330 CIRCULATION OF BLOOD. LECT. XVII. 



The muscular contraction of the heart, and the contrac- 

 tion of the arterial coats, are then the principal motive 

 powers of the sanguineous circulation. The combination 

 of these two forces in the mechanism of this function is so 

 perfect, that the movement of an intermitting jet, produced 

 by the alternate contractions of the heart, is transformed 

 into a continued movement by the elastic force with which 

 the arterial coats are endowed. By this force the arteries, 

 which were at first dilated, recover themselves, and conse- 

 quently propel the blood forward, at the moment when the 

 contraction, which has thrown the blood into the arteries 

 and caused them to dilate, ceases. 



Let us imagine, then, a circu larsystem of tubes of diffe- 

 rent diameters, and having elastic sides ; and that the two 

 extremities or apertures of this system open into two cavi- 

 ties separated from each other, and of which the walls can 

 approximate and separate like those of a pair of bellows. 

 When we fill this tube with a liquid, and rapidly close one 

 of these cavities by depressing its moveable wall, the liquid 

 which it contains is urged into the tube, and thus pushes 

 onward the liquid column. This advances by a movement 

 which soon becomes uniform, and is rapidly communicated 

 to the entire mass. In the mean time, the other part of the 

 bellows opens ; the liquid which filled the tube in the op- 

 posite extreme, advances also, and is easily projected into 

 the dilated cavity. If the walls of the tube were not elastic, 

 the movements would be intermittent, and would cease as 

 soon as the bellows could no longer open; but they become 

 continuous, because the walls are endowed with that pro- 

 perty which begins to be exercised precisely at the moment 

 when the bellows shuts, and the action lasts during the 

 whole time that they remain at rest. The function which 

 I have described, by supposing it to take place with a pair 

 of bellows, is that which is performed by the heart. The 



