CHAP, v.] NUTRITION. 467 



the production of carbonic acid, sinks also ; and that the latter is 

 the cause not the effect of the former is shewn by the fact that 

 the metabolism continues to fall though loss of heat be prevented 

 by surrounding the animal with wrappings of cotton wool. In 

 such a urarized animal, exposure to higher temperatures augments 

 and exposure to lower temperatures diminishes metabolism ; the 

 urarized warm-blooded animal in fact behaves like a cold-blooded 

 animal. Similar, but perhaps not such striking results are gained 

 by division of the medulla oblongata. After this operation the 

 temperature of the body sinks, and the fall, though partly due to 

 increased loss of heat by the skin, caused by the dilated condition of 

 the cutaneous vessels, is also accompanied by diminished metabo- 

 lism and is therefore in part due to diminished production of heat. 

 And when an animal is in this condition, exposure to higher tem- 

 peratures increases and exposure to lower temperatures diminishes 

 the bodily metabolism. We can best explain these results by 

 supposing that, under normal conditions, the muscles, which as we 

 have seen contribute so largely to the total heat of the body, are 

 placed, by means of their motor nerves and the central nervous 

 system, in some special connection with the skin, so that a lowering 

 of the temperature of the skin leads to an increase, while a heigh- 

 tening of the temperature of the skin leads to a decrease, of the 

 muscular metabolism. Further, though the matter has not yet 

 been fully worked out, the centre of this thermotaxic reflex mecha- 

 nism appears to be placed above the medulla oblongata, possibly in 

 the region of the pons varolii. When urari is given, the reflex 

 chain is broken at its muscular end ; when the spinal cord is divided 

 the break is nearer the centre. Whether we should conclude that 

 the working of this reflex mechanism is of such a kind that cold to 

 the skin excites the centre to a heat-producing activity, or of such 

 a kind that warmth to the skin inhibits a previously existing auto- 

 matic activity of the centre, may be left for the present undeter- 

 mined. 



We may add that the muscular metabolism which thus helps 

 to regulate temperature need not involve visible muscular con- 

 tractions, though the heat given out by a muscle will be temporarily 

 increased at every contraction; and that the regulative nervous 

 mechanism may apparently be overborne by an exposure to too 

 great heat or cold. When for instance the cold to which the 

 animal is exposed becomes excessive, the reaction of the thermo- 

 taxic nervous system is powerless against the depressing direct 

 effects, and the metabolism, together with the temperature, sinks. 



Lastly, we have increasing evidence that the phenomena of 

 fever are due, not merely to a derangement of the regulation by 

 loss, though this may be a factor, but also, and indeed chiefly, 

 to an increased production of heat ; for in fever, the production of 

 carbonic acid, and the consumption of oxygen, that is to say, the 

 metabolic changes of the tissues, are increased. 



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