General Principles of Physiolog}/ . 99 



influence of the will, and it is by exciting or suspending the 

 action of that power on the irritability of such or such muscles, 

 that the will causes any particular part to act, or be at rest. 

 It is not thus Tvith the muscles of involuntary motion. These 

 are affected by stimuli of different kinds which are appro- 

 priated to their different functions, and altogether different 

 from the nervous power. It is the blood which is the natural 

 stimulus of the irritability of the heart, alimentary substances 

 of that of the intestinal canal, <^c 



We easily deduce from these principles the explanation of 

 the leading circumstances which we observe in the motions of 

 the heart. Thus, its motions are involuntary, because they are 

 independent of the nervous system. They take place without 

 interruption during life, because the irritability which produces 

 them belongs essentially to the fibres of the heart, and the 

 blood which excites them is constantly supplied to this organ 

 by the veins as it is carried off by the arteries. The systole 

 and diastole succeed each other alternately and regularly, be- 

 cause the stimulus of the blood always occasions the former 

 both in the auricles and ventricles, and the systole itself, by 

 expelling the stimulus, occasions the diastole, which renews 

 the systole by allowing access to new blood. 



Such is a summary view of the celebrated Hallerian theory 

 of irritability. That theory was not contrived in the closet, 

 like the others of which we have spoken ; it was founded, as 

 we have said, on experiments made by Haller and the most 

 distinguished of his scholars. The inferences from these expe- 

 riments, which were repeated throughout Europe, found almost 

 every where supporters, but they found also some opponents of 

 the greatest reputation. The principal cause of this difference 

 of opinion, and that respecting which authors have not yet been 

 able to come to any agreement, is the question whether the mo- 

 tions of the heart are really independent of the nervous system. 



We may reduce to three heads the facts by which the school 

 of Haller has supported the affirmative. 1. If we interrupt all 

 communication between the heart and the brain, the only sup- 

 posed source of nervous power, by dividing the nerves which. 

 H 2 



