I have selected, for the purpose of recording, the contraction of the pectoralis major.
This muscle was chosen for several reasons :
- as it passes the front of the arm-pit, it is superficial enough, and its contraction sufficiently marked to transmit readily the movement to an appropriately arranged tambour;
- the second place, because the pectoralis major is directly involved in the lateral shifts of the arm (octave leaps and the like), that form an important part of pianistic movements, as well as in all forced arm-drops, a type of movement described in detail later.
The muscle is situated below the collar-bone, spreading out from the forward side of the arm-pit in a fan-like manner over the upper side of the chest. It condenses considerably as it passes beneath and around the lower front of the arm-pit to its insertion in the humerus, and the turn taken at the arm-pit makes its contraction very noticeable.
The method used for recording the muscular contractions was the following:
To one end of a rubber tube a tambour was attached.
This consisted of a shallow disk covered with very thin sheet rubber, to the center of which a writing lever was fastened.
To the other end of the tube a specially constructed plunger was fastened.
This consisted of a small cylinder carrying a piston head and rod working with a minimum of friction.
The return of the piston to any original position was insured by the attachment of a sensitive spring.
The whole apparatus thus made practically an air-tight chamber and any outside pressure forcing the piston into the cylinder would send a wave of condensation to the tambour, pressing out the sheet rubber and lifting the writing end of the lever.
Such an instrument is sufficiently sensitive to record the contractions here measured.
The writing lever rested upon a revolving drum which recorded any deflections of the point.
When the piston is held firmly, and at the proper place and angle, against the muscle or tendon whose contractions are to be recorded, it will transmit these to the tambour, as it is pressed in when the muscle contracts.
Its position against the muscle may be better fixed by a small biped resting upon two points of the chest or side.
Then any bodily movements (other than the muscular contraction to be recorded) will not cause deflection of the recording needle.