346 



NA TURE 



[September 9, 1922 



Some Aspects of Animal Mechanism. 1 



By Sir C. S. Sherrington, G.B.E., Sc.D., D.Sc, LL.D., Pres.R.S., President of the Association. 



IT is sometimes said that science lives too much in 

 itself, but once a year it tries to remove that 

 reproach. The British Association meeting is that 

 annual occasion, with its opportunity of talking in wider 

 gatherings about scientific questions and findings. 

 Often the answers are tentative. Commonly questions 

 most difficult are those that can be quite briefly put. 

 Thus, " Is the living organism a machine ? " " Is life 

 the running of a mechanism ? " The answer cannot 

 certainly be as short as the question. But let us. in the 

 hour before us, examine some of the points it raises. 



Of course for us the problem is not the " why " of the 

 living organism but the " how " of its working. If we 

 put before ourselves some aspects of this working we 

 may judge some at least of the contents of the question. 

 It might be thought that the problem is presented at its 

 simplest in the simplest forms of life. Yet it is in 

 certain aspects more seizable in complex animals than 

 it is in simpler forms. 



Our own body is full of exquisite mechanism. Many 

 exemplifications could be chosen. There is the mech- 

 anism by which the general complex internal medium, 

 the blood, is kept relatively constant in its chemical 

 reaction, despite the variety of the food replenishing it 

 and the fluctuating draft from and input into it from 

 various organs and tissues. In this mechanism the 

 kidney cells and the lung cells form two of the main 

 sub-mechanisms. One part of the latter is the delicate 

 mechanism linking the condition of the air at the 

 bottom of the lungs with that particular part of the 

 nervous system which manages the ventilation of the 

 lungs. ( In that ventilation depends the proper respira- 

 tory condition of the blood. The nervous centre 

 which manages the rhythmic breathing of the chest is 

 so responsive to the respiratory state of the blood 

 supplied to itself that, as shown by Drs. Haldane and 

 Priestley some years ago, the very slightest increase in 

 the partial pressure of carbon dioxide at the bottom of 

 the lungs at once suitably increases the ventilation of 

 the chest. Dovetailed in with this mechanism is yet 

 another working for adjustment in the same direction. 

 As the lung is stretched by each inbreath the respiratory 

 condition of the nervous centre, already attuned to the 

 respiratory quality of the air in the lungs, sets the degree 

 to which inspiration shall fill them ere there ensue the 

 opposite movement of outbreath. All this regulation, 

 although the nervous system takes part in it. is a 

 mechanism outside our consciousness. Part of it is 

 operated chemically ; part of it is reflex reaction to a 

 stimulus of mechanical kind, though as such unper- 

 ceived. The example taken has been nervous mech- 

 anism. If, in the short time at our disposal, we confine 

 our examples to the nervous system, we shall have the 

 advantage that in one respect that system presents our 

 problem possibly at its fullest. 



To turn therefore to another example, mainly nervous. 

 Muscles execute our movements ; they also maintain 

 our postures. This postural action of muscles is pro- 

 duced by nerve-centres which form a system more or 

 less their own. One posture of great importance thus 



1 Presidential Address delivered at the Hull Meeting of the British 

 :i 'ii on Sept. 6. 



NO. 2758, VOL. I lo] 



maintained is that of standing, the erect posture. This 

 involves due co-operation of many separate muscles in 

 many parts. Even in the absence of those portions of 

 the brain to which consciousness is adjunct, the lower 

 nerve-centres successfully bring about and maintain the 

 co-operation of muscles which results in the erect 

 posture ; for example, the animal in this condition, if 

 set on its feet, stands. It stands reflexly ; more than 

 that, it adjusts its standing posture to required con- 

 ditions. If the pose of one of the limbs be shifted a 

 compensatory shift in the other limbs is induced, so 

 that stability is retained. A turn of the creature's neck 

 sidewise and the body and limbs, of themselves, take up 

 a fresh attitude appropriate to the side-turned head. 

 Each particular pose of the neck telegraphs off to the 

 limbs and body a particular posture required from them, 

 and that posture is then maintained so long as the neck 

 posture is maintained. Stoop the creature's neck and 

 the forelimbs bend down as if to seek something on the 

 floor. Tilt the muzzle upward and the forelimbs 

 straighten and the hind limbs crouch as if to look at 

 something on a shelf. Purely reflex mechanism pro- 

 vides all kinds of ordinary postures. 



Mere reflex action provides these harmonies of 

 posture. The nerve-centres evoke for this purpose in 

 the required muscles a mild, steady contraction, with 

 tension largely independent of the muscle length and 

 little susceptible to fatigue. Nerve-fibres run from 

 muscle to nerve-centre, and by these each change in 

 tension or length of the muscle is reported to the 

 activating nerve-centre. The)' say " tension rising, you 

 must slacken," or conversely. There are also organs 

 the stimulation of which changes with any change of 

 their relation to the line of gravity. Thus, a pair of 

 tiny water-filled bags is set one in each side of the skull 

 and in each is a patch of cells endowed with a special 

 nerve. Attached to hairlets of these cells is a tiny 

 crystalline stone the pressure of which acts as a stimulus 

 thn nigh them to the nerve. The nerve of each gravity- 

 bag connects, through chains of nerve-centres, with the 

 muscles of all the limbs and of one side of the neck. 

 In tin ordinary erect posture of the head, the stimula- 

 tion by the two bags right and left is equal, because the 

 twi 1 gravity-stones then lie symmetrically. The result, 

 then, is a symmetrical muscular effect on the two sides 

 of the body, namely, the normal erect posture. But 

 the right and left bags are mirror pictures of each other. 

 If the head incline to one side, the resulting slip, micro- 

 scopic though it be, of the two stones on their nerve- 

 patches makes the stimulation unequal. From that 

 slip there results exactly the right unsymmetrical action 

 of the muscles to give the unsymmetrical pose of limbs 

 and neck required for stability. That is the mechanism 

 dealing with limbs and trunk and neck. An additional 

 one postures the head itself on the neck. A second pair 

 of tiny gravity-bags, in which the stones hang rather 

 than press, are utilised. These, when any cause inclin- 

 ing the head has passed, bring the head back at once 

 to the normal symmetry of the erect posture. These 

 same bags also manage the posturing of the eyes. The 

 eye contributes to our orientation in space ; for example, 

 in] hii i.ption of the vertical. For this the eyeball, that 



