being higher in pitch the vibrations are more rapid and hence less readily felt.

Fremitus is Pathologically Decreased. (a) In obstruction of a bronchus asthma, compression, occlusion. (In these cases it may return after coughing.)

(b) From increased reflection or diffusion (pleural thickening, effusion, or pneumothorax).

(c) Over cavities, not surrounded by consolidation.

(d) In cases of dysphonia-weakness or paralysis of the vocal cords. Exceptionally fremitus may be present over pleural effusions; this is due to the factors which have been discussed on pages 60, 61.

FIG. 53. Palpation furnishes the most satisfactory method of examining the liver. The patient lies on his back with the knees slightly flexed, and is told to breathe slowly and deeply with the mouth open. If the liver is enlarged, or displaced downward, its margin can be felt to push against the right hand during the descent of the diaphragm. The examination is rendered more satisfactory if the examiner's left hand presses upward on the posterior costal margin. Splenic enlargement may be determined in a similar manner.

The most intense fremitus is encountered in robust, deep-chested men. The lower the pitch of the voice, the slower the sound vibrations and the greater their amplitude, and hence, provided the thickness of the chest wall remains constant, the more marked the fremitus. This is especially the case on the right side, for the reasons already mentioned. Tactile vibrations may also be produced by:

(c) Mechanical friction-friction fremitus, pleural or pericardial fremitus.

The costal and visceral pleuræ during respiration glide with smooth, mirror-like surfaces over each other. The greatest degree of excursion is vertical (3 cm.). They also move horizontally, their motion being downward and forward during inspiration (see Figs. 9, 10, 11). In health no palpable thrill or audible sound is produced. But the pleural surfaces roughened by disease may produce palpable vibrations as well as sounds (friction fremitus and friction sounds), both being most marked in the mid- and lower-axillary regions.

CHAPTER III

ACOUSTICS IN PHYSICAL DIAGNOSIS

For the ready comprehension of physical diagnosis a superficial knowledge of acoustics is necessary. The phenomena of percussion, auscultation and to a considerable extent palpation, cannot otherwise be grasped or properly interpreted. We have, therefore, been led to briefly review some of the more important laws of sound. Physical diagnosis is for the most part based upon a foundation of acoustics. Our interpretation of the signs elicited from both healthy and diseased tissues is but a manifestation of the fact that physical alterations of the tissues cause corresponding changes in the vibrations which they are capable of assuming or transmitting. The fact that all the phenomena met with cannot as yet be satisfactorily explained is due to the limitations of our knowledge and is not to be attributed to any mysterious manifestations. Acoustics has not developed into such a lofty, rich, well-rounded form as has her sister science, Optics.

1

Rhythmic Vibrations. When the equilibrium of an elastic body is momentarily displaced, it vibrates back and forth until its equilibrium is regained. Thus a bass viol string when on a stretch, if plucked, can be seen to vibrate. The motion imparted to a localized area is gradually transmitted throughout its whole length. The rate of the vibration is increased as the string is rendered more taut, until separate vibrations can no longer be seen. At the rate of 16 per second the lowest audible tone is produced. For the human ear the highest perceptible note corresponds to 36,000 vibrations per second. The vibrations are perceptible to touch (vocal fremitus) as well as hearing, but touch perceives the slow, and hearing the rapid, vibrations more readily. The musical range of vibrations lies between 40 and 4000 per second. The greater the tension, the more rapid the vibrations, and the higher the pitch. When an elastic tissue (e.g. violin string), vibrates rhythmically and with sufficient rapidity a tone is produced.

But a string vibrates both as a whole, and simultaneously in its aliquot parts, at its nodal points, each of which represents an octave of the fundamental note (the note of string vibrating as a whole). These partial vibrations produce additional tones which are known as overtones. Overtones bear a simple relation to the fundamental note: 2, 3, 4, 5, 7, etc. The first six overtones are harmonious, above this they are generally not so. The combination of the fundamental note plus the overtones produces what is known as klang (timbre). Klang, then (the musical quality) results from rhythmic vibrations such as occur when the fundamental note vibrates together with harmonious overtones.

