CAUSE OF THE CONTRACTIONS OF THE HEART. 53 



direct voluntary effort. The last-mentioned fact illustrates the difference be- 

 tween the heart and all other striated muscles. All of them, in order to con- 

 tract, must receive a stimulus, either natural or artificial. The natural 

 stimulus comes from the nerve-centres and is conducted by the nerves. If 

 the nerves going to any of the respiratory muscles, for example, be divided, 

 the muscle is paralyzed and will not contract without some kind of stimula- 

 tion. Connection with the central nervous system does not seem necessary 

 to the action of the heart, for it will contract, especially in the cold-blooded 

 animals, some time after its removal from the body. If the supply of blood 

 be cut off from the substance of the heart, especially in the warm-blooded 

 animals, the organ soon loses its contractility. 



Erichsen, after exposing the heart in a warm-blooded animal and keeping 

 up artificial respiration, tied the coronary arteries, thus cutting off the 

 greatest part of the supply of blood to the muscular fibres. He found, as 

 the mean of six experiments, that the heart ceased pulsating, although 

 artificial respiration was continued, in twenty-three and a half minutes. 

 After the pulsations had ceased, they could be restored by removing the liga- 

 tures and allowing the blood to circulate again in the substance of the heart. 



The regular and powerful contractions of the heart are promoted by 

 the circulation of the blood through its cavities. Although the heart, 

 removed from the body, will contract for a time without a stimulus, it can 

 be made to contract during the intervals of repose by an irritant, such as the 

 point of a needle or a feeble electric current. For a certain time after the 

 heart has ceased to contract spontaneously, contractions may be produced 

 in this way. This can easily be demonstrated in the heart of any animal, 

 warm-blooded or cold-blooded. This excitability, which is manifested, under 

 these conditions, in the same way as in ordinary muscles, is different in 

 degree in different parts of the organ. Haller and others have shown that 

 it is greater in the cavities than on the surface ; for long after stimulation 

 applied to the exterior fails to excite contraction, the organ will respond to 

 a stimulus applied to its interior. The experiments of Haller also show that 

 fluids in the cavities of the heart have an influence in exciting and keeping 

 up its contractions. This observation is important, as showing that the 

 presence of blood is necessary to the natural and regular action of the heart. 

 Schiff succeeded in restoring the pulsations in the heart of a frog, which 

 had ceased after it had been emptied, by introducing a few drops of blood 

 into the auricle. Experiments upon alligators and turtles show that 

 when the heart is removed from the body and emptied of blood, the pul- 

 sations are feeble, rapid and irregular; but when filled with blood, the 

 valves being destroyed so as to allow free passage in both directions 

 between the auricles and ventricle, the contractions become powerful 

 and regular. In these experiments, when water was introduced instead 

 of blood, the pulsations were more frequent and not so powerful as 

 when blood was used (Flint, 1861). These experiments show, also, 

 that the action of the heart may be affected by the character, particularly 

 the density, of the fluid which passes through its cavities, which may ex- 



