OF THE SOUNDS OF RESPIRATION.—The' breaking up of the air into minute streams was discovered by Laennec to produce certain sounds, named " breathing sounds:" which sounds are now made available in detecting organic disease in the lungs. As the air penetrates the lungs, it is divided and sub divided until it enters the minute air vesi cles. The air passes, 1st, through the trachea, producing "tracheal sounds," — a hollow rough blowing ; 2nd, through the next divi sion of vessels (bronchial), producing " bron chial sounds," less hollow and termed "whiff ing or tubular ;" and, 3rd, into the air vesicles, producing " vesicular sound," —a soft, silky murmur like a gentle breeze among the leaves of trees. Dr. Jackson discovered that which Laennec overlooked : — this murmur is not heard in expiration, while the other two sounds are. Hence the expiratory murmur is a morbid sign, and if heard on the left side below the acromial end of the clavicle, is a sure sign of some altered condition of the air tubes, not compatible with a healthy lung. This expiratory mur mur may sometimes be heard faintly on the right side, and not be a morbid sound ; but if strongly heard there, it is a morbid sign. A question now arises : Why is there a murmur ing sound with inspiration and not with expira tion ? First, let us inquire what is the dif ference between the inspiratory and the expiratory act ? They differ in two ways : • 1st. In Inspiration the lungs are passive ; the chest threatens a vacuum, and the air enters a rarefied space. In Expiration the lungs are. active ; there is no rarefied space ; the air is squeezed out into the atmosphere. This does not affect our question.
2dly. In Inspiration a volume of air is broken up into smaller and smaller streams. In Expiration these small streams are collected up into the original volume by larger and larger streams. This answers the question.
The hollow blowing sound in the trachea is caused by the friction of the air against the sides of the tube. The relation of the friction to the stream is the same whether the air passes into or out of the lungs, therefore the tra cheal sounds are equally heard in expiration and in inspiration. But not so in the lungs ; here, as the stream of air proceeds it is subdivided, and with every subdivision the friction is increased ; so that with every advance of the stream into the substance of the lung the sound is increased, and becom ing more and more buried in the substance of the lung is heard as in a continual murmur.
In expiration the very contrary happens. The friction is as quickly diminished, until the substance of the lung entirely masks what remains. In the larger vessels when the volume of the returning air becomes great, and the diameter of the tube more uniform, the friction is the same whichever way the air passes, and here tracheal and bronchial ex piration are audible, during inspiration as well as expiration. If we take a sheep's lung and inflate it, we hear the inspiratory murmur; let go the air and we do not hear it ; but contract part of the lung, say with the edge of a paper knife, and you hear the mur mur during the lung's collapsing, showing that by increasing the friction you produce the expiratory murmur.
When any disease thickens or diminishes the diameter of the air tubes, or when one part of the lung is obliterated and another part has to do double work, then the friction is increased, and thus expiratory murmur is a true sign of some change in the minute air tubes of the lungs. Sometimes the breathing murmur is so gentle and the thickness of the muscles so great that we have even in health known the inspiratory murmur quite inaudible.
In organic change of the lung these sounds become changed in their intensity, rhythm, and character: The cause of the change of sound is yet involved in much obscurity ; hence some persons have been said to have had tubercular lungs, when such has not been the case; or even extensive cavities, a., yet time has shown that there never had been cavities. All the morbid true sounds yet require to be verified as to their cause. As we have natural changes in the character of breathing, so are there changes in the sounds of breathing, as follows : — When there is fluid or disorganisation in the substance of the lung, there are certain crack lings, crepitations, and gurglings, causing certain other sounds not included in the above list. Unfortunately authors differ in the ap plication of the names for these sounds. They may be all classed under two heads, the dry and the moist, whether the tubes be large or small in which the sounds are pro duced.
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