76 REPORT— 1842. 



The view which I have always given of the principle of the stethoscope, represents 

 its operation as varying with the source of sound ; that sounds produced in air 

 (vocal and breath sounds) are best transmitted by an enclosed column of air ; those 

 produced by solids (those of the heart, bronchi, friction) are better communicated by 

 rigid solids of moderate density. This view I still hold, and I proceed to show how 

 the principle may be brought into the best operation. 



I shall first point out the conditions by which the stethoscope may conduct aerial 

 sounds. It has been lately questioned, that the air contained in the central canal 

 and excavation in any degree assist in conducting sound. This doubt has arisen 

 chiefly from the observation first made by Dr. Cowan, that plugging the central 

 canal with cork or putty does not much impair the power of the instrument. Pro- 

 fessor Forbes has repeated the same remark. 



I have made many experiments on this point* and will now state some of the re- 

 sults. Corking the pectoral end of the instrument decidedly impairs its conducting 

 power, but stopping the ear-end does so in a much slighter degree. But in any way 

 stopping the tube does impair the passage of sound ; and, to be assured of this, it is 

 necessary to try a test sound (as opticians use a test object), a sound just within the 

 bounds of audibility, such as the sound of expiration, or a very faint cardiac mur- 

 mur. But the impairing effect of such a stoppage is most obvious in the flexible 

 stethoscope, in which, if used near the pectoral end, a cork in great measure stops 

 the sound. That the stethoscope really conducts sound by its closed column of air, 

 as well as by its solid walls, is further proved by the following facts : — Loud pectoral 

 sounds, particularly of the heart and its murmurs, may be heard by bringing the ear 

 end close to the ear, without touching it ; the sound is then conveyed exclusively by 

 the air, and may be totally intercepted by a plug. If a hole be made in the side of a 

 stethoscope, its conducting power is greatly impaired, especially for aerial sound ; 

 and it is at once restored by closing the aperture with the finger. This depends not 

 only on the accession of extraneous sounds through the orifice, but chiefly on the 

 much lower power of conduction which an open column of air possesses. 



The closed state of the column of air is the chief condition necessary to give air a 

 high conducting power. Following the assertions of acoustic writers, that the pulses 

 of sound pass through air in straight lines like rays of light, I formerly recommended 

 that the pectoral end of the stethoscope, instead of being made with a parabolic hol- 

 low as directed by Laennec, should open by a very tapering cone, and that the whole 

 interior should be made as smooth as possible to promote the most direct reflexion 

 of the waves of sound. But this principle is more applicable to ear-trumpets, which 

 receive sounds from the open air, than to the stethoscope, which receives vibrations 

 from a solid surface. Air confined in a close tube vibrates as a whole, and its vibra- 

 tions pass over angles and through the coils of a flexible tube with a facility which 

 supersedes the idea of straight reflexion ; and although they must be more freely 

 transmitted through a straight smooth tube than through a crooked and rugged one, 

 the difference is less than might be expected without a knowledge of the properties 

 of close tubes. 



The chief object in the formation of the hollow part of the stethoscope is to bring 

 into closed contact with the walls of the chest a surface of air as large as possible, 

 and to convey the pulses of this air as directly as possible to the ear. It is at the 

 same time desirable to avoid a large hollow within the instrument, because such a 

 hollow causes a conchal or tinkling echo, from the repeated transverse reflexion of 

 the vibrations. For this reason the parabolic cavity is bad. The conical cavity is 

 much better, and for the aerial vibrations is perhaps the best ; but the trumpet or 

 bugle-end does not appear to be inferior, and answers better than the conical end for 

 transferring the vibrations of solids. 



We now proceed to consider the office of the solid walls of the stethoscope. I 

 have before noticed the inferiority of the solid stethoscope. Those who consider the 

 stethoscope to conduct only by its solid walls, ascribe this inferiority to the weight of 

 its mass, and suggest that, if its weight be reduced by hollowing, the simple solid is 

 still the best instrument. On this principle I had an instrument constructed, closed 

 at the pectoral end with a thin plate of wood, but it proved to be much inferior to 

 the open kind. 



But finding the considerable share which the solid walls have in communicating 

 the sounds, I have devised a form for the pectoral end of the instrument which qua- 



