SOUNDS OF THE HEART. 131 



cause of the second of these sounds, there is still great dis- 

 crepancy of opinion respecting the real agent concerned in 

 the production of fi&Q first of them. 



The most rational theory, however, to be at present 

 adopted, with regard to the cause of the first sound, is 

 one which admits the co-operation of several coincident 

 agencies in its production. The chief of these, probably 

 by far the chief, appears to be the vibration of the 

 auriculo -ventricular valves, and also, but to a less extent, 

 of the ventricular walls, and coats of the aorta and pul- 

 monary artery, all of which parts are suddenly put into a 

 state of tension and stretching at the moment of ventricular 

 contraction. 



This view, long ago advanced by Dr. Billing, is sup- 

 ported by the fact observed by Valentin, that if a portion 

 of a horse's intestine, tied at one end, be moderately filled 

 with water, without any admixture of air, and have a 

 syringe containing water fitted to the other end, the first 

 sound of the heart is exactly imitated by forcing in more 

 water, and thus suddenly rendering the walls of the 

 intestine more tense. 



Although, however, it is most likely that the vibration 

 first referred to, is the chief element in the production of 

 the first sound of the heart, it must be allowed that there 

 are others which may assist, 'if only in a small degree, in 

 producing the effect. Of these, (i) the noise or Iruit 

 resulting from the sudden and forcible contraction of the 

 large mass of muscular fibres composing the walls of the 

 ventricles, and (2), the vibration of the blood itself in its 

 rapid compression and ejection from the heart into the 

 arteries (Leared) with the coincident throwing back of 

 the semilunar valves, are probably the chief. The share 

 which these severally take in the production of the first 

 sound, is, however, very uncertain, and their effect alto- 

 gether is probably but very slight. 



The cause of the second sound is more simple than that 



