TONUS 



535 



the wood with so powerful a crash that their edges are splintered. The wood is 

 then held as in a vice. One can, however, pull it out by twisting it about 

 backwards and forwards, and then one is surprised to see that the shells remain 

 motionless, just as would the jaws of a vice if an object clamped between them 

 had been forced out. The shell movement shows not the least degree of elasticity. 

 The muscular fibres seem to have been suddenly frozen solid." If one next tries 

 to open the shell, no effect can be produced, but even the pressure of a finger is 

 sufficient to press them nearer together, and in this position they remain fixed 

 again, so that they cannot be brought back. The nearest mechanical illustration 

 that can be given is that of two racks with saw teeth, as in Fig. 1 69 ; these will 

 glide over one another if pulled in the direction of the arrow, but resist any pull 

 in the opposite direc- 

 tion. The fact that 

 the animal itself can 

 allow the shells to 

 open, shows that the 

 " catch " can be re- 

 moved by some means. 

 This, as we shall see 

 presently, is done by 

 " inhibition " from the 

 central nervous system. 

 In the model, it might 

 be supposed to be 

 effected by separation 

 of the two racks to the 

 extent of the depth of 

 a tooth. The device 

 of Fig. 170 may per- 

 haps assist in under- 

 standing the process. 



If a flat piece of soft 

 iron be arranged so as 

 to be able to move 

 around an axis at one 

 end, and an electro- 

 magnet fixed at a short 

 distance above it, on 

 sending a current 

 through the coils of 

 the magnet the weight 

 of the piece of iron is 

 raised ; but, in order to 

 hold it up, energy 

 must be continually 



FIG. 170. APPARATUS TO ILLUSTRATE THE MECHANISM OF A 

 TONUS MUSCLE. If the upper electro-magnet is actuated by 

 closing the lower key on the right, the iron lever, with the 

 weight attached, is raised. In this, it pushes back the thin 

 vertical steel spring, until the end of the lever has passed the 

 tooth. When this happens, the spring flies back and the 

 lever is now supported on the top of the tooth, so that the 

 magnet may now be put out of action without the weight 

 falling. To release the weight, so that it may fall again, 

 the electro-magnet on the left must be actuated by means 

 of the upper key. This attracts the spring and draws 

 away the support from the lever, which then falls until it 

 reaches the pointed support. Finally, the circuit of the 

 second electro-magnet is broken, the spring flies back and 

 the apparatus is in its original state. The two keys may 

 be supposed to be nerve centres. 



supplied to the magnet, 

 and this energy is dissipated as heat. This part of the process corresponds to 

 the behaviour of the sartorius muscle of the frog in A. V. Hill's experiments 

 (page 450). Suppose, however, that a thin strip of steel spring, with a little 

 projection on it, is arranged at right angles vertically at the free end of the 

 piece of iron, and in such a position that the projection together with the spring 

 can be pushed back by the iron weight as it rises. When the weight has passed 

 over the projecting bit, the spring flies back underneath the weight, and the latter 

 Avill remain suspended when the current is switched off the magnet. In order to 

 allow it to fall again, a second electro-magnet is fixed at the back of the spring 

 and, when this magnet is actuated, the support is drawn back with the spring and 

 the weight falls. This last mechanism corresponds to the impulses from the 

 central nervous system, which release the adductor muscle of the mollusc. 



Now the muscle which has this remarkable property must be able also to 