Not only the rate and amplitude of the vibrations, however, determines their audibility, their duration is perhaps of even greater importance. Recent investigations (Gianfranceschi) have shown that vibrations must last one-fortieth of a second to be audible, and that duration is a much more constant factor in perceptibility than

rate.

On different, though accurately attuned instruments-flute, violin, clarionet the same fundamental note may be sounded, yet the individuality of each instrument remains distinct; we can distinguish one from the other. Now this individual difference-timbre, klang, quality-is dependent upon the character of the overtones. The individual quality of different voices is dependent upon similar factors.

Unrhythmic Vibrations.-Theoretically if a string could be struck at a minute point, by a hard hammer, in 0 time, the vibrations would tend to remain localized, and those points of the string which had not been directly struck would begin to vibrate slowly and unrhythmically. As a result the overtones and especially the higher overtones--the unharmonious ones-would disproportionately increase in strength and an unpleasant metallic note result (Helmholtz). In the case of the piano this is obviated by having the strings struck by soft broad hammers,

FIG. 54.-Diagrammatic illustrations of a vibrating string. First, as a whole; second, with one nodal point; third, with two nodal points; fourth, with three nodal points.

which remain in contact long enough to ensure a continuous vibration of the whole string, and are so placed as to dampen the unharmonious overtones by eliminating their nodal points. If in any sound-producing body the elastic equilibrium be briefly and locally disturbed, unrhythmic vibrations result. Very unrhythmic vibrations allow the distant overtones to preponderate, and a metallic quality is produced. This occurs regardless as to whether we are practicing auscultation or percussion, and regardless as to whether these higher overtones are heard together with the fundamental (amphoric breathing) or separately. All the metallic sound phenomena, of auscultation and percussion are thus produced (metallic ring, bell tympany, amphoric breathing, cracked-pot sound) (see Fig. 55).

Vibrations in Tense Membranes.-Tense membranes such as a kettle drum or the distended stomach, tend to vibrate with very diverse and variable nodal points, and hence the relationship between the fundamental note and the overtones is a very variable one. This may be illustrated by throwing stones into a pool of water. Each stone will produce its own circle (vibrations) and these circles will mingle and interweave without losing the original identity.

Sympathetic Vibrations.-Vibrations may be set up in neighboring tissues not only by the direct conduction of the sound-producing impact, but also by what is known as sympathetic vibration. It has been stated that overtones accompany the fundamental note, but it is also

true that if an overtone be produced the fundamental note which corresponds to it will begin to vibrate. Thus if a certain note on a piano be struck and suddenly damped, certain other strings can be heard to vibrate. If iodide of nitrogen be painted upon the "G" string of a bass viol and allowed to dry, a violent detonation will occur if a similarly pitched string of another instrument in the neighborhood is set in vibration; while vibrations of the "E" string are without effect. It is also a wellknown fact that the vocal or instrumental production of certain musical notes may, to the chagrin of the musician, crack glass vases on the nearby mantelpiece. How important a rôle sympathetic vibrations play in physical diagnosis cannot be stated, but that they have some bearing cannot be questioned.

FIG. 55.-Diagram illustrating the difference in the sounds produced by vibrations of different kinds.

Interference Waves. When two waves travel through a vibrating string in opposite directions, they tend to nullify each other if their nodal points be similar. This phenomenon has been used to explain some physical signs.

Loaded Strings. If a piece of wax be attached to one of two tuning forks, or to musical strings of a similar pitch, the vibrations of the structure thus treated become slower than those of its fellow and the note which it gives forth correspondingly lower in pitch. A bottle filled with soapsuds gives forth a much lower note when percussed, than a similar body when empty. Here the suds act as a "load" and retard the vibra