588 



NATURE 



[October 20, 1892 



In a of Z is * 



substitute X for Z, 



and we have a of X is e. 



Or (2) a combination of what Jevons calls Immeiiate Inference 

 by Complex Conception (which I should like to class with some 

 other Immediate Inferences as Extraversion, which is largely 

 used in mathematics) and Mediate Inference ; thus— 



Z is X (a) 



.-. aof Zisa of X \b) 



But a of Z is ^ \c) 



. • . a of X is £ {d) 



{b) is Inference by Complex Conception from (a) ; (b) and {c) are 

 the premisses which give {d) as their (syllogistic) conclusion. 

 Cambridge, October ii. E. E. Constance Jones. 



The Temperature of the Human Body. 



Mr. Cumming's second or "physical" query will, I think, 

 require no answer if his first or "physiological" question is 

 replied to. If an isolated muscle from which evaporation was 

 prevented could go on working in a heat enclosure, and always 

 remain at a lower temperature than the enclosure (which it could 

 only do by transferring heat from itself to its surroundings), we 

 should have to ask m good earnest how this was consistent with 

 the Second Law of Thermodynamics. We are quite certain, 

 however, that the temperature of the working muscle would 

 always, when a steady state of things had been reached, be above 

 that of the enclosure. 



The temperature of an isolated muscle during activity 

 (assuming that it could be kept alive and evaporation prevented) 

 would, of course, not only be very much higher " at the equator " 

 than "at the pole," but also somewhat above that of the sur- 

 rounding bodies in either latitude. The intact homoiothcrmal 

 animal, even when the temperature of the air is greater than that 

 of its blood, is on the whole, within the limits which can be borne, 

 always losing more heat to its surroundings than it receives from 

 them. For heat is still becoming latent at the evaporating 

 surfaces of the body, the skin and the respiratory mucous mem- 

 brane, even when the balance of gain and loss by radiation, &c., 

 is telling the other way ; and, indeed, in general more evapora- 

 tion than usual is going on when this is the case. The tem- 

 perature of these surfaces is always kept below that of the blood 

 which comes to them. The blood, therefore, always loses heat 

 here, and gains it from the muscles, which accordingly transfer 

 heat to a medium colder than themselves, even when the external 

 temperature is higher than that of either. 



If, of two similar and similarly situated men, A and B (I ask 

 pardon for degradmg an austere geometrical phrase to such 

 loose and vulgar application), exposed to the same high tem- 

 perature (above that of the blood, say), A sweats little and B 

 much, while the blood-temperature of both remains constant, 

 A must either produce less Iheat than B or lose more in other 

 ways than evaporation of sweat. lie may produce less either 

 because he works less than B, or because even at rest his 

 metabolism is not so active. Or an extra loss of water-vapour 

 from the lungs may make up for the diminished loss from the 

 skin. For example, in the dog, which has but few sweat- 

 glands, nearly the whole of the evaporation takes place 

 in the respiratory tract. Of course much water is evapor- 

 ated from the skin which never appears as visible sweat ; 

 and it is possible that some persons give off a greater pro- 

 portion of the total perspiration in this way than others do, 

 the quiet steady sweater, if one may be allowed the expression, 

 getting through as much work on the whole as the steaming 

 paroxysmal kind of fellow who breaks out into dewdrops on the 

 smallest provocation. But it should be clearly recognized that 

 an air temperature equal to or above that of the blood is 

 occasional, and not permanent in any latitude, and that men, 

 and even animals, adopt expedients to avoid such extremes and 

 to tide them over. 



Any good recent text-book of physiology will give the in- 

 formation asked for as to what is known of the mechanism by 

 which the temperature of warm-blooded animals is kept approxi- 

 mately constant. It is too wide a subject to be entered into 

 here. In man the regulation of the heat loss seems to be far 

 more important than any regulation of the production of heat. 

