A modified crepitation (dry crepitant râle of Delafond) is usually heard over an emphysematous lung. The noise in this case has been compared to that induced by handling a sheet of paper.
The subcrepitant râle is another modification holding a place intermediate between the crepitant and the mucous râles. It has been likened to the sound of a moderate effervescence in beer or other liquid. It is referable to the presence of mucus in the smaller bronchial tubes and indicates bronchitis or broncho-pneumonia.
Still other sounds are heard in diseased conditions of the pleuræ. These are friction sound, creaking, metallic tinkling, and gurgling or splashing.
A friction sound is heard in the early stages of pleurisy and is caused by the dryness of the pleural surfaces from the absence of the halitus or vapor which normally moistens them and the deposition of layers of lymph by which the surfaces are rendered rough and uneven. An approximate sound may be observed by placing the palm of the left hand on the right ear and drawing a finger of the right softly over its back. The sound is quick and jerking, one or a few jerks only being heard with each inspiration as the act is cut short on account of the pain attending the friction. It is rarely heard in expiration. It is chiefly heard at the lowest part of the chest where the lungs have the greatest freedom of movement. The thinness of the walls of the chest above the breast bone in cattle and dogs permits the friction sound to be heard more distinctly than in the horse. After the lapse of twelve, twenty-four or forty-eight hours the friction sound disappears, the surfaces of the pleuræ being separated by the liquid effusion, but it may reappear when the fluid is absorbed in the process of recovery. Sometimes the friction is further manifested by vibration of the walls of the chest perceptible to the touch.
The creaking sound, as from the bending of a piece of strong leather is caused by the movement of a thick and solid false membrane binding the lungs to the side of the chest. This is often confounded with crepitation.
Metallic tinkling is only heard when liquid and gas both exist in the pleural sac and is due to the falling of a drop from the shreds of false membrane above into the fluid contents below. The sound is somewhat like the falling of drops in a closed cask half full of water, or it may be fairly exemplified by placing the palm of the left hand flat on the right ear and striking the back of the hand smartly with the middle finger of the right. The sound is chiefly heard after the patient has changed its position and especially after rising. The explanation of this is that in the recumbent position the liquid changes its place and bathes parts which in standing are surrounded by gaseous products only. Drops accordingly fall into the liquid for some time with diminishing rapidity until they cease altogether. Other explanations of the sound but which less frequently exist are: the ascent of a bubble through the liquid and its bursting on the surface; and the sudden recoil of air from one wall of the plueral cavity to the other as the result of movement or sound generated in the deeper seated solid structures.
A gurgling or splashing sound is equally indicative of the presence of fluid and gas in the pleural sac. It is almost never heard unless after a sudden movement on the part of the patient causing considerable commotion in the contained liquid. Gurgling sounds transmitted from the abdomen are too often mistaken for this. In small animals with hydro-pneumothorax a quick shaking of the patient will develop it.
Auscultation of the Cough is sometimes valuable, though more difficult and less satisfactory in the lower animals than in man, chiefly because of the extensive movement of the ribs in the former. As conveyed through a healthy lung to the ear applied on the side of the chest, the sound is short, dull and indistinct. When the lung is more solid from hepatisation, pleural exudation or other cause, or when the bronchi are dilated the sound is loud and strong. The extent over which it may be heard thus forcibly agrees with the area of lung in a state of consolidation. When a considerable cavity or canal communicates with a bronchial tube and extends to near the surface of the lung the sound is loud and ringing. The note is specially clear and metallic when such a cavity opens into the bronchus by a narrow orifice; an apt illustration of this noise may be obtained by coughing into a narrow necked vessel.
The results obtained by auscultation should be confirmed by percussion before arriving at any definite conclusion as to the state of the chest. Consolidated lung tissue is a much better conductor of sound than the healthy, and sounds conveyed through this may be heard at a considerable distance from their point of origin. Thus the heart sounds are frequently heard over any part of the right side of the chest, and crepitation and other sounds may be heard in the centre of a hepatized portion. On all such occasions the dull sound elicited on percussion will not fail to correct the fallacy.