228 CIRCULATION. 



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 main- 

 tained 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 enjoyed 

 by all muscular fibres. The irritability 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 stimulus in a remarkable degree, and is 

 even capable of restoring irritability after it has become ex- 

 tinct. 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 pulsation s, though 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 satisfactory manner by Har- 

 vey in the case of the Yiscount Montgomery. In this case 

 the heart was exposed ; and Harvey found that it could be 

 touched and handled without even the knowledge of the sub- 

 ject. This has been verified in other instances in the human 

 subject. Its physiological movements are capable of being 

 influenced in a remarkable degree through the nervous sys- 

 tem, notwithstanding this insensibility, and in spite of the 

 fact that the muscular fibres composing it are capable of 

 contraction when removed from all connection with the 

 body, and that the regular pulsations can be kept up for a 

 long time by the mere passage of blood through its cavities. 

 The influence thus exerted is so great, that some eminent au- 



