EXPEEIMENTAL PHYSIOLOGY 



the tendon, or this is pierced by a bent pin, and thus the lower end 

 attached to the lever of a myograph (see fig 31, p. 25). 



If a crank lever is to be used it is not necessary to thoroughly 

 isolate the femur, but its lower end can be directly fixed to the cork 

 plate of the myograph by a needle which is passed through the bone. 



In many cases, too, it is not 

 necessary to completely iso- 

 late the nerve up to the 

 vertebrae. 



DIRECT AND INDIRECT 

 EXCITATION OF MUSCLE. 

 A muscle may be made to 

 contract by a stimulus applied 

 to the muscle mass itself, 

 when the excitation is termed 

 direct, or it may be caused to 

 contract by a stimulus applied 

 to its nerve, which stimulus 

 then travels down to the 

 muscle. This is indirect ex- 

 citation. Test this by apply - 

 and then to the muscle, and 



Fig. 24. — Gastrocnemius-sciatic Preparation. 



ing the electrodes first to the nerve 



sending an induced current through the electrodes. 



THE MOIST CHAMBER 



In all instances in which we are experimenting upon an excised 

 muscle and nerve, it is of the greatest importance that they should be 

 protected from drying. To secure this it is necessary either to im- 

 merse them in some fluid which exerts no harmful effect upon them, 

 such as defibrinated ox-blood, or to place them in an enclosed air- 

 chamber in which the air is kept moist. This latter is termed a moist 

 chamber, and is of different form according to the myograph employed. 

 It consists of a glass cover to the myograph, in which is an aperture 

 through which a thread may pass to connect the muscle to the record- 

 ing lever. The air in the chamber is kept moist by placing in it a 

 few pieces of blotting-paper wetted with normal saline solution. 



Experiment 1. — Utilise this nerve-muscle preparation to prove that the 

 break shock is stronger than the make shock. Arrange the apparatus in 

 the same way as in Experiment 1, p. 14 (see fig. 20), placing the nerve upon 

 the pair of electrodes. Gradually decrease the distance between the two coils 

 as in that experiment, and make notes of the positions of the secondary coil 

 when a twitch occurs — (1) at break of the primary circuit, (2) at make. 



Experiment 2. — By using the arrangement previously described and 

 shown in fig. 8, p. 7, show that, by the introduction of a deriving circuit of 



