ACTION OF THE HEART. 363 



II. Action of the Heart. 



480. The Heart is endowed in an eminent degree with the property of 

 irritability, by which is meant the capability of being easily excited to move- 

 ments of contraction alternating with relaxation ( 366). 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 vigour until 

 they cease. To this property, the contact of blood with the membrane lining 

 its cavity, appears to be the usual stimulus ; and, when this is withdrawn, its 

 action will cease after a certain time ; whilst its movements may be prolonged, 

 by means of artificial respiration (which assists in maintaining the circulation 

 through the lungs), for a much greater duration, even after the Brain and 

 Spinal Cord have been removed, and when animal life is, therefore, completely 

 extinct. Hence we see that the Irritability of this organ must be an endow- 

 ment properly belonging to it,' and not derived from the Nervous System. 

 Like the contractility of other muscles, it can only be sustained for any great 

 length of time by a supply of Arterial blood to its own tissue ( 392). It is 

 much less speedily lost in cold-blooded animals, however, than in warm- 

 blooded ; the heart of the Frog, for example, will go on pulsating for many 

 hours after its removal from the body ; and it is stated by Dr. Mitchell* that 

 the heart of a Sturgeon, which he had inflated with air, continued to beat until 

 the auricle had absolutely become so dry as to rustle during its movements. 

 It is commonly supposed that when it is empty of blood, the contact of air 

 with its internal cavities is the stimulus by which the irritability is excited ; 

 but Dr. J. Reid has proved that this is not a sufficient explanation, by placing 

 under an air-pump a Frog's heart in a state of activity, which still continued 

 after the receiver had been exhausted.! It is thought by Dr. Alison, that the 

 succession of movements may be in some degree accounted for, by the pecu- 

 liar arrangement of the fibres of the heart, which may cause one set, in con- 

 tracting, to press on and irritate another ; and this idea may be considered as 

 by no means unworthy of adoption, although it can scarcely account for the 

 whole of the phenomena. In all experiments made upon the cause of the 

 Heart's contraction, "it must be carefully borne in mind, that the slightest dis- 

 turbance of the organ will frequently renew its motions, after they have ceased 

 for some time ; the neglect of which fact has led to several erroneous conclu- 

 sions. It has been thought by some that the contraction of the ventricle is 

 the necessary sequence of the contraction of the auricle, a doctrine which 

 might seem to follow inevitably from the circumstance (ascertained by Dr. 

 Knox) that when the irritability is nearly exhausted, contractions excited 

 in the auricle are sometimes followed by contractions of the ventricle, when 

 irritation of the outer surface of the ventricle itself produced no effect. But it 

 is to be remembered that the irritability of the internal surface is much greater 

 than that of the external ; and that the movement of the auricle will excite 

 that of the ventricle, by forcing blood into its cavity, and thus renewing the 

 usual stimulus. That this is the true explanation, is shown by the facts observed 

 by Dr. Reid and others, that the usual relation between the movements of 

 the auricles and ventricles is often so much disturbed, when the irritability 

 is becoming exhausted, that these do not regularly alternate with each other, 

 that the- contraction of the auricle frequently ceases before that of the ven- 

 tricle, on the left side particularly, and that both sets of movements will con- 

 tinue, when the auricle and ventricle have been separated from each other. 



* American Journal of the Medical Sciences, vol. vii. p. 58. 

 f Cyclopaedia of Anatomy and Physiology, vol. ii. p. 611. 



