THE CIRCULATION. 277 



The passage of the blood through the heart is accomplished by 

 alternate contraction and relaxation of its muscular walls ; by which 

 successive portions are delivered from the auricles into the ventricles, 

 and ^hence into the arteries. Each movement of this kind is called a 

 beat or pulsation of the heart. The cardiac pulsations are accompanied 

 by certain phenomena dependent on the structure of the organ and its 

 mode of action. 



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

 heart are two in number, differing from each other in time, tone, and 

 duration. They are known as the first and second sounds, and may 

 be heard on applying the ear to the chest at the cardiac region. The 

 first sound is loudest over the anterior surface of the heart, particularly 

 at the situation of the apex beat, over the fifth rib and fifth intercostal 

 Space. It is comparatively long and dull in tone, and occupies one-half 

 the duration of a beat. It corresponds in time with the impulse of the 

 heart in the preeordial region, and with the stroke of the large arteries 

 in the vicinity of the chest. The second sound follows almost immedi- 

 ately upon the first. It is most audible 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 about one- 

 quarter of the time of a pulsation. It is followed by an equal interval 

 of silence ; after which the first sound recurs. The whole time of a 

 pulsation may be divided into four quarters, of which the first two are 

 occupied by the first sound, the third by the second sound, and the 

 fourth by an interval of silence, as follows : 



TIME AXD DURATION OF THE HEART-SOUNDS. 



f lst **) First sound. 



2d " \ 

 Cardiac pulsation J ~ f 



\ od Second sound. 



|^ 4th " Interval of silence. 



The first sound of the heart is mainly produced by the tension and 

 consequent vibration of the auriculo-ventricular valves and chorda? 

 tendineae at the time of ventricular systole. It may be imitated by 

 alternately loosening and extending a tape or ribbon, with its ends 

 firmly held between the fingers of the two hands. According to Chau- 

 veau and Faivre,* when the tension of the auriculo-ventricular valves 

 is prevented, in the horse, either by dividing the chordae tendineae, or 

 by inserting into the auriculo-ventricular orifice a short tube, from 1-J- 

 to 2 centimetres in diameter, the sound is changed in character, and 

 replaced by a soft murmur ; a reflux of blood, at the same time, taking 

 place into the auricle. Valvular tension is therefore generally admitted, 

 as a cause for the first sound, and by many observers is regarded as 

 fully sufficient to account for it. There is a difference of opinion as to 



* Dictionnaire Encyclopedique des Sciences Medicales. Paris, 1876, tome xviii., 

 p. 344. 



