168 



CIRCULATION OF THE BLOOD 



The cause of the first heart sound is to be sought chiefly in the so-called 

 muscle tone ; that is to say, in the tone or noise which is to be heard whenever 

 a muscle contracts (cf. Chapter XV). The first sound is clearly audible in 

 the case of a heart which is almost entirely empty of blood and air, and in 

 which accordingly the valves cannot be stretched and cannot therefore be set 

 in vibration (Ludwig and Dogiel). 



Other factors cooperate in the production of the first sound, notably the 

 closure of the atrio-ventricular valves at the beginning of systole, and the vibra- 

 tions set up in them and in the blood by their 

 closure. It is not impossible that vibrations 

 which are caused by the opening of the semilunar 

 valves play a certain part also in the production 

 of this sound. However the most prominent fac- 

 tor of all is a muscle sound with which these 

 other sounds are associated. 



The second heart sound is produced by the 

 sudden tension of the semilunar valves, and by 

 other simultaneous vibrations in the blood con- 

 sequent upon their closure. Sudden stretching 

 of these valves in an excised aorta produces a 

 sound which agrees exactly in character with 

 the second heart sound (Carswell and Rouanet). 



It will be apparent at once that we have in 

 reality four sounds, two first and two second. 

 Practical experience teaches however that the 

 first two occur simultaneously and the second two 

 simultaneously: and this is also evidence of the 

 fact that both ventricles begin their contractions 

 simultaneously and that the semilunar valves of 

 . I, the two sides are closed at the same instant. 



4. PRESSURE CHANGES IN THE 

 HEART DURING ITS ACTIVITY 



FIG. 55. Heart sound, after Chau- 

 veau and Marey. 



A. TECHNIQUE 



In order to measure the pressure and its vari- 

 ations in different chambers of the heart, it is 

 necessary that these should be connected with a 

 manometer. This can be done in the closed 

 thorax by passing a cannula or sound from the 

 carotid through the aorta into the left ventricle, 



in which one must so far as possible avoid injury to the semilunar valves (Chau- 

 veau and Marey). Sounds can be passed likewise by the jugular vein into the 

 right auricle and right ventricle. In the opened thorax sounds can be thrust 

 either directly through the walls into the different heart cavities, or they can 

 be passed into the ventricles first through the walls of the auricles, then through 

 the atrio-ventricular openings. 



Various instruments have been constructed for the study of the pressure 



