ANALOGIES BETWEEN NERVOUS AND IMPONDERABLE ACTION. 



larger bronchial tubes. In an instant, by diffusion (Ap., 50), the oxygen of this air leaves 

 its nitrogen, which in these circumstances is a more slowly-diffusing gas, finds its way 

 to the minutest air-cells, passes in an instant (Ap., 53) through the thin vascular coats, 

 interchanges w ith carbonic acid, and the act of arterialization is complete. The arte- 

 rialization of the blood is, therefore, nothing more than an ordinary chemical process, 

 and the nervous agencv which has been brought into action is for no other end than 

 the production of a mechanical effect ; for, though the act of respiration is eventually 

 carried on under the operation of chemical laws, mechanical movements have to pre- 

 cede. A division of those nerves puts an end to the process, not because respiration 

 is the result of a vital, a nervous, or any other such agency, but simply because the 

 necessarv movements which end in the introduction of oxygen gas can no longer be 

 accomplished. 



399. Now let us turn to the vegetable kingdom. As long as the sun is above the 

 horizon, his beams, impinging upon everything, occasion an elevation of temperature. 

 The various objects exposed the leaves of trees, the grass, and the surface of the 

 ground become warm. Participating in this elevation of temperature, the atmo- 

 spheric air has its capacity for vapour increased, and that transpiration of steam which 

 is copiously going forward from the exposed vegetable surfaces, from the soil, and 

 from water, raises the dew point. But on the going down of the sun, and even before 

 the close of day, a reverse action begins to be established. Radiation into space is ac- 

 complished, the temperature of the ground and of the leaves begins to fall, and by-and- 

 by the dew point is reached, and now drops of w ater form on those organs, and under 

 its influence they begin to recover the parching effect of the preceding day. The re- 

 duction of the atmospheric temperature down towards the dew point, thus brought 

 about by the conjoint agency of the ground and the exposed parts of plants, in an in- 

 stant puts a check upon evaporation from the leaves ; this, in its turn, reacts upon the 

 ascent of the sap: it is like putting the cover over the burner of a spirit lamp-; evapo- 

 ration from the reservoir is stopped, and simultaneously, also, the current along the cap- 

 illary fibres of cotton is checked. With this check upon the ascent of the sap, that 

 store of carbonic acid gas which exists in the soil, and which arises from the decay of 

 humus, or from artificial manures, is also preserved ; carbonic acid which, during the 

 day, comes up to the leaveswith the circulating juice, and is decomposed by solar light. 

 But, now. if a cloud intervenes, Or the sky at night be uniformly obscured, no radiation 

 can be accomplished, no reduction of temperature takes place, the dew point is never 

 reached, and evaporation goes on from the leaves, and carbonic acid gas, drawn up 

 from the spongioles. is expended. The reflex action of that radiant heat which came 

 from the sun during the day is cut off, its passage outward into the regions of space 

 arrested, and the dependant physiological phenomena cease to be performed. There 

 is ah analogy of effect between the temporary action of an overhanging canopy of 

 clouds at night on the vegetable world, and the permanent injury which results to an 

 animal when the par vagum. one of the offices of which is, at proper moments, to in- 

 troduce a measured quantity of oxygen gas into the system, is divided. 



400. Where, then, do these things carry our reflections ? What are the elevated 



O 



