INCOME AND OUTGO 339 



special case of muscular activity. The simple fact is 

 that cold leads to an increase in reflex tonus, to shiver- 

 ing, and to instinctive briskness of movement. The 

 result is thus a form of exercise. 



The change from fasting to feeding influences the 

 metabolism much less than might be anticipated. It 

 is true that a fasting animal will usually be found to 

 have a low metabolism but this is because it is disposed 

 to be quiet. If it is given food, and remains as inactive 

 as before, the increase in oxidation will be very moderate. 

 It is not likely to exceed 20 per cent., whereas mild 

 muscular activity will double the original quantity. 

 The tendency of protein food to increase the metabolism 

 is much more marked than that of the non-nitrogenous 

 types. 



Changes in the state of the tissues and fluids of the 

 body as a result of the altered activity of certain organs 

 may considerably change the total metabolism. This 

 is particularly true of the thyroid gland, a structure 

 which will be more fully dealt with by and by. The 

 general statement may be made here that abnormal 

 activity of the thyroid means a high rate of decomposi- 

 tion among the stores of the body. Accordingly, people 

 with overactive thyroids become emaciated and the 

 extract of the organ is often used in patent medicines 

 intended to reduce weight. Deficient thyroid activity 

 may help to cause obesity in certain cases, the oxidation 

 in the tissues lagging behind the assimilation of new 

 material. 



Small animals have a greater average metabolism, 

 weight for weight, than larger ones. This is true whether 

 we have reference to the young and adults of one species 

 or to the adults of two species, as mice and rats. It 

 has been shown that the metabolism is more nearly 

 proportional to surface than to weight. It is a principle 

 of geometry that the smaller one of two solids of similar 

 form has more surface in proportion to its mass than 

 the larger. If a mouse is half as long as a rat and of 



