182 



COMPARATIVE PHYSIOLOGY. 



action, followed by death, is essential. A moist chamber con- 

 sists essentially of an inclosed cavity, in which is placed some 

 wet blotting-paper, etc., and is usually made with glass sides. 

 The air in such a chamber must remain saturated with moisture. 



A good knowledge of the subject of electricity is especially 

 valuable to the student of physiology. But there are a few ele- 

 mentary facts, it is absolutely necessary to bear in mind : 1. An 

 induced current exists only at the moment of making or break- 

 ing a primary (battery) current. 2. At the moment of making, 

 the induced current is in the opposite direction to that of the 

 primary current, and the reverse at breaking. 3. The strength 

 of the induced current varies with the strength of the primary 

 current. 4. The more removed the secondary coil from the 

 primary the weaker the current (induced) becomes. 



The clock-work mechanism and its associated parts, as seen 

 in Fig. 170, on the right, is usually termed a myograph. 





Fig. ITO.—Arrangement of apparatus for transmission of muscular movement by tam- 

 bours (after McKendriclc). a, galvanic element; 6, primary coil; c, secondary coil 

 of inductorium; d, metronome for interrupting primary circuit when induction 

 current is sent to electrodes &; h, forceps for femur; the muscle, which is not 

 here represented, is attached to the receiving tambour p', by which movement is 

 transmitted to recording tambour e, which writes on cylinder/. 



Instead of muscular or other movements being communi- 

 cated directly to levers, the contact may be through columns 

 of air, which, it will be apparent, must be capable of communi- 

 cating very slight changes if the apparatus responds readily to 

 the alterations in volume of the inclosed air. 



Fig. 171 represents a Marey's tambour, which consists essen- 



