A Lecture On the Preservation of Health

to be applied, and fome of them, for inftance heat, as the fummer advances become increaf- ed, they firft loofe their verdure, then grow brow n, and at the end of fummer ceafe to live ; becaufe their excitability is exhaufted by the long continued action of the exciting powers; and this does not happen merely in confequence of the heat of fummer decreafing, for they grow brown and die, even in a greater degree of heat than that which in fpring made them grow luxuriantly. Thefe are examples of the finite, or irreparable exhauftion of the excitability ; but Section 4but we find alfo, that it may be ex- haufted for a time, and accumulated a- gain. Though the eye has been fo dazzled by the fplendour of light, that it cannot fee an object moderately illuminated, yet, if it be fhut for fome timer the excitability of the optic nerve accumulates again, and we are again capable of feeing with an ordinary light. We find, that we are not always equally capable of performing the functions of life. When we have been engaged in any exertion, either mental or corporeal, for fome hours only, we find ourfelves fatigued, and unfit to purfue our labours much longer; if in this ftate, feveral of the exciting powers, particularly light and noifer be withdrawn ; and if we are laid in a pofture which does not require much mufcular exertion, we foon fall into that ftate which nature intended for the accumulation of the excitability, and which we call fleep. In this ftate, many of the...