REPORT OF MEDICAL SECTION. 159 
the tube, or at the mouth of a gum elastic bottle, where there 
is no tube or wall beyond to cause them: usually, this is the 
situation where they can be most readily produced, because 
here the current has acquired its greatest momentum, and finds 
a free exit beyond the obstructing point. The more flaccid state 
of the portion of a tube beyond a partially obstructed point is a 
necessary effect of the scantier supply of water beyond the im- 
pediment. It is therefore a necessary concomitant of the ob- 
struction and its sound, but is not the cause of the sound. When, 
however, the sound occasioned by the obstruction is strong, its 
vibrations may be communicated to the whole contents and walls 
of the tube beyond, which will then vibrate i system with it, 
and be capable of modifying its note, just as the tube of a reed 
instrument affects the note which is exclusively generated in the 
reed. On the other hand, when the sound generated in the ob- 
struction is weak and varying, the condition of the tube or walls 
beyond it will not affect it. 
In short, the laws of the production of sound by liquids so 
nearly resemble those which regulate the same phenomenon 
in air, that illustrations for the one may be taken for the other. 
_ It may be proper to advert to an objection to this view, that 
a murmur is sometimes caused where there is no impediment 
to the course of a liquid, as when it passes suddenly from a small 
into a large tube, or into a sac. Now it is not true that in 
such a case there is no impediment, for the liquid in the large 
tube or sac, having less velocity, must in itself be an impediment. 
Besides this, the course of the smaller swift current becomes 
changed by spreading into the larger channel; and instead of 
running smoothly parallel to the tube, now strikes its walls at 
an angle, causing a series of impulses and resistances, which, if 
forcible and rapid enough, constitute the vibrations of sound. 
It may be remarked, however, that this modification of a moving 
current is not so constantly attended with the production of 
sound as the direct obstacle presented by a narrowing of or pro- 
jection into the caliber of the tube. A current issuing from a 
tube or orifice into a larger vessel or sac, is also capable of pro- 
ducing a sound by impinging against an opposite surface. 
Experiments on the production of murmurs in the living body. 
About two inches of the length of the common carotid artery 
of a young ass was laid bare. Different degrees of pressure, 
either by the stethoscope or by a probe passed under it, occa- 
sioned a variety of murmurs, blowing, sawing, filing, and musical 
cooing at each pulse. When the stethoscope was merely placed 
