ACTION OF THE HEART. 



heart will do the same, when separated from the rest of the ventricle. The 

 stimulus of the contact of the blood with the lining membrane of the heart, to 

 which its regular actions have been commonly referred, can have no influence in 

 producing such movements ; nor does it appear that the contact of air can take 

 its place ; since, as Dr. J. Reid has shown, the rhythmical contractions of the 

 heart of a frog will continue in vacua. 1 Nor is there any evidence that the flow 

 of blood through the cavities has the effect of securing the regularity of their 

 successive contractions in the living body; for this regularity is equally marked 

 in the contractions of the excised heart, when perfectly emptied of blood, so 

 long as its movements continue vigorous. But when its irritability is nearly 

 exhausted, the usual rhythm is often a good deal disturbed, so that the contrac- 

 tions of the auricles and ventricles do not regularly alternate with each other ; 

 and one set frequently ceases before the other. The difficulty of finding any 

 other satisfactory solution of the problem has recently led many Physiologists 

 to recur to the idea that the Heart's action is dependent upon Nervous power; 

 this power being supposed to be derived, however, not from the Cerebro-spinal 

 system, but from the ganglia of the Sympathetic system which are found in the 

 organ itself. For the proper estimation of the evidence favorable to this view, 

 it is requisite that we should bring together the principal facts which indicate 

 the relation of the Heart's action to Nervous influence, from whatever source 

 this proceeds. 



496. It has been asserted by Valentin and other experimenters, that mechani- 

 cal irritation of the Pneumogastric nerves, especially at their roots, has a tend- 

 ency to excite or accelerate the heart's action; numerous experimenters, how- 

 ever, have obtained none but negative results. Admitting, what seems probable, 

 that the Cardiac branches of the Pneumogastric have some influence upon the 

 Heart's action, it remains to inquire whether that influence be essential to its 

 movements ; and whether these nerves form the channel through which they are 

 affected by emotions of the mind, or by conditions of the bodily system. In 

 regard to the first point, no doubt can be entertained; since the regular move- 

 ments of the heart are but little affected by section of the Pneumogastrics. 

 With respect to the second, there is more difficulty ; since the number of causes 

 which may influence the rapidity and pulsations of the heart is very considera- 

 ble. For example, when the blood is forced on more rapidly towards the heart, 

 as in exercise, struggling, &c., its contractions are rendered more frequent; and 

 when the current moves on more slowly, as in a state of rest, their frequency 

 becomes proportionably diminished. If the contractions of the heart were not 

 thus in some degree dependent upon the blood, and their number were not regu- 

 lated by the quantity flowing into its cavities, very serious and inevitably fatal 

 disturbances of the heart's action would soon result. That this adjustment takes 

 place otherwise than through the medium of the nervous centres, is evident from 

 the fact, that, in a dog, in which the Pneumogastric and Sympathetic had been 

 divided in the neck on each side, violent struggling, induced by alarm, raised 

 the number of pulsations from 130 to 260 per minute. 3 It is difficult to ascer- 

 tain, by experiments upon the lower animals, whether simple emotion, unat- 

 tended with struggling or other exertion, would affect the pulsation of the heart, 



1 " Cyclopaedia of Anatomy and Physiology," vol. ii. p. 611. This experiment has been 

 since repeated by Prof. Tiedemann ("Muller's Archiv.," 1847) and by Drs. Mitchell and 

 Bache (Prof. Dunglison's " Human Physiology," vol. ii. p. 150) with a different result; the 

 pulsations being speedily brought to a stand by the exhaustion of the air, and being renewed 

 when it was re-admitted. This, however, does not invalidate the positive fact that the 

 pulsation may continue in vacuo, which proves that the stimulus of air cannot be its main- 

 taining power ; and only shows that the presence of oxygen is essential to the continuance 

 of the heart's movements, as to muscular action in general ( 324). 



2 See Dr. J. Keid's " Anat. Phys. and Path. Researches," p. 170. 



