646 SPECIAL PHYSIOLOGY. 



base of the heart descends within the thorax ; moreover, the apex is 

 tilted upwards, and lifted a little towards the left ; whilst the entire 

 heart, owing, perhaps, to the obliquity of its muscular bands, rotates 

 slightly on its axis, and undergoes a screw-like motion to the right. 

 The base of the heart appears to move to a greater extent than the 

 apex, because the downward movement of the entire heart lessens the 

 change of position upwards, caused by the contraction of the ventri- 

 cles. Some even maintain that the apex is moved a little downwards 

 and to the left. 



These changes in the form and position of the heart, bring its ante- 

 rior surface a little above the apex, up to the walls of the chest, the 

 pericardium, however, intervening, and produce a slight concussion 

 against them. This, well known as the impulse of the heart, is felt 

 most distinctly opposite a point a little below the middle of the ven- 

 tricles, between the fifth arid sixth ribs of the left side, 2 inches from 

 the sternum. The impulse coincides with the systole of the ventricles. 



The auriculo-ventricular valves, and the semilunar valves, are open 

 and shut alternately at different moments of the heart's action ; for 

 the atfriculo- ventricular valves are open, and the semilunar valves 

 closed, during the diastole of the ventricles ; whilst the former are 

 shut, and the latter open, during the systole of those cavities. Their 

 joint action is remarkable, for they determine, like the valves of the 

 force-pump, the direction of the blood through the two sides of the 

 heart, and hence the course of the circulation generally. The segments 

 of the auriculo ventricular valves, connected by the chordae tendineae 

 with the sides, or with the musculi papillares of the corresponding ven- 

 tricles, are first opened by the torrent of blood rushing from the auri- 

 cle into the ventricle ; then they are gradually raised from the sides 

 of the ventricle, and suddenly closed, as is now generally maintained, 

 by the action of the blood itself, during the ventricular diastole ; 

 although Haller, from observations on a living animal, believed that 

 their closure was in part due to the action of the papillary muscles. 

 Immediately the ventricles begin to contract, so as to change the form 

 and size of the cavities and auriculo-ventricular orifices, and press 

 forcibly upon the contained blood, the valve-segments, as already stated, 

 are prevented from being driven back into the auricle by the chordae 

 tendineae, which are kept tense by the musculi papillares contracting 

 simultaneously with the walls of the ventricles. Were it not for the 

 existence of the papillary muscles, the chordae tendineae would be 

 relaxed, and the segments of the valves would be more or less forced 

 into the auricular cavities. The tricuspid valve is said to close less 

 perfectly than the mitral (Hunter); thus a certain quantity of blood 

 is driven back into the right auricle, and also beyond it, upon the blood 

 in the great veins, producing a slight regurgitant flow, as far as the 

 valves of the jugular veins, and causing a venous pulse, synchronous 

 with the pulse in the carotid arteries. The reflux thus permitted from 

 the right ventricle is probably important, preventing the over-disten- 

 sion of that cavity by any temporary obstruction to the circulation 

 through the lungs, from muscular effort or cold ; it has been found ex- 

 perimentally, that when the heart is forcibly distended, its pulsations are 



