

MOVING POWERS OF THE CIRCULATION. 321 



action consists in this, that the whole mass of fibres of each division 

 of the organ contract and relax together. The contraction of the two 

 ventricles is perfectly synchronous, as is that of the two auricles ; but 

 the contraction of the auricles is synchronous with the dilatation of the 

 ventricles, and vice versa. The regularity of this alternation, however, 

 is somewhat disturbed, when the irritability of the heart is becoming 

 exhausted ; and both sets of movements will continue, when the auricle 

 and ventricle have been separated from one another. Their regular suc- 

 cession, in the natural state, is doubtless in part due to the fact, that 

 the transmission of blood from the auricle into the ventricle, by the 

 contraction of the former, is the stimulus which most effectually excites 

 the latter to contraction ; whilst the ventricle is contracting, the auricle, 

 now free to dilate, is distended by the flow of blood from the veins that 

 open into it ; and this flow stimulates it to renewed contraction, just at the 

 time when the contraction of the ventricle has been completed, and its 

 state of relaxation enables it to receive the blood poured in through the 

 orifice leading from the auricles. 



569. In the living animal, the auricular and ventricular movements 

 succeed one another with great regularity ; and, when the circulation 



proceeding with vigour, scarcely any appreciable pause can be dis- 

 ;overed between the different acts. The contraction or systole of the 

 jiricles takes place precisely at the same moment with the dilatation or 

 liastole of the Ventricles ; and, as soon as the latter are full, and the 

 former are empty, the diastole of the Auricles and the systole of the 

 ~ T entricles, immediately succeed. The systole of the Ventricles occa- 

 dons the propulsion of blood into the arterial system; and this action 

 produces the pulse, as will be explained hereafter. And it also corre- 

 sponds with the impulse, or stroke of the heart against the parietes of 

 the chest. This impulse is not produced, as some have supposed, by 

 the swinging of the entire heart forwards ; but by the peculiar mode in 

 which the Ventricular systole takes place. In the contraction of its 

 walls, every dimension is lessened ; but shortening is the most percep- 

 tible change, the vertical diameter of the Ventricle being the greatest. 

 Owing to the peculiar spiral disposition of the fibres of the heart, its 

 apex is not simply drawn upwards by their contraction, but it is made 

 to describe a spiral movement, from right to left, and from behind for- 

 wards ; and it is in this manner, that it is caused to strike against the 

 side of the chest. 



570. The systole of the Ventricles is immediately followed by their 

 diastole ; but the commencement of this has been observed to occur at 

 a small interval previous to the contraction of the Auricles ; and some- 

 times a brief interval of repose may be noticed, separating the first stage 

 of the Ventricular diastole, which may be partly due to the simple elas- 

 ticity of the walls of the Ventricles, from the second, which is accompa- 

 nied by the systole of the Auricles, and in which the blood of the latter 

 is forcibly propelled into them. When the circulation is being carried 

 on regularly, the blood is propelled into the Ventricles with sufficient 

 force to dilate them strongly ; so that the hand closed upon the heart 

 is opened with violence. Even the auricles dilate with more force than 

 it seems easy to account for by the vis a tergo of the blood in the venous 



