THE SIMPLE MUSCLE CONTRACTION 



525 



acting as a specific poison for the motor end-plates. If the motor end-plates 

 are destroyed, then forms of stimuli which produce contractions of the muscle 

 must act directly on muscle substance, proving that muscle substance, as such, 

 is irritable. 



5. The Simple Muscle Contraction. Striated muscle responds to 

 electrical stimuli even of almost instantaneous duration. The response 

 which the muscle gives to a single stimulus is called a simple muscle contrac- 

 tion, and is demonstrated as follows: 



Make a muscle-nerve preparation with the tendon isolated and the skin 

 removed, and mount it in a moist chamber, figure 350. Connect the tendon 



FIG. 350. Moist Chamber. 



with a recording lever by short copper-wire hooks. Lay the nerve across a 

 pair of platinum electrodes, shake a little water on the sides of the cover of 

 the moist chamber, and place it over the preparation so as to prevent drying 

 of the nerve and of the muscle. Arrange an induction coil with its keys, 

 battery, and electrodes connected as shown in the diagram, figure 351. Set 

 the secondary coil at a position which will give a strong contraction of the 

 muscle, and record this contraction on the smoked paper of an ordinary re- 

 cording cylinder. Whenever the induction shock is sent through the nerve 

 there will be a single contraction of the muscle. If this contraction is re- 

 corded on the drum standing still, then the record will be a vertical line, the 

 height of which can be measured. From it and the arms of the lever the 

 exact shortening of the muscle can be computed. Repeat the stimulus with 

 weaker and weaker currents, until no contraction is produced. As the 

 stimulus becomes weaker a point is reached at which the contractions rapidly 

 decrease in height and cease altogether. If, on the other hand, the stimulus 

 is stronger the contractions only slightly increase. 



Arrange the apparatus so as to stimulate the muscle by an automatic key 



