ELEMENTARY EXPERIMENTAL PHYSIOLOGY 21 



it becomes maximal and of about the same height as the break 

 contraction. 



The higher the contractions become the more obvious is it that the 

 writing point describes on the stationary drum, not a straight line, but 

 an arc of a circle. The shortening of the muscle, after allowing for 

 the magnification by the lever, is measured not by the length of this 

 arc but by a perpendicular line dropped from its highest point on to 

 the base line. 



It is necessary to point out here that, when the primary circuit is 

 made and the same is true if it be broken a momentary induced 

 current is both made and broken through the nerve, and yet there 

 is only one contraction of the muscle. It has been found that in 

 a current of such short duration the break stimulus is ineffective 

 because it falls within the refractory period of the make stimulus (see 

 Chap. VII., p. 40). In both cases, whether the primary circuit is made 

 or broken, the effective stimulus to the nerve is only the make stimulus 

 of the induced current. 



Unipolar Excitation. Connect a battery to a coil so as to give 

 tetanising shocks ; connect a wire to one pole of the secondary coil and 

 place its free end on the tongue. If the secondary coil be moved 

 completely over the primary, faint shocks will be felt. The explana- 

 tion of this phenomenon is that the making and breaking of the 

 primary circuit causes free electricity to collect at the end of the wire 

 connected with the secondary coil; when the E.M.F. of this charge is 

 sufficient to overcome the resistance of the tissues of the body, the 

 circuit is completed through the body, the floor and desk, and so back 

 to the other pole of the secondary coil. With the wire still on the 

 tongue, touch the other pole of the secondary coil with a moistened 

 finger; much more powerful shocks are felt because a more direct 

 circuit from one pole to the other of the secondary coil has been 

 provided. 



Repeat the experiment on a sciatic-gastrocnemius preparation in the 

 following way, with either tetanising or single-induction shocks. Lay 

 the preparation on a perfectly clean and dry glass-plate and place a 

 wire connected with one pole of the secondary coil under the nerve; no 

 contraction of the muscle takes place because the dry plate insulates 

 the preparation and the secondary circuit cannot be completed. Now 

 touch the muscle with a wire, the other end of which rests on a gas or 

 water pipe ; the muscle contracts because the circuit is completed 

 through the earth. It is not even necessary that the conductor should 

 touch the preparation, for, if a moistened finger is brought as near the 

 muscle as possible without touching it, the muscle contracts, especially 



