58 CIRCULATION OF THE BLOOD. 



cle, and its walls descended toward the middle line of the heart : but this ventricle did 

 not delay to lose its movement the first. As for the other ventricle, which could no 

 longer empty itself into the aorta, it was filled with blood and its movement continued 

 for four hours. ..." 



This experiment was confirmed by numerous others. It will be observed that one 

 side of the heart was made to cease its pulsations, while the other side continued to con- 

 tract, by simply removing the blood from its interior ; which conclusively proves that, 

 although the heart may act for a time independently, the presence of blood in its cavities 

 is a stimulus capable of prolonging its regular pulsations. Schiff has gone still farther, 

 and has 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. Our 

 own experiments upon the hearts of alligators and turtles show that, when removed 

 from the body and emptied of blood, the pulsations are feeble, rapid, and irregular ; but 

 that 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 pulsa- 

 tions became more regular, but were more frequent and not so powerful as when blood 

 was used. 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 it, which may 

 explain its rapid and feeble action in anaemia. 



It seems well established that the heart, although capable of independent action, is ex- 

 cited to contraction by the blood as it passes through its cavities. A glance at the suc- 

 cession of its movements, particularly in cold-blooded animals, in which they are so slow 

 that the phenomena can be easily observed, will show how these contractions are in- 

 duced. If we look at the organ as it is in action, we see first a distention of the auri- 

 cle, and this is immediately followed by a contraction filling the ventricle, which in its 

 turn contracts. Undoubtedly, the tension of the fibres, as well as the contact of blood 

 in its interior, acts as a stimulus ; and, as all the fibres of each cavity are put on the 

 stretch at the same instant, they contract simultaneously. The necessary, regular 

 distention of each cavity thus produces rhythmical and forcible contractions ; and the 

 mere fact that the action of the heart alternately empties and dilates its cavities in- 

 sures regular pulsations, so long as blood is supplied and no disturbing influences are in 

 operation. 



The muscular fibres of the heart seem to be endowed with an inherent property, 

 called irritability, by virtue of which they will contract for a certain time without the 

 application of a stimulus. Irritability, manifested in this way, continues so long as, by 

 the processes of nutrition, the fibres are maintained in their integrity. The muscular 

 tissue, however, may be thrown into contraction, during the intervals of repose, by the 

 application of a stimulus, a property which is observed in all muscular fibres. The irri- 

 tability manifested in this way is much more marked in the interior than on the exterior 

 of the organ. Blood in contact with the lining membrane of the heart acts as a stimu- 

 lus in a remarkable degree and is even capable of restoring irritability after it has be- 

 come extinct. The passage of blood through the heart is the natural stimulus of the 

 organ and may be said to be the cause of its regular pulsations, although it by no means 

 endows the fibres with their contractile properties. 



Influence of the Nervous System on the Heart. 



The movements of the heart, as we have seen, are not directly under the control of 

 the will ; and observations on the human subject, as well as on living animals, have 

 shown that the organ is devoid of general sensibility. The latter fact was demonstrated 

 in the most satisfactorv manner by Harvey, in the case of the Viscount Montgomery. 

 In this case, the heart was exposed, and Harvey found that it could be touched and 

 handled without even the knowledge of the subject. This has been verified in other in- 



