THE HEART. 159 



tricular systole has begun. According to some authors, however, the closure 

 of the valve is effected, at the very beginning of the ventricular systole, by 

 the contraction of the papillary muscles ; the chordae tendineae of a papillary 

 muscle are attached to the adjacent edges of two flaps, so that the shorten- 

 ing of the muscle tends to bring these edges into apposition. 



The auricular systole is, as we have said, immediately followed by that of 

 the ventricle. Whether the contraction of the ventricular walls (which as 

 we shall see is a simple though prolonged contraction and not a tetanus) 

 begins at one point and swiftly travels over the rest of the fibres, or begins 

 all over the ventricle at once, is a question not at present definitely settled ; 

 but in any case the walls exert on the contents a pressure which is soon 

 brought to bear on the whole contents and very rapidly rises to a maximum. 

 The only effect upon the valve of this increasing intra-ventricular pressure 

 is to render the valve more and more tense, and in consequence more secure, 

 the chordae tendinese (the slackening of which through the change of form of 

 the ventricle is probably obviated by a regulative contraction of the papillary 

 muscles) at the same time preventing the valve from being inverted or even 

 bulging largely into the auricle, and indeed, according to some observers, 

 keeping the valvular sheet actually convex to the ventricular cavity, by 

 which means the complete emptying of the ventricle is more fully effected. 

 [Figs. 50, 51.] The connection, to which we have just referred, of the chordae 

 of the same papillary muscle with the adjacent edges of two flaps, also assists 

 in keeping the flaps in more complete apposition. Morever the extreme 

 borders of the valves, outside the attachments of the chordae, are excessively 

 thin, so that when the valve is closed, these thin portions are pressed flat 

 together back to back ; hence, while the tougher central parts of the valves 

 bear the force of the ventricular systole, the opposed thin membranous 

 edges, pressed together by the blood, more completely secure the closure of 

 the orifice. 



At the commencement of the ventricular systole the semilunar valves of 

 the pulmonary artery are closed, and are kept closed by the high pressure of 

 the blood in the artery. As, however, the ventricle continues to press with 

 greater and greater force on its contents, making the ventricle hard and 

 tense to the touch, the pressure within the ventricle becomes greater than 

 that in the pulmonary artery, and this greater pressure forces open the semi- 

 lunar valves and allows the escape of the contents into the artery. The 

 ventricular systole may be seen and felt in the exposed heart to be of some 

 duration ; it is strong enough and long enough to empty the ventricle com- 

 pletely ; indeed, as we shall see, it probably lasts longer than the discharge 

 of blood, so that there is a brief period during which the ventricle is empty 

 but yet contracted. 



During the ventricular systole the semilunar valves are pressed outward 

 toward but not close to the arterial walls, reflux currents probably keeping 

 them in an intermediate position, so that their orifice forms an equilateral 

 triangle with curved sides ; thus they offer little obstacle to the escape of 

 blood from the cavity of the ventricle. The ventricle, as we have seen, pro- 

 pels the blood with great force and rapidity into the pulmonary artery, and 

 the whole contents are speedily ejected. Now, when a force which is driving 

 a fluid with great rapidity along a closed channel suddenly ceases to act, 

 the fluid, by its momentum, continues to move onward after the force has 

 ceased ; in consequence of this a negative pressure makes its appearance in 

 the rear of the fluid, and, sucking the fluid back again, sets up a reflux cur- 

 rent. So when the last portions of blood leave the ventricle a negative 

 pressure makes its appearance behind them, and leads to a reflux current 

 from the artery toward the ventricle. This alone would be sufficient to bring 



