+--------------------------------+--------------------------------+ | _Hours _Tempera- _Amplitude | _Hours _Tempera- _Amplitude | | of ture._ of | of ture._ of | | day._ Response._ | day._ Response._ | +--------------------------------+--------------------------------+ | 5 p.m. 28° C. 28 mm. | 5 a.m. 20° C. 5 mm. | | 6 " 25.5° " 28 " | 6 " 20.5° " 4.2 " | | 7 " 24.5° " 27 " | 7 " 21° " 3.5 " | | 8 " 23° " 23.5 " | 8 " 22° " 2.5 " | | 9 " 22° " 21.5 " | 9 " 24° " 0 " | |10 " 21° " 18 " |10 " 26° " 6 " | |11 " 20.5° " 15 " |11 " 26.5° " 15.5 " | |12 " 20° " 13 " |12 " 28° " 22.5 " | | 1 a.m. 20° " 10 " | 1 p.m. 28° " 26 " | | 2 " 20° " 8 " | 2 " 28.5° " 28 " | | 3 " 20° " 7.5 " | 3 " 28.5° " 28 " | | 4 " 19.5° " 6 " | 4 " 29° " 28 " | +--------------------------------+--------------------------------+ From the data given in the table, two curves have been obtained. One of these shows the relation between the hours of the day and temperature; the other exhibits the relation between the hours of the day and the excitability as gauged by the amplitude of response (Fig. 28). It will be seen that there is, broadly speaking, a marked resemblance between the two curves, which demonstrate the predominant influence of temperature on diurnal variation of excitability. [Illustration: FIG. 28. The continuous curve shows the relation between the hour of the day and temperature. The dotted curve exhibits relation between the hour of the day and excitability.] EFFECT OF PHYSIOLOGICAL INERTIA. It has been shown (page 59) that owing to physiological inertia, the change of excitability, generally speaking, lags behind the inducing cause. This fact finds striking illustration in the lag exhibited by the curve of excitability in reference to the temperature curve. The minimum temperature was attained at about 4 A.M., but the excitability was not reduced to a minimum till four hours later and again there is a marked fall of temperature after 5 P.M., but the excitability did not become depressed till two hours later. There is again the factor of variation of light, the effect of which is not so great as that of temperature. The periods of maximum of light and temperature are, however, not coincident.