481. The ultimate object of respiration, then, is to prepare and to preserve in a state of purity a fluid capable of affording to all the parts of the body the materials necessary to maintain their vital endowments. By the exhalation of oxygen and water, and the absorption of carbon, under the agency of light, the plant elaborates such a fluid from its nutritive sap, and out of this elaborated sap forms terniary combinations, the organic elements of all vegetable solids. By the absorption of oxygen and azote, and the exhalation of carbonic acid and water, probably under the influence of electricity, conducted and regulated by the nervous system, the animal elaborates such a fluid from its aliment, and out of this elaborated fluid forms quaternary combinations, albumen, and fibrin, the organic elements of all animal solids. CHAPTER IX. Of the temperature of living bodies—Temperature of plants—Power of plants to resist cold and endure heat—Power of generating heat—Temperature of animals—Warm-blooded and cold-blooded animals—Temperature of the higher animals—Temperature of the different parts of the animal body—Temperature of the human body—Power of maintaining that temperature at a fixed point whether in intense cold or intense heat—Experiments which prove that this power is a vital power—Evidence that the power of generating heat is connected with the function of respiration—Analogy between respiration and combustion—Phenomena connected with the functions of the animal body, which prove that its power of generating heat is proportionate to the extent of its respiration—Theory of the production of animal heat—Influence of the nervous system in maintaining and regulating the process—Means by which cold is generated, and the temperature of the body kept at its own natural standard during exposure to an elevated temperature. 482. Closely connected with the function of respiration, is the power which all living beings possess of resisting within a certain range the influence of external temperature. The plant is warmer than the surrounding air in winter, and colder in summer. A thermometer placed at the bottom of a hole bored into the centre of a living tree, precaution being taken to keep off as much as possible all external influence either of heat or cold, does not rise and fall according to the changes of external temperature; but rises when the external air is cold, and falls when it is warm. Thus, in a cold day in spring, the wind being north, at six o’clock in the evening, the temperature of the external air being 47°, that of a tree was 55°. On another cold day in the same month, there being snow and hail, and the wind in the north-east, at six o’clock in the evening, the external temperature being 39°, that of the tree was 45°. On the contrary, in one experiment, when the temperature of the air was 57½°, that of the tree was only 55°; and when the temperature of the air was 62°, that of the tree was 56°.