WHY BO WE EAT OUR DINNER? 805 



of the heart, with the consequent propulsions of blood through the 

 system ; the expirations and inspirations of the lungs, which supply us 

 with the oxygen for carrying on these processes; the act of digestion 

 and assimilation ; and many other minor functions of like sort. But 

 just as in the case of the steam-engine, so in the human or animal body, 

 the union of the oxygen with the hydrocarbons, besides producing 

 motion, liberates heat. This heat keeps the bodies of birds, quadru- 

 peds, and human beings, which are all very active in their automatic 

 movements, at a much higher temperature than the surrounding medium ; 

 while reptiles, fishes, and other " cold-blooded " creatures, having much 

 less energetic motions of the heart and lungs — which of course betokens 

 much less oxidation of food-stuffs — have bodies comparatively little 

 different in warmth from the air or water about them. We thus see in 

 part why it was that the anaconda and the desert-snail could go so long 

 without food ; though we can not quite understand that question till 

 we have examined the voluntary movements as well. It should be 

 added that, though the latter class of actions also produce heat — as we 

 all know when we walk about on a cold day to warm ourselves — yet 

 the temperature induced by the automatic activities of the body alone 

 is generally sufficient under normal circumstances to keep us comfort- 

 ably warm. Thus, while we are asleep, only the actions of breathing 

 and the beating of the heart continue ; but the union of oxygen with 

 the food-stuffs to produce these movements suffices as a rule to make 

 bed quite hot enough for all healthy persons ; and if we ever wake up 

 cold after a good night's rest, we may be sure that our automatic ac- 

 tivities are not what they ought to be. 



The voluntary activities of the body are brought about in a slightly 

 different manner. Directly or indirectly, they depend upon the union 

 of oxygen and food-stuffs within the tissues of our locomotive muscles, 

 the energy so liberated being made use of to bend or extend our bones 

 or limbs in the particular way we desire. The muscles always contain 

 (in a healthy and well-fed person) large quantities of s;ich stored-up 

 food-stuffs ; and the blood supplies them from moment to moment with 

 oxygen which may unite with the food-stuffs whenever occasion de- 

 mands. But the union does not here take place regularly and con- 

 stantly, as in the case of the automatic organs ; it requires to be set up 

 by an impetus specially communicated from the brain. That seat of 

 the will is connected with the various voluntary muscles by the living 

 telegraphic wires which we call nerves ; and when the will determines 

 that a certain muscle shall be moved, the nerves communicate the dis- 

 turbance to the proper quarter, the necessary oxidation takes place, and 

 the muscle contracts as desired. We do not quite know how the nerves 

 and muscles perform these functions ; but it is pretty certain that the 

 nitrogen of our foods plays an active part in the process, and that, as I 

 have already hinted, it acts in a manner somewhat analogous to that of 

 a match. We may suppose, to put the matter in a familiar form, that 



