Io8 PHYSIOLOGICAL TRIGGERS 



where: / is frequency in cycles per second; Po is maximum departure of tension 

 plus or minus the equilibrium value, T'; and Xd is maximum departure of 

 length plus or minus equilibrium value, L'. The work done is therefore a func- 

 tion of the phase angle, the maximum value of the tension above or below the 

 equilibrium value, and the maximum length above or below the equilibrium 

 length. The frequency of the vibration depends chiefly upon the mass and the 

 elastic constant of the spring. A stififer spring and smaller mass will increase 

 frequency. 



In the foregoing discussion the behavior of a model self-excited system is 

 reviewed. The oscillating motion is seen to be maintained by an internal force 

 component that changes sign and magnitude throughout the cycle and so aids 

 the movement on both the shortening and lengthening strokes. This compo- 

 nent arises as the result of a shift in the phase relations of tension and length, 

 the length reaching its maximum and minimum values before tension. To 

 what extent does fibrillar muscle resemble this model? 



EXPERIMENTAL STUDY OF THE MECHANICAL PROPERTIES OF THE 

 BUMBLE BEE FIBRILLAR MUSCLE PREPARATION 



To study fibrillar muscle as a mechanical system, it was necessary to find a 

 muscle that was large, easily attached to the recording system, and readily 

 available. A satisfactory preparation was finally made using the longitudinal 

 flight muscle of the common large bumble bee, Bombus. The head and abdo- 

 men are first removed and the posterior phragma to which the muscle is 

 attached, exposed. The thorax is then impaled, posterior end up, on two needles 

 projecting vertically from a small mounting board. With a third needle inserted 

 at right angles to the other two, the thorax is rigidly held. A small wire hook, 

 carefully inserted into each side of the saddle-shaped phragma, is attached 

 through a light chain to an RCA mechano-transducer tube for recording 

 tension. As a last step the arms of the phragma that extend forward to the 

 articulation are cut. Stimulation is accomplished through the two vertical 

 needles. 



In the first series of experiments with this preparation, the results obtained 

 with fly muscle (8) and with the tymbal muscle of the cicada (12) were con- 

 firmed. The isometric twitch tension was small compared to the tetanus 

 tension. Fusion of the contractions was complete at about 40 stimuli per 

 second. The build-up of tension on stimulation to maximal tetanus, and its 

 decrease at the end of stimulation, were relatively slow; contraction time 0.4 

 seconds, relaxation time 0.5 second. 



The static tension-length curve was determined by allowing the muscle to 

 shorten without a load to the desired length and then to build uj) tension 

 without shortening. The results are shown in figure 2 and are typical for many 

 types of striated muscle. The active tension curve (OA) is for the muscle 



