58 ELEMENTARY PHYSIOLOGY less. 



booming sound ; then a short sharp sound, then a pause, 

 then the long, then the sharp sound, then another 

 pause ; and so on. Tliese sounds are usually likened to 

 the pronunciation of the syllables "lubb," "dup." There 

 are many different opinions as to the cause of the tirst 

 sound ; some physiologists regard it as a muscular sound 

 caused by the contraction of the muscular fibres of the 

 ventricle, while others believe it to be due to the vil)ration 

 of the auriculo-ventricular valves, when they become 

 suddenly tense or stretched as the ventricles begin to 

 contract. In reality tlie first sound has probably a douljle 

 origin, being partly muscular and partly valvular, and 

 this view is borne out by the following facts. The sound 

 is given out during the ventricular systole and is most 

 plainly heard at the spot where the cardiac impulse is-most 

 readily felt. It is greatly altered in character and 

 obscured in cases of disease, or experimental injury of the 

 auriculo-ventricular valves ; but on the other hand it may 

 be heard, although modified, in a beating heart through 

 whose cavities the passage of blood is temporarily [ire- 

 vented. 



The second sound is without doubt caused by the 

 membranes of tlie semilunar valves becoming tense, and 

 thus thrown into vibrations, on their sudden closure at 

 the end of the ventricular systole. This is proved by the 

 facts that the sound is loudest at that point on the chest- 

 wall under which the semilunar valves lie ; that it is 

 modified and obscured l)y disease of these valves ; and that 

 it may be made to cease by experimentally hooking 

 back the semilunar valves in a living animal. 



11. Blood-pressure.— When an artery is cut, the 

 outflow of blood is nt)t uniform and smooth, but takes 

 place in jerks which correspond to each beat of the heart. 

 Moreover the blood spurts out iCUh considerable force, 

 which although it is greater at each jerk is still persistent 

 and large betwctni the jerks. The obvious conclusion to 

 be drawn from the above observation is that the blood in 



