346 



SCIENCE 



[Vol. LVI, No. 1448 



proper respiratory condition of the blood. The 

 nervous center which manages the rhythmic 

 breathing of the chest is so responsive to the 

 respiratory state of the blood supplied to it- 

 self that, as shown by Di-s. Haldane and 

 Priestley some years ago, the very slightest 

 increase in the partial pressure of carbon di- 

 oxide at the bottom of the lungs at once suit- 

 alDly increases the ventilation of the chest. 

 Dovetailed in with this mechanism is yet an- 

 other working for adjustment in the same di- 

 rection. As the lung is stretched by each in- 

 breath the respiratory condition of the nei"vous 

 center, 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 sys- 

 tem 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 

 unperceived. The example taken has been 

 nervous mechanism. 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, main- 

 ly nervous. Muscles execute our movements; 

 they also maintain our postures. This postural 

 action of muscles is produced by nerve-centers 

 which form a system more or less their own. 

 One posture of great importance thus main- 

 tained is that of standing, the erect posture. 

 This involves due cooperation of many sepa- 

 rate muscles in many parts. Even in the ab- 

 sence of those portions of the brain to which 

 consciousness is adjunct, the lower nerve- 

 centers successfully bring about and maintain 

 the cooperation 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 stand- 

 in postui-e to required conditions. 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 them- 

 selves, take up a fresh attitude appropriate 



to the side-turned head. Each particular pose 

 of the neck telegi-aphs off to the limbs and 

 body a particular posture required from them, 

 and that posture is then maintained so long 

 as the neck postui'e 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 

 provides all kinds of ordinary postures. 



Mere reflex action provides these harmonies 

 of posture. The nerve-centers evoke for this 

 purpose in the required muscles a roild, steady 

 contraction, with tension, largely independent 

 of the muscle length and little susceptible to 

 fatigue. Nerve-fibers run from muscle to 

 nerve-center, and by these each change in 

 tension or length of the muscle is reported to 

 the activating nerve-center. They say "ten- 

 sion rising, you must slacken," or conversely. 

 There are also organs the stimulation of whicti 

 changes with any change of their relation to 

 the line of gi-avity. 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 pres- 

 sure of which acts as a stimulus through them 

 to the neiwe. The nerve of each gi-avity-bag 

 connects, through chains of nerve-centers, with 

 the muscles of all the limbs and of one side 

 of the neck. In the ordinary erect posture of 

 the head, the stimulation by the two bags right 

 and left is equal, because the two 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. Bu't the right and left bags are 

 mirror pictures of each other. If the head in- 

 cline to one side, the resulting slip, micro- 

 scopic though it be, of the two stones on their 

 nerve-patelies makes tlie stimulation unequal. 

 From that sUp there results exactly the right 

 unsymmetrieal action of the muscles to give 

 the unsymme'trical pose of limbs and neck re- 

 quired 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 



