Responses of Higher Animals: The Effectors - 433 



(quadriceps femoris, Fig. 24-2) of the knee, receives a single excitation. Mounted in a 



for example, act against an adverse lever ratio moist chamber to prevent drying, the muscle 



of more than 10:1. Yet an average man hangs by its origin from an immovable rod 



weighing 150 pounds can straighten his (Fig. 24-3). A cord, tied to the tendon of the 



knees, lifting not only his own body but also muscle, penetrates a hole in the floor of the 



that of another man. This means that the chamber, connecting the muscle to a lever, 



right and left extensors together can exert The marking point of this muscle lever 



force enough to lift more than 3000 pounds, makes contact with the recording paper, 



or at least 1500 pounds apiece. which covers the surface of a revolving drum. 



Contraction: The Single Twitch. When a Thus each contraction lifts the lever and 



muscle is removed from the body, it retains marks a record of the height and duration of 



its capacity to contract, and may survive for the single twitch while the drum is turning, 



several days. Special precautions to maintain Usually the muscle is stimulated by an 



the temperature must be taken in the case of electric shock, which is discharged directly 



muscles from warm-blooded animals; but into the muscle, as shown in the diagram; or 



"cold-blooded muscles" keep very well at the shock may be applied to the motor nerve, 



room temperatures. Thus frog muscles are if it is still connected with the muscle. The 



studied most frequently; and usually it is the instant of stimulation is recorded on the 



powerful gastrocnemius, or "calf muscle," drum by an electrical signal marker, which 



that is selected for experimentation. 



The apparatus shown in Figure 24-3 is de- 

 signed to make a record of a single contrac- 

 tion, or twitch, which results when a muscle 



is included in the stimulating circuit. Very 

 frequently the timing of the twitch is also 

 shown by a tuning fork, which makes a trac- 

 ing of its vibrations (usually 100 per second) 



MUSCLE 



MOIST 

 CHAMBER 



WRITING POINTS 

 ROTATING DRUM 



MOTOR MECHANISM 



Fig. 24-3. Simple mechanical method of recording the contraction of a muscle. The moist 

 chamber protects the muscle from drying; the signal magnet, which is in parallel circuit with 

 the stimulating electrode, records the instant of stimulation; and the lever, lifted by the con- 

 tracting muscle, records the extent and duration of the contraction on the paper cover of the 

 moving drum. 



