72 PHYSIOLOGICAL PHYSICS. [Chn P . vn. 



another can also be regulated by it to a large ex- 

 tent. The metronome (Fig. 39) consists of a box 

 containing clockwork, which causes the oscillations 

 of a rod t. The rod carries a small weight c, which 

 may be moved down the rod, causing the rod to 

 oscillate faster, or up the rod, when it will beat more 

 slowly. A scale fixed behind has marked on it the 

 number of oscillations per minute, corresponding to 

 different heights of the weight. On a little shelf at 

 the side of the metronome is a cup of mercury into 

 which dips one of the wires b of the primary circuit 

 of the inductorium. The other wire a is connected 

 by a binding screw with the oscillating rod. The rod 

 carries a projecting wire g, which, with one oscilla- 

 tion, is dipped into the mercury, forming the circuit, 

 and with the next is carried out of it, breaking the 

 circuit. Thus a definite number of contacts per 

 minute can be easily arranged, and consequently a 

 definite number of single induction shocks. 



Secondary contraction. An arrangement for 

 showing a very interesting experiment is represented 

 in Fig. 40. Two muscle telegraphs FI and F2 are so 

 placed that the muscle preparations fixed in them are 

 brought close to one another. The muscle mi of the 

 first telegraph is prepared without the nerve, that of the 

 second mz with the nerve. The nerve of ma is so laid 

 over mi that part touches the tendon of mi and part 

 the muscular fibres, mi has attached to it wires from 

 a secondary coil n, the primary of which receives a 

 current arranged for single shocks from a Daniell, a key 

 x being interposed, as shown in the figure. Muscle 

 2 receives no current. Then, by slowly approximating 

 the secondary to the primary coil and opening and 

 closing the key, a place will be found where a single 

 shock produces not only contraction of mi, but of 

 m2 also. The explanation is, that certain electrical 

 variations in mi, discussed in chapter ix., produced by 



