ACTION OF THE HEART. 297 



2. Action of the Heart. 



234. The Heart is endowed in tin eminent degree with the property of 

 Irritability, by which is meant the capability of being easily excited to 

 movements of contraction alternating with relaxation. Thus, after the 

 Heart has been removed from the body and has ceased to contract, a slight 

 irritation will cause it to execute, not one movement only, but a' series of 

 alternate contractions and dilatations, gradually diminishing in vigor until 

 they cease. The contraction begins in the part irritated, and then extends 

 to the rest. It appears, however, from Mr. Paget's experiments, 1 that it is 

 necessary for the propagation of this irritation, that the parts should be con- 

 nected by muscular tissue, of which a very narrow isthmus will suffice; and 

 that the propagation will not take place if the connecting isthmus be com- 

 posed of tendon, even though this be a portion of the auriculo-veutricular 

 ring, which has been supposed by some to be peculiarly efficacious in this 

 conduction. Like the contractility of other muscles, that of the Heart can 

 only be continuously sustained by a supply of arterial blood to its own tissue. 

 This is shown not only by the serious effects produced by disease of the 

 coronary arteries, but also by the experiments of Erichsen 2 and Schiff; 3 the 

 former of whom found that ligature of the coronary arteries rapidly pro- 

 duced cessation of the movements of the heart, especially if the cardiac veins 

 were also opened; whilst the latter obtained a local paralysis of either ven- 

 tricle, according as the artery distributed to the one or the other was tied. 4 

 The contractility of the Heart is much less speedily lost in cold-blooded ani- 

 mals than in warm-blooded ; the heart of a Frog, for instance, will continue 

 to pulsate for as many as twelve hours after its removal from the body, par- 

 ticularly if kept in an atmosphere of oxygen, though it ceases in a few min- 

 utes if immersed in Carbonic acid gas. 5 It has further been shown by Mr. 

 Tod, that the irritability of the heart is of great duration after death in very 

 young animals; which, as was long since demonstrated by Dr. Edwards, 

 agrees with the cold-blooded Vertebrata in their power of sustaining life for 

 a lengthened period without oxygen. 



235. It is difficult to account for the long continuance of the alternate 

 contractions and relaxations of the muscular parietes of the Heart, after all 

 evident stimuli have ceased to act upon it; and many theories have been 

 offered on the subject, none of which afford an adequate explanation. 6 The 



1 Brit, and For. Med. lleview, vol. xxi, p. 551. 



2 Lond. Med. Gaz., 1842, vol. ii, p. 561. 3 Arch. f. Phys. Heilk., torn. ix. 



4 v. Bezold, however, found that ligature of the coronaries produced but little 

 effect for a considerable period in rabbits. 



5 Caste!!, Muller's Archiv, 1854, p. 226. 



6 The rhythmical movements of the heart may in part be accounted for by regard- 

 ing them as an expression of the peculiar vital endowments of its muscular tissue, 

 and as the ordinary muscles of the body may, under certain conditions, be conceived 

 to contract spontaneously in consequence of being charged with motility or motor 

 force engendered by previous acts of nutrition, so the rhythmical movements of the 

 heart may be due to a simple excess of 1,his motility, continually supplied by the nu- 

 tritive operations, and as constantly discharging itself in contractile action. This 

 view is supported by cases of muscular action where no nerves are apparent, as in the 

 embryonic Heart at an early period, and the hearts of the lower animals (the Tuni- 

 cata for instance), and also by cases of local cramp and spasm which cannot be fairly 

 attributed to a perverted reflex action of the nervous system. M. Brown-Sequard has, 

 however, attributed the intermittent action of the Heart and other muscles to the 

 presence of venous blood in their capillaries, which he believes to exert a stimulant 

 action on muscular fibre. (See Experimental Researches, etc., 18-33, p. 114 ; Comptes 

 Rendus, 1857; and Journal de la Physiol., 1858, p. 95 et seq.) Dr. Radcliffe, on the 

 other hand, has adduced evidence to 'the effect that the true stimulus to the contrac- 



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