METABOLISM 237 



if the body's heat increases from any cause whatever, metabolism is 

 heightened. In addition to these modes of increasing heat-production 

 there is apparently, in small animals in particular, an influence exerted 

 by the nervous system directly on the heat-forming tissues. This acts 

 especially on the muscles, and causes them to increase the body-tempera- 

 ture without actual (visible) contraction. This has been called "chemic 

 tone," and in case of muscle differs little from muscular tone (see below, 

 page 390), but when effected in other tissues, directly increasing metab- 

 olism, it is doubtless a portion of that little-understood system of trophic 

 influences (see page 56). Various drugs, finally, increase metabolic 

 heat, these being largely those like strychnine, e. g., which increase mus- 

 cular tone or muscular activity of a more obvious sort. 



Muscular movements are instinctively employed by all animals, 

 homo therms at least, for increasing their temperature. If one compares, 

 for example, in this respect the people on the pavements in winter and in 

 summer, the difference in their muscular activities is obvious. Shivering 

 is a reflex action with considerable thermogenic powers. 



THE REGULATION OF HEAT-LOSS is probably a more active function 

 than is the control of thermogenesis. It is largely performed by means 

 of vaso-motion (enlargement and narrowing of the arterioles) in connec- 

 tion with the secretion of sweat, both being under the direction of the 

 nervous system. (See page 300.) 



When the body-temperature tends to become unduly low from any 

 cause, impulses are sent out from the medulla's vaso-constrictor centers 

 to the arterioles and capillaries of the skin all over the body, and these 

 thereupon become smaller in diameter. This drives much of the blood 

 then on the surface into the body's interior. As will be recalled, about 

 88 per cent, of the total heat lost is expended from the skin, the amount 

 depending on the quantity of warm blood the surface contains. By this 

 peripheral vaso-constriction radiation and conduction are lessened as 

 well as the production of "insensible" sweat, by whose evaporation 

 much heat is lost. It is by a too sudden and too vigorous action of this 

 vaso-constrictor mechanism that congestions of the nasal mucosa, lungs, 

 kidneys, or ovaries are sometimes produced. Normally the process 

 takes place gradually and the circulation adjusts itself so that no 

 organ is harmfully over-charged with blood. By this means the latter, 

 carrying so much vital heat with it in the course of a minute, 

 is removed from the surface, whence that heat would be partly 

 lost. Exposure to cold causes the skin to become blanched, but if it be 

 excessive the nerves or muscles of the arterioles or both are paralyzed, 

 and the blood-vessels expand widely under the pressure from the heart, 

 making the skin red. Chronic alcoholism, because of its continued 

 surface vaso-dilating effect, has the same influence on the skin. Owing 

 to the reciprocal action between the skin and the kidneys, the vaso- 

 constriction in the former tends to increase the flow of urine. Only 

 1 or 2 per cent., however, of the body's heat is given off in the urine, so 

 that this opposing effect counts but little in increasing thermolysis. 



