28 LIFE: ITS NATUEE, OEIGIN AND MAINTENANCE 



prevention of the contraction of others. For our present knowledge of 

 these conditions we are largely indebted to the researches of Professor 

 Sherrington. 



A less conspicuous but no less important part played by the nervous 

 system is that by which the contractions of involuntary muscles are 

 regulated. Under normal circumstances these are 

 always independent of consciousness, but their regula- 

 tion is brought about in much the same way as is that 

 of the contractions of voluntary muscles viz., as the result of impressions 

 received at the periphery. These are transmitted by afferent fibres to 

 the central nervous system, and from the latter other impulses are sent 

 down, mostly along the nerves of the sympathetic or autonomic system 

 of nerves, which either stimulate or prevent contraction of the involun- 

 tary muscles. Many involuntary muscles have a natural tendency to 

 continuous or rhythmic contraction which is quite independent of the 

 central nervous system ; in this case the effect of impulses received from 

 the latter is merely to increase or diminish the amount of such contrac- 

 tion. An example of this double effect is observed in connection with 



the heart, which although it can contract regularlv 



Effects of emotions. , , ' . . , J 



and rhythmically when cut off from the nervous 



system and even if removed from the body is normally stimulated to 

 increased activity by impulses coming from the central nervous system 

 through the sympathetic, or to diminished activity by others coming 

 through the vagus. It is due to the readiness by which the action 

 of the heart is influenced in these opposite ways by the spread of 

 impulses generated during the nerve-storms which we term ' emotions ' 

 that in the language of poetry, and even of every day, the word ' heart ' 

 has become synonymous with the emotions themselves. 



The involuntary muscle of the arteries has its action similarly balanced. 

 When its contraction is increased, the size of the vessels is lessened and 

 they deliver less blood ; the parts they supply accordingly become pale in 

 colour. On the other hand, when the contraction is diminished the 

 vessels enlarge and deliver more blood; the parts which they supply 

 become correspondingly ruddy. These changes in the arteries, like the 

 effects upon the heart, may also be produced under the influence of 

 emotions. Thus ' blushing ' is a purely physiological phenomenon due to 

 diminished action of the muscular tissue of the arteries, whilst the pallor 

 produced by fright is caused by an increased contraction of that tissue. 

 Apart, however, from these conspicuous effects, there is constantly pro- 

 ceeding a less apparent but not less important balancing action between 

 the two sets of nerve-fibres distributed to heart and blood-vessels ; which 

 are influenced in one direction or another by every sensation which we 

 experience and even by impressions of which we may be wholly un- 

 conscious, such as those which occur during sleep or anaesthesia, or which 

 affect our otherwise insensitive internal organs. 



