At the same time the amount of carbonic acid expired is increased. Its amount bears a nearly constant relation to the amount of muscular exercise, and consequently the amount of carbonic acid eliminated in various forms of exercise affords a just estimate of their relative value. The increased elimination of carbonic acid, the corresponding increased absorption of oxygen, and the absence of increase of elimination of urea are shown in the following summary of observations by Pettenkofer and Voit:— ┌──────────┬─────────────┬───────────────────────────┐ │ │ABSORPTION OF│ ELIMINATION IN GRAMMES OF│ │ │ OXYGEN ├──────────────┬──────┬─────┤ │ │ IN GRAMMES.│CARBONIC ACID.│WATER.│UREA.│ ├──────────┼─────────────┼──────────────┼──────┼─────┤ │_Work day_│ 955 │ 1284 │ 2042 │ 37·0│ │_Rest day_│ 709 │ 912 │ 828 │ 37·2│ └──────────┴─────────────┴──────────────┴──────┴─────┘ The above amounts are for the entire day. During actual exercise the excess of elimination of carbonic acid is much greater. Thus, Dr. E. Smith experimentally found that if the amount of carbonic acid eliminated during rest be represented by one, the amount walking at two miles an hour and carrying 7 lbs = 1·85, the amount walking at three miles an hour = 2·64. Alcohol diminishes the excretion of carbonic acid, and should therefore be avoided during muscular training. By muscular exercise the size of the lungs is increased, and their _vital capacity_, that is, the amount of air capable of being expired after a forced inspiration, is considerably increased. Corresponding with this increase of vital capacity, exercise, especially that in which the arm and chest muscles are systematically developed, increases the size of the chest. A perceptible difference in the circumference of the chest may be noticed after only a few weeks’ methodical exercise. 3. _The action of the skin is increased._—Sensible perspiration is commonly induced, but less readily in those habituated to hard work. Insensible perspiration is always increased. 4. _The temperature of the body_ is not increased, so long as perspiration occurs. Every muscular contraction involves the production of heat; but this is counteracted by increased evaporation from the skin, and by the circulatory current carrying the hotter blood to every part of the body, and so rapidly equalising its temperature. Chilblains are due to the defective circulation of the blood, and can in most cases be cured by active exercise aided by warmer clothing and an abundant supply of oxidisable food. 5. _The Heart and Blood-vessels._—By exercise the heart’s action is increased in frequency and force. The pulse usually increases from ten to thirty beats per minute above the rate while at rest. After prolonged exercise it may temporarily fall below the normal standard.