62 CIRCULATION OF THE BLOOD. 



intermittent dilatation of its cavities makes these contractions assume a certain regular- 

 ity ; but the quantity and density of the hlood are subject to very considerable variations 

 within the limits of health, which, without some regulating influence, would undoubtedly 

 cause variations in the heart's action, so considerable as to be injurious. This is shown 

 by the comparatively-inefficient and palpitating action of the heart when the pneumogas- 

 trics are divided. These nerves convey to the heart a constant influence, which we may 

 compare to the insensible tonicity imparted to voluntary muscles by the general motor 

 system. For we know that when a set of muscles on one side is paralyzed, as in facial 

 palsy, their tonicity is lost, they become flaccid, and the muscles on the other side, with- 

 out any effort of the will, distort the features. We can imitate an exaggeration of this 

 force by a feeble current of galvanism, which renders the pulsations of the heart less 

 frequent and more powerful, or exaggerate it still more by a more powerful current, 

 which arrests the action of the heart altogether. Phenomena are not wanting in the 

 human subject to verify these views. Causes which operate through the nervous sys- 

 tem frequently produce palpitation and irregular action of the heart. Cases are not 

 uncommon in which palpitation habitually occurs after a full meal. There are instances 

 on record of immediate death from arrest of the heart's action as a consequence of 

 fright, anger, grief, or other severe mental emotions. Syncope from these causes is by 

 no means uncommon. In the latter instance, when the heart resumes its functions, 

 the nervous shock carried along the pneumogastrics is only sufficient to arrest its action 

 temporarily. When death takes place, the shock is so great that the heart never 

 recovers from its effects. 



Summary of certain Causes of Arrest of the Action of the Heart. 



In warm-blooded animals, the heart's action speedily ceases after it is deprived ot 

 its natural stimulus, the blood. It is not from experiments on the inferior animals 

 alone that we derive proof of this fact. It is well known that, in profuse haemorrhage 

 in the human subject, the contractions of the heart are progressively enfeebled, and, 

 when the loss of blood has proceeded to a certain extent, are permanently arrested. 

 Cases of transfusion after haemorrhage show that when blood is introduced the heart 

 may be made to resume its pulsations. The same result takes place in death by asthe- 

 nia ; and cases are on record in which life has been prolonged, as in haemorrhage, by trans- 

 fusion of even a small quantity of healthy blood. These facts have been demonstrated 

 on the inferior animals by experiments already cited. The experiment of Haller, in 

 which the action of the right side of the heart of a cat was arrested by emptying it of 

 blood, while the left side, which was filled with blood, continued to pulsate, showed 

 that the absence of blood in its cavities is competent of itself to arrest the heart. The 

 experiments of Erichsen, who paralyzed the heart by ligating the coronary arteries, and 

 of Schiff, who produced a local paralysis by ligating the vessel going to the right 

 ventricle, show that the heart may also be arrested by cutting off the circulation of 

 blood in its substance. Both of these causes must operate in arrest of the heart's 

 action in haemorrhage. 



The mechanical causes of arrest of the heart's action are of considerable pathologi- 

 cal importance. The heart, in common with other muscles, may be paralyzed by me- 

 chanical injury. A violent blow upon the deltoid paralyzes the arm ; a severe strain 

 will paralyze the muscles of an extremity ; and, in the same way, excessive distention 

 of the cavities of the heart will arrest its pulsations. This is shown by arrest of the 

 circulation in asphyxia. We have already seen that, under these circumstances, the heart 

 is incapable of forcing the unaerated blood through the systemic capillaries. The heart 

 finally becomes enormously strained and distended and is consequently paralyzed. The 

 same result follows the application of a ligature to the aorta. This effect may be pro- 

 duced, also, in the cold-blooded animals, in which, if the heart be left undisturbed, the 



