512 PRINCIPLES OF GENERAL PHYSIOLOGY 



the means of influencing it reflexly. We shall see the part played by these 

 receptors in discussing " plastic tonus " in the next chapter. 



An illustration may assist here. Suppose a general commanding a battle in which s. \, r,il 

 regiments ar engaged and extending over a wide area. It is clear that, in order that a 

 particular movement may be effectively ordered, it is necessary for the general to know that 

 the previous order has been carried out. The message may, perhaps, not have reached the 

 body of troops in question. Again, since we judge of the distance between one inaccessible 

 object and another by movement of the eyes, the centres must be made aware that the eye 

 muscles have actually executed the movement intended ; knowledge of the sending of an 

 efferent impulse is not sufficient of itself, for the nerve may have been unable to conduct it. 



As regards the variety of external forces for which sense organs have been 

 evolved, it is .not to be forgotten that there may be forms of energy for which 

 we have no appropriate receptors. Moreover, it is possible that other animals 

 than ourselves may be able to appreciate phenomena of which we are unaware. 

 Quantitative differences of this kind are familiar. We have no appropriate 

 receptors for ultra-violet light. The cat is able to hear notes higher in pitch than 

 man can, and so on. 



Further, a form of energy which we know by physical experiments to be one 

 and the same, such as radiation from the sun, is known to us as light or as heat, 

 according to the particular receptors on which it falls, although the only difference 

 is in rate of vibration. More precisely, a particular, somewhat narrow, range of 

 rate of vibration or wave length, appears to the eye as light, longer or shorter 

 wave lengths are unappreciated ; but the long waves which have no effect on the 

 retina are able to stimulate certain receptors in the skin and are then called heat. 



We see thus, that, in order that a particular form of energy may be able to 

 produce a propagated disturbance in the receptor neurone, what is necessary is 

 that a mechanism of some sort shall be present in which changes shall be produced 

 of sufficient magnitude to excite the nerve fibres, although the incident energy 

 itself may be far too small to do so if it acted directly on the nerve. How small 

 an amount of energy is able to produce such a nerve impulse, if it acts through 

 an appropriate receptor, is shown by the calculation of V. Henri et Larguier des 

 Bancels (1911, p. 856), who found the retina to be sensitive to an amount of light 

 energy as small as 5 x 10~ 12 ergs. This is about three thousand times as sensitive 

 as the most rapid photographic plate. These considerations naturally lead us to 

 expect that the nature of the receptor organ will vary greatly according to the 

 particular form of energy to be detected ; just as we use a galvanometer to detect 

 electrical currents, a thermometer for heat, an actinometer for light, and so on. 

 It is also instructive, in view of the facts to be considered in the next paragraph, 

 to remember that it is possible to detect and measure all forms of energy by 

 converting them into electric currents. Heat, by the use of a thermopile as 

 receptor, sound by a telephone, ordinary kinetic energy by using it to turn a 

 dynamo, chemical energy, as that of the neutralisation of a base by an acid, by 

 the use of the hydrogen electrode, or indirectly by conversion to heat and then 

 by the thermopile, can all be converted into electric currents in a wire, just as 

 in our receptor organs of sense they are converted into nerve impulses. There is, 

 however, a difference to be noted ; in the sense organ a small incident energy is 

 caused to set free a larger amount of energy by what we may call a " trigger " 

 action ; as if, in our physical instruments, we always made use of what is known 

 as a " relay," such as is done to magnify the energy of the electrical waves 

 received in wireless telegraphy, where a minute amount of energy is caused to 

 complete the circuit of an independent battery, and to set in action a much 

 greater quantity of energy, which can readily be detected. 



Mi/fler's Law. -We saw in Chapter XIII. that all evidence points to the fact 

 that there is no difference between forms of nerve disturbances. Impulses may 

 follow one another at different rates, within the limitations of the refractory 

 period, but the individual impulses are in all cases alike, with the single exception 

 of those that follow immediately in the wake of a previous disturbance. Even 

 here, the first disturbance is a normal one and those that follow only differ from it 

 in magnitude. This being so, impulses produced in the optic nerve by light on 

 the retina are identical with those produced by sound in the auditory nerve, by 



