650 



CIRCULATION. 



chest, its parietes become flaccid, and it as- 

 sumes a flattened form. The pulse in the 

 arteries, which is in truth nothing more than 

 the communication of the impulse of the heart 

 along the blood in these vessels, corresponds, 

 at least in the larger arteries near the heart, very 

 exactly in time with the ventricular systole and 

 the beat on the walls of the chest. The action 

 of the heart is accompanied by two sounds, 

 that can be heard on applying the ear to the 

 cardiac region. The first of these sounds is 

 synchronous with the systole of the ventricles, 

 the second with their diastole ; the second 

 follows the first immediately, and is succeeded 

 by an interval of silence. Of the space of 

 time in which a full action of the heart is 

 completed, the systole of the ventricle occu- 

 pies nearly a third, the systole of the auricle 

 less than a quarter ; the dilatation of the ven- 

 tricle and repose taken together must be 

 effected in the remainder. 



The heart, from its structure and action, may 

 justly be considered as a living or self-moving 

 double forcing-pump, which is continually 

 filled at one part and emptied at another. 

 During one-third of the time of a complete 

 action of the heart, the blood in the arteries is 

 impelled onwards by the direct impulse of the 

 ventricles at their systole. During the other 

 two-thirds of the time, while the ventricle is 

 inactive, the communication between its cavity 

 and the great arteries is stopped by the closure 

 of the semilunar valves, and the blood must, 

 therefore, at this time be propelled by the 

 elastic and other forces of the arteries them- 

 selves. But the heart continues to receive 

 blood from the veins during a longer time than 

 it gives out any of that fluid, for the auricles 

 offer a resistance to the entrance of blood du- 

 ring only a space of less than a quarter of the 

 time employed in a complete action of the 

 heart, and the blood is continually impelled 

 into the auricles as well as the ventricles du- 

 ring the whole time that these cavities are not 

 contracted, although more blood enters the 

 auricles immediately after their relaxation, and 

 more is propelled into the ventricles just be- 

 fore their contraction than in the rest of the 

 time. 



During the systole of the ventricles, while the 

 stream of blood issues from their cavities into 

 the first adjoining parts of the large arteries, 

 the folds of the semilunar valves are laid close 

 to the inner side of these vessels. As soon as 

 the contractile force of the ventricles ceases, 

 the free edges of the semilunar valves are 

 brought towards the middle of the vessel, and 

 applied firmly against one another so as to 

 close the ventriculo-arterial orifices : this is 

 effected by the pressure of the column of 

 blood acted upon by the elastic coats of the 

 arteries, assisted perhaps by the elasticity of 

 the borders of the valves themselves and by 

 the change of position consequent on dilatation 

 of the ventricles. 



During the systole of the ventricles, the 

 auriculo-ventricular or tricuspid and mitral 

 valves are closed, so as to prevent in a great 

 measure regurgitation of the blood from the 



ventricles into the auricles. When the ven- 

 tricles are in the relaxed state, the valves 

 are opened by the stream of blood flowing 

 from the auricles. The circumstance that 

 the free margins of the mitral and tricuspid 

 valves are bound down to the inner walls 

 of the ventricles by the tendinous cords at- 

 tached to the fleshy pillars, and that, by the 

 contraction of these pillars, the free margins 

 of the valves mnst be pulled further down 

 into the ventricle than in the relaxed state, 

 has occasioned to some a difficulty in under- 

 standing their action, and led them to suppose 

 that the columnae carneae must necessarily be 

 relaxed at the time of the ventricular systole, 

 and that by contracting while the ventricle is 

 in its diastole, the fleshy pillars contribute to 

 open the valves. The direct observation of 

 the contraction of the columns carneae in the 

 heart of an animal taken from the body, and 

 an attentive observation of the structure of 

 these valves, from which it appears that the 

 tendinous cords passing to opposite flaps of 

 the valves frequently come from the same 

 columnae carneae or point of attachment in the 

 ventricular paries, sufficiently prove that these 

 fleshy pillars actually contract at the same mo- 

 ment as the rest of the parietes of the ven- 

 tricles, and that their contraction, besides 

 drawing the free margins of the valves down- 

 wards into the ventricles, must also tend to 

 make them approach one another more nearly ; 

 and we are therefore entitled to form the con- 

 clusion, that, while the tendons serve to fix 

 the valves, the action of the columnae carneae is 

 to draw these down so as to allow the blood 

 to pass behind them, and to press them to- 

 gether and close them in the same manner as 

 the semilunar valves of the aorta and pulmo- 

 nary artery are shut. 



The apparently greater facility of the en- 

 trance of blood into the heart at one time than 

 at another, has given rise to the opinion enter- 

 tained by some physiologists that the dilatation 

 of the heart is, like the contraction, accom- 

 panied with the production of a new force, 

 which draws the blood from the veins towards 

 the heart. Some who regard muscular elon- 

 gation as a source of new power have gone so 

 far as to suppose that this force is even greater 

 than that accompanying contraction, but it is 

 manifest that such a view is opposed by every 

 thing we know of muscular action, which leads 

 to the belief that the shortening of muscular 

 fibre ought alone to be considered as an active, 

 and the subsequent elongation as entirely a 

 passive change. Others suppose the ventricles 

 of the heart to dilate in consequence of elas- 

 ticity, in the same manner as a bag of caout- 

 chouc does after being compressed with some 

 degree of force. Attempts have even been 

 made to measure the extent of the force pro- 

 duced during the dilatation of the ventricles, 

 by endeavouring to ascertain the weight which 

 is displaced by this motion of the heart. We 

 would not wish to be understood to deny the 

 possibility of the heart's exerting some slight 

 force in this way during its dilatation, but it 

 appears very clear that a measurement of the 



