The subject of vegetations upon the valves was very fully considered by Corvisart in 1806. Corvisart was the first to mention the importance of what is now called the purring thrill. He stated that "it probably came from a difficulty experienced by the blood in going through an orifice disproportionate to the amount of fluid." Laennec regarded murmurs or bruits as the result of spasmodic contraction of the heart or arteries. Corrigan in 1829 defined murmurs as "the result of the development of currents and the intrinsic collision of the moving liquid."
In 1842, Gendrin gave cardiac murmurs as bruits de frottement endocardiaques, and established the friction theory. He also called attention to the fact that alteration in the constituents of the blood will produce murmurs which are heard in arteries of medium calibre.
Bouillaud describes a murmur as an exaggeration of the normal bruit caused by blood-friction against the segments of the heart, and he says that according to the size or condition of the orifice the murmur will be rasping, sawing, or blowing.
Chauveau states that bruit de souffle is produced by the vibration of a nei e fluide, always formed when blood rushes through a part of the circulatory system actually or relatively dilated.
This nei e fluide has its best development in anæmia, when it is termed the bruit du diable, for the jugular veins do not collapse and the volume of blood in anæmia is diminished. Chauveau's theory is applicable to anæmic murmurs, but not to all cardiac murmurs.
Hope states that "valve murmurs are produced by collision of the blood-particles against one another, or that either the liquid alone or the liquids and solids conjointly may develop murmurs."
There are many who have advocated the tension theory—viz. that an increase in tension and force can so exaggerate a normal sound as to produce a murmur. This theory has no clinical foundation. Often, however, valve-lesions may exist, and the blood-current be so weak, the propulsive force so feeble, that no murmurs are audible.
Some observers are of the opinion that spasm of the papillary muscles and chordæ tendineæ and weakening of these structures through fatty degeneration can cause temporary murmurs.
The conditions that determine the character of a cardiac murmur, its pitch, quality, and intensity, are subject to the same physical laws as govern the formation and quality of sound elsewhere. They are the rapidity and force of the moving body, the obstructions which it meets, and the physical properties of the media of conveyance. The same vibration that produces a murmur may produce an endocardial thrill, called sometimes purring thrill.
Far more important, however, than loudness, pitch, or quality of a murmur are its rhythm, its point of maximum intensity, and the area of its diffusion, all of which can best be considered in connection with the physical signs of each valvular lesion.