52 CIRCULATION OF THE BLOOD ACTION OF THE HEART. 



If respiration be resumed before the heart's action has entirely ceased, 

 the organ in a few moments will resume its contractions. There is observed 

 first a change from the dusky hue it had assumed, to a vivid red, which is 

 owing to the circulation of arterial blood in its capillaries. The distention 

 then becomes gradually relieved, and for a few moments, the pulsations 

 are abnormally frequent. The arteries will then be found to contain red 

 blood. An instrument applied to an artery will show a diminution in ar- 

 terial pressure and in the force of the heart's action, if the arrest of respi- 

 ration have been carried only far enough to moderately distend the heart ; 

 or there is an increase in the pressure and force of the heart, if its action 

 have been nearly arrested. A few moments of regular insufflation will cause 

 the pulsations to resume their normal character and frequency. 



In the human subject, the effects of temporary or permanent arrest of 

 respiration on the heart are undoubtedly the same as those observed in ex- 

 periments upon the warm-blooded animals. In the same way, also, it is pos- 

 sible to restore the normal action of the organ, if respiration be not too long 

 suspended, by the regular introduction of fresh air into the lungs. Examples 

 of animation restored by artificial respiration, in drowning etc., are evidence 

 of this fact. In cases of asphyxia, those measures by which artificial respira- 

 tion is most effectually maintained have been found most efficient. 



CAUSE OF THE RHYTHMICAL CONTRACTIONS OF THE HEAKT. 



The question of the actual cause of the rhythmical contractions of the 

 heart is one of great importance and has long engaged the attention of physi- 

 ologists. While researches have resulted in much positive information with 

 regard to influences which regulate or modify this action, there seems to be 

 little known, even now, concerning the main question, why the fibres of the 

 heart, unlike the ordinary muscular fibres, seem to contract spontaneously. 



The heart in its structure resembles the voluntary muscles ; but it has a 

 constant office to perform and seems to act without any palpable excitation, 

 while the latter act only under the influence of a natural stimulus, like the 

 nervous impulse, or under artificial excitation. The movements of the heart 

 are not the only examples of what seems to be spontaneous action. The cili- 

 ated epithelium is in motion from the beginning to the end of life, and will 

 continue for a certain time, even after the cells are detached from the organ- 

 ism. This motion can not be explained, unless it be called an explanation to 

 say that it is dependent upon vital properties ; but if the actual cause of 

 the rhythmical contraction of the heart be unknown, physiologists are ac- 

 quainted with certain influences which render its action regular, powerful 

 and sufficient for the purposes of the economy. 



The action of the heart is involuntary. Its pulsations can be neither 

 arrested, retarded nor accelerated by an effort of the will, excepting, of 

 course, examples of arrest by stoppage of respiration or acceleration by 

 violent muscular exercise etc. In this respect the heart differs from cer- 

 tain muscles, like the muscles of respiration, which act automatically, but 

 the movements of which may be temporarily arrested or accelerated by a 



