CIRCULATION. 145 



a tendency to perform their movements in a uniform manner, and in 

 a successive order ; that they contract and dilate in regular alterna- 

 tion, and at equal intervals ; but, moreover, they continue these 

 movements equally without rest and without fatigue. On go the 

 motions, night and day, for eighty years together, at the rate of a 

 hundred thousand strokes every twenty-four hours, alike without 

 disorder, cessation, or weariness. The muscles of the arm tire 

 after an hour's exertion, are exhausted after a day's labor, and can 

 by no effort be made to work beyond a certain period. There is no 

 appreciable difference between the muscular substance of the heart 

 and that of the arm. It is true that the heart is placed under one 

 condition which is peculiar. Muscles contract on the application 

 of stimuli ; and different muscles arjeobedient to different stimuli, 

 the voluntary muscles to the stimiffis of volition, and the heart 

 to that of the blood. The exertion of volition is not constant, but 

 occasional ; the muscle acts only when it is excited by the applica- 

 tion of its stimulus : hence the voluntary muscle has considerable 

 intervals of rest. The blood, on the contrary, is conveyed to the 

 heart without ceasing, in a determinate manner, in a successive 

 order ; and this is the reason why through life its action is uniform : 

 it uniformly receives a due supply of its appropriate stimulus. But 

 why it is unwearied, why it never requires rest, we do not know. 

 We know the necessities of the system which render it indispensa- 

 ble that it should be capable of untiring action, for we know that 

 the first hour of its repose would be the last of life ; but of the 

 mode in which this wonderful endowment is communicated, or of 

 the relations upon wTiioh it is dependent, we are wholly ignorant. 



The force exerted by the heart is vital. It is distinguished from 

 mechanical force in being produced by the very engine that exerts 

 it. In the best-constructed machinery there is no real generation 

 of power. There is merely concentration and direction of it. In 

 the recoil of the spring, in the re-action of condensed steam, the 

 energy of the expansive impulse is never greater than the force 

 employed to compress or condense, and the moment this power is 

 expended, all capacity of motion is at end. But the heart produces 

 a force equal to the pressure of sixty pounds by the gentlest appli- 

 cation of a bland fluid. Here no force is communicated to be again 

 given out, as in every mechanical moving power ; but it is new 

 power, power really and properly generated ; and this power is the 

 result of vital action, and is never in any case the result of action 

 that is not vital. 



10 



