OF THE CIRCULATION OF THE BLOOD. 



2. Action of the Heart. 



494. The Heart is endowed in an eminent degree with the property of irrita- 

 bility; by which is meant, the capability of being easily excited to movements 

 of contraction alternating with relaxation ( 315). 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 contrac- 

 tions and dilatations, gradually diminishing in vigor until they cease. The con- 

 traction begins in the part irritated, and then extends to the rest. It appears 

 from Mr. Paget's experiments, 1 that it is necessary for the propagation of this 

 irritation, that the parts should be connected by muscular tissue, of which a very 

 narrow isthmus will suffice; and that the propagation will not take place, if the 

 connecting isthmus be composed of tendon, even though this be a portion of the 

 auriculo-ventricular ring, which has been supposed by some to be peculiarly effi- 

 cacious in this conduction. -That the irritability of the Heart is not dependent 

 upon the Cerebro-spinal system, appears not merely from the manifestation of it 

 when the organ is altogether removed from the body; but also from the fact, 

 that if the flow of blood through the lungs be kept up by artificial respiration, 

 the heart's action will continue for a lengthened period, even after the Brain 

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

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

 endowment properly belonging to itself, and not derived from that portion of the 

 Nervous System. 3 Like the contractility of other muscles, it can only be sus- 

 tained for any great length of time, by a supply of Arterial blood to its own 

 tissue ( 323, 324). It is much less speedily lost in cold-blooded animals, how- 

 ever, 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 

 l)r. Mitchell 3 that the heart of a Sturgeon, which he had inflated with air, con- 

 tinued to beat, until the auricle had absolutely become so dry, as to rustle dur- 

 ing its movements. It has further been shown by Mr. Tod, that the irritability 

 of the heart is of long duration after death in very young animals : which, as 

 long since demonstrated by Dr. Edwards, agree with the cold-blooded Vertebrata 

 in their power of sustaining life, for a lengthened period, without oxygen. 



495. It is difficult to account for the long continuance of the alternate con- 

 tractions 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. The extraordinary 

 tendency to rhythmical action, which distinguishes the heart from nearly all 

 other muscles ( 318, note), is shown by the fact, that not only do the entire 

 hearts of cold-blooded animals continue to act, long after their removal from 

 the body, but even separated portions of them will contract and relax with great 

 regularity for a long time. Thus the auricles will persist in their rhythmical 

 action, when cut off above the auriculo-ventricular rings; and the apex of the 



1 "Brit, and For. Med. Review," vol. xxi. p. 551. 



2 It was formerly supposed, that the movements of the Heart were dependent upon its 

 connection with the centres of the Cerebro-Spinal nervous system ; and the experiments 

 of Legallois and others, who found that they were arrested by crushing, or otherwise sud- 

 denly destroying, large portions of these centres, appeared to favor the supposition. But 

 it has been shown by Dr. Wilson Philip and his successors in the same inquiry, that the 

 whole Cerebro-Spinal axis might be gradually removed, without any such consequence ; 

 which fact harmonizes perfectly with the "experiments prepared for us by Nature," in 

 the production of monsters destitute of these centres, which nevertheless possessed a 

 regularly-pulsating heart. 



3 "American Journal of the Medical Sciences," vol. vii. p. 58; see also Prof. Dungli- 

 son's "Human Physiology," 7th edit., vol. ii. p. 149. 



