HEART. 



613 



perfectly impossible to insulate that organ from 

 the nerve and experiment upon it; but we 

 think we are justified in concluding from ob- 

 servations and experiments derived from other 

 sources, that in all probability the contractility 

 of the heart depends upon a property possessed 

 by the muscular fibre itself without any neces- 

 sary intervention of its nerves. The possibility 

 of exciting or increasing the action of the heart 

 by stimuli applied to its nerves has been mixed 

 up with this question. Though it must be 

 admitted that mechanical and chemical stimu- 

 lants applied to a considerable surface of the 

 central organs of the nervous system quicken 

 the heart's action, yet experimenters have gene- 

 rally acknowledged that these stimulants applied 

 to the nerves of the heart produce no effect 

 upon its movements. Burdach,* however, 

 maintains that he has quickened the heart of a 

 rabbit deprived of sensation by applying caustic 

 potass to the trunk of the sympathetic, or its in- 

 ferior cervical ganglion. That the heart can be 

 excited to contraction by the application of 

 galvanism has had many supporters, and many 

 celebrated names are arranged both on the 

 affirmative and negative sides of the question. 

 That the movements of the heart may be in- 

 creased or renewed by the application of gal- 

 vanism as the experiment is usually performed, 

 there can be no reasonable doubt ; for if one 

 wire is placed upon the nerve and the other 

 upon the heart, the moist nerve will act as a 

 conductor to the electricity, and the effect pro- 

 duced will be the same as if the stimulant had 

 been applied to the substance of the heart itself. 

 Nysten admits that movements of the heart 

 were excited by the galvanism when one of the 

 wires was applied to one of the large arteries 

 from which all the visible filaments of the 

 nerves had been dissected off. Dr. C. Holland,! 

 in a number of experiments, satisfied himself 

 that the tissues of the body conduct galvanism 

 with so much facility, thai the heart's action 

 could readily be excited, when one wire was 

 placed upon the heart and the other in the nose, 

 mouth, and even among the moist food in the 

 stomach. I have performed similar experiments 

 with the same results. Ilumboldt and Brachet 

 assert that they have quickened the movements 

 of the heart by applying both wires to one of 

 the cardiac nerves. If these and the experi- 

 ments of Burdach could be relied upon, they 

 would be sufficient to prove that the heart could 

 be occasionally stimulated through the cardiac 

 nerves, but the negative experiments on the 

 other side are so numerous, and the sources of 

 fallacy in judging in this manner of the relative 

 quickness of the heart's action between one 

 time and another so obvious, that we must be 

 allowed to distrust them unless they should be 

 confirmed by other accurate observers. 



Constancy of the heart's action. The con- 

 stancy of the heart's action is more apparent 

 than real. After each contraction a state of 

 relaxation follows. The relative duration of 



* Traite de Physiologic, torn. vii. p. 74, traduit 

 par Jourdan. 



t Experimental Inquiry, &c. p. 275. 



the contraction of the auricles and ventricles, 

 according to Laennec, appears to be as fol- 

 lows : a third at most, or a fourth or a little 

 less by the systole of the auricles ; a fourth or 

 a little less by the state of quiescence ; and the 

 half or nearly so by the systole of the ventricles. 

 From this he calculates that the ventricles, 

 when the heart is acting with its usual frequency, 

 rest twelve hours out of the twenty-four, and 

 that in those individuals in whom the pulse is 

 naturally below 50, it must be in a state of 

 relaxation sixteen hours out of the twenty-four.* 

 Now this is a degree of contraction of which 

 many muscles of the body are probably suscep- 

 tible, such as the muscles which support the 

 trunk when we sit or walk, and which some, 

 as the diaphragm and intercostals, generally 

 perform. 



Regularity of the heart's movements. The 

 regularity of the heart's movements, so essential 

 to the welfare of the animal, has appeared, 

 even to many modern physiologists, to be inti- 

 mately connected with some peculiarity in its 

 structure. We are inclined, however, to agree 

 with Haller, that this is perfectly explicable on 

 the known laws of muscular contractility 

 in other parts of the body. The regularity of 

 the heart's action was another fertile subject of 

 hypothesis to the older physiologists ; and even 

 in the present day we find the term " organic 

 instinct" employed to designate it. 



The contractions of the heart take place in 

 the order in which the blood flows into its 

 different cavities; and if the blood be the habi- 

 tual stimulant upon which its movements 

 depend, this is exactly what we would expect.f 

 The blood forced in greater quantity into the 

 auricles by the contraction of the termination 

 of the cavae and pulmonary veins, stimulates 

 the auricles to contract and propel an additional 

 quantity into the ventricles ; and this, acting as 

 a stimulant upon the ventricles, excites them to 

 contract and drive the blood into the arteries, 

 when the same series of phenomena is renewed 

 and repeated in the same succession. 



The continuance of the heart's action after 

 the circulation has ceased, we have already 

 attempted to explain ; and if these contractions 

 depend upon the presence of a stimulus, they 

 must evidently be in the same order as in the 

 natural state of the organ, as these have not 

 been interrupted. The continuance of the re- 

 gular order of the contractions of the heart 

 after its removal from the body can in general, 

 we think, be satisfactorily accounted for by 

 the substitution of a new stimulant for that 

 of the blood ; the cavities are then occu- 

 pied with air instead of blood, and each 



* We have "not here given Laennec's calculations 

 of the relative duration of the contraction and 

 relaxation of the auricles, as they must be founded 

 on false data on the supposition that the second 

 sound of the heart marked the duration of the con- 

 traction of the auricles. 



t This was also the doctrine maintained by Se- 

 nac, op. cit. torn. i. p. 325. Senac, however, was 

 opposed to the doctrine of Haller, that the contrac- 

 tility of the heart was a property inherent in the 

 muscular fibre, and independent of the nerves. 

 Tom. i. p. 451. 



