, to be 

 the more cautious in admitting the existence 

 of this innate moving power, since it is in 

 opposition to a well-known law in the animal 

 economy, that though the various tissues of 

 an organized body are endowed with certain 

 vital properties, yet the application of certain 

 external and internal stimuli is necessary to 

 produce their manifestations of activity. In 

 fact it is from the action and reaction of these 

 tissues and excitants upon each other, that the 

 phenomena of life result.* 



* The remarks which we have made above, 

 illustrating the great length of time which the heart 

 will continue to contract after being removed from 

 the body, and when all communication between the 

 nerves ramified in ics substance and the sympathetic 

 ganglia and the central organs of th? nervous sys- 

 tem have been cut off, when taken along with the 

 equally well ascertained fact, that its contractions 

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