570 



NA rURE 



[April i6, 1908 



■different muscles. The nervous system has been called, 

 with a picturesque truth, the master-system of the body. 

 It controls the action of organs; it controls, quite 

 especially, the activity of the muscles. This heart which 

 we see beating here receives nerves. One of those nerves 

 when stimulated will cause it to contract less, the other 

 to contract more. The contraction of the heart is its 

 "beat." The vagus nerve slows the beating, the other 

 nerve quickens the beating. 



The heart is a tubular muscle ; it drives blood through 

 itself. When it contracts it squeezes the blood from it 

 into the arteries, and so the blood flows to feed all the 

 myriads of minute lives — cells — composing the whole com- 

 plex living animal. The lives of these myriad minute 

 entities all depend on their supply of blood, and therefore 

 the life of the whole creature depends on the contraction 

 of the heart. At each beat the heart by squeezing the 

 blood out of its arterial end maintains the flow of blood, 

 and this flow resulting from its own contraction refills it, 

 because the blood returns to it by the veins. 



This beating is all which the heart has to do. What- 

 •cver happens it must continue to do this, or the creature 

 perishes. Life-long, night and day, winter and summer, 

 It must^ do this. Whatever act the creature mav be 

 accomplishing, sitting, walking, feeding, sleeping, catch- 

 ing its prey, or escaping its enemies, this beating must go 

 on, in the frog about ten times a minute, in ourselves 

 about seventy times a minute. The task is monotony 

 itself. How admirably is the heart muscle adapted to 

 fulfil it ! ■ 



Self-adjustment to meet the environmental conditions 

 differentiates animate from inanimate nature. As 

 characteristic as this self-adjustment itself is its constant 

 trend toward what has sometimes been termed " pur- 

 pose." .\nimate objects are observed to adjust themselves 

 to their own advantage, that is, so as to prolong their 

 individual existence or that of their species. The more 

 we know of them the more complete appears to us this 

 trend in their reactions. The living organism advantage- 

 ously adapts itself to its surroundings ; and every part 

 of a living organism e.xhibits this power. The heart- 

 muscle reveals it clearly. It must not tire, and in normal 

 circumstances the healthy heart, unlike other muscles, 

 shows no fatigue. Its beat must always be strong enough 

 to press its contents over into the artery against consider- 

 able resistance which opposes it. A heart-beat which did 

 not expel the blood would be useless, worse than useless, 

 wasteful, because it would be energy spent in vain. Its 

 task can be roughly likened to that of a man with a 

 bucket who has to keep lifting water from a tank at his 

 feet to pour it over a wall of certain height before him. 

 If he lift the bucket much above the wall he expends more 

 (■nergy than he need do; if he lift it less than the wall's 

 height his work fails altogether. If he still, when the 

 bucket is emptied, keep it above the wall's height, his 

 work stops, although his effort does not. 



The heart, whether its stimulus be weak or strong, beats 

 always with sufficient power ; it thus avoids the useless 

 labour of a beat too weak to fulfil the office of a beat. 

 If the heart were to give too prolonged contraction it 

 would defeat its own purpose ; after its beat, which empties 

 it of blood, it must relax to refill for the next beat ; to 

 keep contracted would be for its purpose as harmful as to 

 cease from beating; it would stop the blood instead of 

 pumping it onward. In harmony with this, we find a 

 prolonged stimulus to the heart does not keep the heart 

 contracted ; after the heart has replied to the stimulus bv 

 a beat it exhibits a refractory phase, during which it pays 

 no attention to the further stimulation, and relaxes ; aiid 

 only after it has fully relaxed does it again pay attention 

 to the stimulus and contract, that is to sav, beat again. 

 In short, it replies rhythmically to a continued stimulus 

 which would keep the other muscle continuously contracted. 



That the heart should go on beating after removal from 

 the body does not seem greatly surprising, because it is 

 si ill then alive. The wonder lies rather in its continuing 

 1(1 live so long when thus removed ; that granted, it seems 

 natur.al that it should do what it has done previouslv all 

 Us life. 



