September, 1910. 



KNOWLEDGE. 



357 



elements in the food of twentv-four hours. Knowing 

 that one gramme of protein (flesh) yields us 4'1 

 calories, one gramme of sugar 4'1, and one gramme 

 of fat 9, it is not difficult to construct a dietary 

 containing potentially the necessary heat. We have 

 further to know that the ratio of the weight of 

 nitrogen - containing to non-nitrogen - containing 

 substances must be about 1 to 3"5 or 4"5. 



The portions of the total heat-loss borne bv each 

 system have been determined as follows : — 



Heat radiated and conducted from the skin. 2190 



calories or 73%. 

 Heat lost in e\aporating water from the skin. 435 



calories or 14'5%. 

 Heat lost in evaporating water from the lungs. 216 



calories or 7'2%. 

 Heat lost in warming the e.xpired air tobody-temperature, 



105 calories or y5%. 

 Heat lost in warming the dejecta. 54 calories or r8%. 



In human beings, then, the skin is the system of 

 heat-loss, being responsible for the loss of 87'5% of 

 the total heat lost. This is not so in some animals : 

 the dog, for instance, whose hairv coat does not 

 permit its skin to perspire. The dog loses heat 

 largely by its expired air, and bv the radiation and 

 evaporation of water from its tongue : thus it pants 

 and puts out its tongue on a hot day. 



Obviously our clothes prevent the loss of heat, and 

 the more effectually as the\- are bad conductors of 

 heat. For this reason flannel, wool and furs are so 

 much "warmer" than linen or cotton, materials 

 from the vegetable kingdom, and therefore not the 

 natural clothing of animals. 



An animal is not only a transformer of energv. but 

 it is the most economical transformer known : for 

 whereas in the very best steam-engine we obtain 

 about twelve per cent of the original potential energv 

 as motion (external work), in the animal we get as 

 much as twenty-five per cent in the form of useful 

 work. And whereas in the engine a great deal of 

 the heat set free is lost or wasted from the engineer's 

 point of view, the animal heat, so far from being 

 wasted, is all essential to the protoplasm to provide 

 it with the optimum temperature for the performance 

 of its vital activities. 



The muscles as energy- transformers difter from an 

 engine in possessing two distinct powers, heat- 

 producing and work-producing, or the thermogenic 

 and dynamogenic respectively. Now the interesting 

 thing is that, although a muscle cannot do work 

 w ithout producing heat, it can continue to produce 

 heat without actively contracting, without doing 

 work. In this latter state it is said to be in a 

 condition of tonus or tone. No doubt when 

 sufficiently analysed tonus is discovered to be a state 

 of imperceptible or incipient contraction. When a 

 muscle having shortened continues to support a 

 weight but shortens no more, it continues to produce 

 heat although in the sense of the physicists it is not 

 doing any external work : thus dynamogenesis and 

 thermogenesis are separate capabilities. But further, 

 the two related capacities of muscle for heat- and 



work-production only van.' pciri passu w ithin certain 

 limits. .\s a muscle lifts heavier and heavier w eights 

 it sets free more and more heat each time, and also 

 if it raises the same weight each time, but under the 

 influence of increasingly strong stimuli, it will evolve 

 more and more heat each time. In this latter case 

 the dynamogenic effects are the same throughout the 

 series, w hile the thermogenic eftects are greater and 

 greater. As fatigue comes on, the heat-producing 

 faculty is impaired long before the work-producing. 

 Thus ver}- tired muscles may be able to lift the load 

 to the height to which they raised it when fresh, but 

 they now do so with the greatest possible econom\- 

 as regards their store of potential energy, for the\' 

 evolve the minimal quantity of heat. Thev oxidise 

 material to as slight a degree as possible. Muscular 

 work and heat are thus not quantitativeh- parallel 

 products : there is nothing comparable w ith this in a 

 machine of human construction. 



As regards the kinds of food from w hich we derive 

 heat, our ideas have undergone changes since the 

 time when the great chemist Liebig divided foods 

 into flesh-formers and heat-givers ; the " meatv "' 

 stuffs and cheese he placed in the former group, the 

 starches, sugars and fats in the latter. After having 

 been much criticised, this classification by Liebig is 

 admitted to be substantiallv correct. While sugars 

 and fats do not build up tissue, but are found to be 

 normally oxidised more or less directly for heat- 

 giving or work-producing purposes, the proteins 

 I flesh-formers) both repair tissue-waste (bv their 

 nitrogen-containing moiety), and also contribute to 

 heat-production by the non-nitrogen-containing 

 substances into which in digestion they are split up. 

 Liebig told the truth but not the whole truth. 



If during a certain time a person's temperature 

 remains constant, that person is losing as much 

 heat as he is producing. In technical language, 

 thermogenesis. or heat-production, is just balanced bv 

 thermohsis. or heat-loss. Obviously, since we are 

 constantly producing heat, unless we were as 

 constantly to lose it, we should have to get hotter 

 and hotter, and would damage our tissues bv fatal 

 fever. Now, it is ver\- well known that the temper- 

 ature of a healthy man (98-4°F, 37°C) hardly differs 

 by a degree from one year's end to the other, or 

 from the poles to the equator. If no heat were 

 lost at all. our temperature would rise one degree 

 Centigrade in half an hour : in thirty-six hours our 

 body fluids would be boiling : in another thirtv-six 

 autocremation would be far advanced. 



\\'e may now ask ourselves the question, how 

 it comes about that some animals have and 

 others have not the power to keep their body 

 temperature constant. Does not a fall of tempera- 

 ture necessarily depress vitality, and a rise, within 

 limits, e.xalt it ? The answer is that protoplasm qua 

 protoplasm is certainly depressed by a fall and 

 stimulated by a rise of temperature, as can be well 

 seen by the slowing effect of chilling an isolated frog- 

 heart and the accelerating effect of warming it. It 

 would seem then that there ought to be no such 