 The former is, of course, largely voluntary, but the quantity of 

 blood going through the skm, an important factor in more than 



one way, is greatly influenced by reflex nervous impulses. It i* 

 doubtful whether the very considerable heat capacity of the 

 bodies of large animals has been sufficiently taken into account 

 in its bearing on the steadiness of the blood temperature. This 

 in itself prevents any sudden change. In some animals, and 

 apparently more especially in small animals — ^.^.,the rabbit and 

 guinea-pig — the production of heat, as well as the loss, is very 

 distinctly under the control of the nervous system, and is 

 increased when the external temperature is lowered, and 

 diminished when it is raised. 



Of course, as your correspondent is doubtless aware, we do 

 not really know what kind of a machine a muscle is, except that it 

 is a machine by means of which the potential energy of the food 

 is partly transformed into mechanical work and to a much greater 

 extent into heat. Up to a certain limit the work and the heat 

 increase together, although less heat is given cjff by an active 

 muscle which is allowed on the whole to do external work than 

 by the same muscle when it constantly undoes its own work. 



G. N. Stewart. 



New Museums, Cambridge, October ii. 



NO. 1 199, VOL. 46] 



The following brief account of the working of the heat 

 mechanism of the human body will, I hope, help to make clear 

 to Mr. Cumming the problems of which he seeks the explana- 

 tion. 



The temperature of a man at the equator is within a degree 

 Centigrade the same as that in the arctic regions. This is 

 because, in the first place, in the arctic regions the loss of heat 

 from the body is very slight, and in the tropics it is very great, 

 for {a) in the tropics more perspiration is secreted by the skin, 

 and this, in consequence of the high temperature of the air, 

 evaporates very quickly, and hence the body is kept cool. It is 

 true, as Mr. Cumming says, that in the tropics people may not 

 be observed to perspire freely, but that is simply because as fast 

 as the perspiration is secreted it is evaporated. It is what is 

 called insensible perspiration, {b) More water is secreted by 

 the bronchial mucous membranes in the tropics, and in con- 

 sequence of the higher temperature of the air it, like the per- 

 spiration, evaporates very quickly. The excessive secretion of 

 moisture by the body when the temperature of the air is high, 

 is shown in a Turkish bath, and leads, in a bath of about two 

 hours' duration, to a loss of weight amounting with some 

 persons to three pounds, and to a great diminution in tiie 

 quantity of urine secreted, (c) In the tropics the vessels of the 

 skin are more widely dilated than in the arctic regions, hence 

 there is more blood in it, and therefore heat is more readily 

 radiated and conducted from the skin to the external atmosphere. 

 {d) The specific heat of the body is very high, and so it cools 

 very slowly in the arctic regions. Judging from some experi- 

 ments I have made on animals, it is, at the usual temperature of 

 the human body, well over I'o. {e) The above facts are cer- 

 tain, but in addition, for all we know to the contrary, the skin 

 may, under difTerent conditions, have difTerent radiating powers 

 quite apart from the quantity of blood in it. 



In the second place, although it has not been calorimetrically 

 proved, it is very highly probable that in the arctic regions the 

 quantity of heat produced by the body is much greater than in 

 the tiopics. 



With regard to the second query of Mr. Cumming, no doubt, 

 as he says, the human body in the tropics must often be the 

 coolest of surrounding objects ; in this case it cannot lose any- 

 thing by radiation or conduction, but it is kept cool by the 

 rapid evaporation of perspiration (usually insensible) and fluid 

 secreted by the bronchial mucou-; membrane. Whether or not 

 a man in the tropics produces any heat under such circumstances 

 has not been demonstrated, but probably, although the produc- 

 tion of heat falls very low, it does not entirely cease. 



65 Harley-street, W. W. Hale White. 



Photographic Dry Plates. 

 I HAVE found great difficulty in obtaining fresh photographic 

 dry plates of whatever maker, from dealers, who frequently pass 

 off upon the purchasers packets of plates which have been in 

 stock for a long time, and consequently unfit for use. It has 

 therefore occurred to me that this trouble might be avoided by 

 the makers dating every packet as issued l)y them, thus following 

 the custom of the Plaiinotype Company with their tins of paper. 

 By such a system the purchaser would be able to protect himself, 



