fingers slightly bent; or with a light elastic hammer called the percussor (Fig. 4). The finger of the other hand or a solid, flattened disk, the pleximeter, must be held firmly against the chest to receive the stroke and to educe the proper resonance. The percussion-sound, though apparently unmusical, must have its intensity, be high or low, and have its peculiar timbre—all requiring acuteness of hearing and judgment in interpretation.
The spirometer is of use for measuring in cubic inches the maximum
Fig. 4.—Flint's Percussor and Pleximeter.
amount of respirable air, of which each individual has his normal quantity, but which is subject to changes from organic affections of the lungs.
The cyrtometer is used for delineating the external contour of the chest and for exact comparison of one side with the other.
Numerous other instruments are of real utility, only one of which, however, will be mentioned.
The sphygmograph is an instrument of somewhat complicated mechanism. It is used to "feel the pulse" and to record its impressions. It will give its frequency and rhythm, its varying tension and strength, the condition of the heart and certain valves, with a delicacy and exactitude which, compared with the results obtainable by the most sensitive finger, are like the perfect work of photography compared with the attempts of the juvenile charcoal artist. With its touch upon the heart or its vessels, and its pen apparently in sympathy and vital connection with them it will record in delicate but infallible tracery the diagnosis, and mayhap the prognosis, of the subject under examination, which may be read with trembling expectation.
In this instrument the impulse of the blood-movements is communicated to the pen by water contained in flexible tubes. The oblong receptacle, also containing water, is connected with one of these tubes. It has on one side an elastic projection which is to be securely fixed upon the pulse to be examined. All vibrations received by it are transmitted by the water through the tube to the chamber. On the upper surface of the chamber is a delicate membrane which receives the vibrations with every requisite as to quality and exactitude. The movement of its wave is, however, microscopic, and, in order to render it visible and legible, an exceedingly light and sensitive lever termi-