240 THE BODY AT WORK 



the nerve to initiate a process which was not occurring before 

 the electric current was passed through it. The muscular 

 spasm equally appears to be an isolated event. As usual, we 

 are misled by the analogy of human inventions. We compare 

 the nerve-impulse to the fall of a hammer, the muscle-spasm 

 to the explosion of gunpowder. We forget that nerve and 

 muscle are in permanent connection ; that the impulse is a 

 sudden exaggeration of an influence which the nerve is con- 

 tinuously exerting, the contraction an exaggeration of meta- 

 bolic changes which are constantly occurring in muscle. (See 

 in this connection the explanation of muscle-tone, p. 273.) In 

 the case of plain muscle, nerve stimuli do not cause contraction ; 

 they merely increase the excitability of the muscle. It may be 

 more difficult for us to figure to ourselves the way in which 

 dilator nerves diminish excitability ; but the existence of such 

 an anabolic influence is beyond the reach of doubt. Heart 

 and bloodvessels are part of the same system. The heart has 

 its accelerator and inhibitory nerves, the bloodvessels their 

 constrictor and dilator nerves. For both vessel-wall and 

 heart the stimulus to contraction is the distending pressure of 

 blood although it is not altogether necessarythat this stimulus 

 should be acting at the time. Sympathetic and vagus nerves 

 can to a certain extent control the beating of a bloodless heart. 

 The heart-tissue has acquired the habit of beating, and the 

 habit of listening to advice conveyed to it through these nerves. 



The self -adjustment of the blood-tubes to the pressure to 

 which they are exposed is exhibited in the adaptation of their 

 degree of contraction to the position of the body to the 

 weight, that is to say, of the column of fluid which they have 

 to support. Everyone has played the game of " right hand 

 or left." When the hand is held above the head the blood 

 leaves it, and the hand becomes cold ; but if there be need for 

 adjustment, and time is given for the mechanism to come into 

 play, it works to perfection. When we are standing erect, there 

 is neither too much blood in the feet nor too little in the head. 

 But after a fortnight in bed a convalescent finds, the first time 

 that he stands upright, that his legs are quickly engorged his 

 slippers after a few minutes feel too tight for him whereas the 

 brain becomes so anaemic that he turns giddy, or even faints. 



Numberless illustrations of vaso-motor action are met with 