But this other muscle, which likewise continues to live 

 when removed from the body ; it, though it can contract, 



NO. 2007, VOL. 77] 



does not. That seems — at least at first sight — the more 

 remarkable. Why does this muscle stop ? So long as it 

 was part of the living creature it showed contraction over 

 and over again. We must turn to the nervous system for 

 our answer. 



In the first place let us note that an animal, unlike that 

 other great example of life, a plant, cannot nourish itself 

 from naked earth and air alone. The plant strikes down 

 roots and throws up leaves, and draws through these 

 material and energy with which it can replenish its own 

 substance and activities. Where it as a seed fell, there its 

 foster-moster Earth gives it the food it wants. Not so 

 the animal. It must have subtler and rarer stuffs, or die. 

 The material it needs is not spread so broadcast. It, to 

 replenish itself, must have more special material ; it must 

 have for food material that is living, or has lived. To 

 obtain this it has to range about. It has to hunt for it ; 

 and it itself is hunted by other animals following the 

 same quest. Therefore its very existence involves locomo- 

 tion. It must find food and seize it, and must itself 

 escape being found and seized. It is both hunter and 

 hunted. Moreover, in a vast number of cases it has to 

 seek its kind to propagate its species. The movement 

 necessary in this great game of life is million-sided — subtle 

 beyond words — and most animal lives are spent in nothing 

 else. Existence for the individual and the race depends 

 upon success in it. Man plays it also — let us hope that 

 sometimes he plays something else as well. In all cases 

 the chief instruments of the game are the skeletal muscles, 

 those muscles of which the biceps of our arm may stand 

 as type. An old philosophic adage has it that all which 

 mankind can effect is to move things. The dictum illus- 

 trates how supremely chief an executant of man's activity 

 his muscles are. AH the things which man can move 

 are moved in the first instance by that prime thing which 

 he can move, his body ; and for this his main agents are 

 his skeletal muscles. These execute his movements, but 

 in doing so are but the instruments of his nervous system. 

 Therefore it is in reality the nervous system which is the 

 player of the game ; and it is because it is really the 

 nervous system which is the player of the game that man 

 is the most successful creature on earth's surface at the 

 present epoch, for his is the nervous system which, on 

 the whole, is the most developed, much best adapted to 

 dominate the environment. 



To understand a little how the nervous system compasses 

 this end we may turn to examine its performance in some 

 of its simpler governing of the muscles. Its main office 

 is to react to changes in the environment. The ' animal 

 body is provided with a number of organs specially 

 attuned to react to changes in the environment. These 

 changes, in so far as they excite these organs, are termed 

 stimuli. Thus, it has organs stimulated by the radiant 

 energy of light and heat, others by chemical particles 

 drifting from odorous objects, others mechanically by 

 objects touching the skin, and so on. These organs, 

 specially adapted to environmental stimuli, are called 

 rt'Lcptors. .■\ttached to them are nerves. Through these 

 the e.xcitement set up in the receptor by a stimulus spreads 

 to the general nervous system. .Arrived there, two kinds 

 of effect ensue from it — one, a change in nerve-cells 

 innervating muscles and glands, the other, a change in 

 consciousness on the basis of sensation. These two effects 

 are separable. The former, or " reflex " reaction, is not 

 necessarily accompanied by any manifestation of the latter, 

 though it may be so, and very often is so. We will 

 confine ourselves to the former, or purely reflex effect, and 

 to its operation on muscle. 



The endowment with receptor organs is not equally rich 

 in all parts of the body. It is the external surface of the 

 animal which, as we might e.xpect, has them in richest 

 profusion ; and the receptors of the external surface are 

 likewise those most developed, specialised, and sensitive. 

 This also we might expect ; for it is the external surface 

 that for countless ages has felt the influences of the illimit- 

 able outside world playing on it. Through refinement of 

 the receptors of its outer surface the animal has been 

 rendered sensitive in many cases to stimuli delivered even 

 by the remotest stars. 



It is a feature of receptoi'S generally that they react 

 most to their agent when the intensity of that agent 



