ACTION OF THE HEART. 303 



the muscles under the control of the will. According to M. Marey, each 

 systole of the heart is accomplished by a single shock or blow, and not by a 

 series of shocks, as in the case of the muscles of animal life. The duration 

 of the shock in the case of the heart of the frog is as much as fifteen times 

 longer than that of the ordinary striated muscle. 



237. The course of the circulating fluid through the Heart, and the action 

 of its different valves, will now be briefly described. The Venous blood, 

 which is returned by the ascending and descending Vena Cava, enters the 

 rif/ht Auricle during its diastole, on the contraction of which it is propelled 

 through the tricuspid valves into the Ventricle, which thus becomes com- 

 pletely distended. The reflux of blood into the veins during the auricular 

 systole, is impeded by the contraction of their own walls, and is limited on 

 the right side by the valves placed at the junction of the jugular and sub- 

 clavian veins ; but these valves are so formed as not to close accurately, 

 especially when the tubes are distended ; so that a small amount of reflux 

 usually hikes place, and this is much increased when there is any obstruc- 

 tion to the pulmonary circulation. Whilst the right Ventricle is contracting 

 upon the blood that has entered it, the carnece, columnce, which contract 

 simultaneously with its proper walls, put the cJwrdce tendinece upon the 

 stretch, preventing the flaps of the Tricuspid valve from being driven back 

 into the auricular cavity. The blood is now expelled by the ventricular 

 systole into the Pulmonary artery, which it distends, passing freely through 

 its Semilunar valves ; but as soon as the vis a tergo ceases, and reflux might 

 take place by the elastic recoil of the arterial walls, the valves are filled out 

 by the backward tendency of the blood, and completely check the return of 

 any portion of it into the ventricle. The blood, after having circulated 

 through the lungs, returns as Arterial blood, by the Pulmonary Veins, to 

 the left Auricle ; whence it passes through the Mitral valve into the left Ven- 

 tricle, and thence into the Aorta through its Semilunar valves. In the 

 same manner with that on the other side, as just described. 1 There is a well- 

 known anatomical difference between the auriculo-ventricular valves on the 

 two sides, which has given rise to the diversity of name ; and this seems, 

 from the researches of Mr. King, 2 to be connected with an important func- 

 tional difference. The Mitral valve closes much more perfectly than the 

 Tricuspid; and the latter is so constructed as to allow of considerable reflux, 

 when the cavities are greatly distended. According to Dr. Flint, 3 an equally 

 well-marked difference in sufficiency also exists between the arterial semi- 

 lunar valves on the two sides, the closure of the pulmonary being much less 

 perfect than the aortic. Many occasional causes tend to produce an accu- 

 mulation of blood in the venous system, and in the right side of the Heart ; 

 thus, any obstruction to the pulmonary circulation, cold, compression of the 

 venous system by muscular action, etc., are known to favor such a condition. 

 This is a state of peculiar danger, from a liability which over-distension of 

 the Ventricular cavity has to produce a state of muscular paralysis ; and in 

 the structure of the Heart itself there seems, as just stated, to be a provision 

 against it. For, when the ventricle is thus distended, the Tricuspid valves 

 do not close properly ; and a reflux of blood is permitted, not only into the 

 Auricle, but also (through the imperfect closure of their valves under the 

 same circumstances) into the large veins. This is proved by the fact, sev- 

 eral times observed by Dr. J. Reid in his experiments upon Asphyxia, etc., 

 that when the action of the right ventricle had ceased from over-distension, 



1 For a full account of the mechiinism of the Semilunar valves of the Heart, see 

 Ceradini, abstract in Centralblatt, 1872, pp. 123 and 135. 



2 Guy's Hospital Reports, vol. ii. 3 Physiology, 1866, p. 203. 



