322 THE CIRCULATION. 



ventricle it turns upon itself and then follows a direction from below 

 upward, from right to left and from before backward, through that part 

 of the right ventricle tying in front of the heart and termed the " conus 

 arteriosus," to the commencement of the pulmonary artery. On return- 

 ing from the lungs to the left auricle, it passes from above downward 

 into the cavity of the left ventricle, when it makes a turn like that upon 

 the right side and is directed again from below upward and from left to 

 right, behind the situation of the conus arteriosus, and crossing it at an 

 acute angle, to the commencement of the aorta. The aorta itself, though 

 its point of origin is placed somewhat posteriorly to that of the pulmo- 

 nary artery, soon comes more to the front in its arched portion, while 

 the pulmonary artery runs almost directly backward. Thus the two 

 blood-currents may be said to twist spirally round each other in their 

 course through the corresponding auricles and ventricles. 



The movement of the blood through the cardiac cavities is not a con- < 

 tinuous and steady flow, but is accomplished by alternate contractions ' 

 and relaxations of the muscular walls of the heart ; by which successive 

 portions of blood are delivered from the auricles into the ventricles, and 

 thence discharged into the arteries. Each one of these successive actions 

 is called a beat or pulsation of the heart. The cardiac pulsations are 

 accompanied by certain physical phenomena dependent upon the struc- 

 ture of the heart and its mode of action. 



Sourtds, Movements, and Impulse of the Heart. The sounds of the 

 heart are two in number. They can be heard by applying the ear over 

 the cardiac region, when they are found to be quite different from each 

 other in position, tone, and duration. They are distinguished as the 

 first and second sounds of the heart. The first sound is heard with the 

 greatest intensity over the anterior surface of the heart, and particularly 

 at the situation of the apex beat, over the fifth rib and the fifth inter- 

 costal space. It is comparatively long, dull, and smothered in tone, 

 and occupies one-half the entire duration of a beat. It corresponds in 

 time with the impulse of the heart in the precordial region, and with the 

 stroke of the large arteries in the immediate vicinity of the chest. The 

 second sound follows almost immediately upon the first. It is heard 

 most distinctly at the situation of the aortic and pulmonary valves, 

 namely, over the sternum at the level of the third costal cartilage. It 

 is short and distinct, and occupies only about one-quarter of the whole 

 time of a pulsation. It is followed by an equal interval of silence ; after 

 which the first sound again recurs. The whole time of a cardiac pulsa- 

 tion may be divided into four quarters, of which the first two are occu- 

 pied by the first sound, the third by the second sound, and the fourth 

 by an interval of silence, as follows : 



KELATIVE TIME AND DURATION OF THE HEART-SOUNDS. 



r 1st quarter ) 



I 2d j First sound. 



Cardiac pulsation \ _ 



3d Second sound. 



1 4th " Interval of silence. 



