28 ESSENTIALS OF PHYSIOLOGY. 



contains lactic acid is exposed for some hours in pure oxygen, the 

 amount of lactic acid in it decreases. Evidently the muscles possess 

 the power of destroying lactic acid. 



This process also takes place in the living animal, but if the supply 

 of oxygen to the muscles is deficient, e.g. during asphyxia, or if the 

 formation of lactic acid is very rapid, for example in actively contract- 

 ing muscles, the acid is not completely oxidised and passes into the 

 blood and may be excreted in the urine. In man the excretion of 

 lactic acid in the urine is 3 to 4 mgr. per hour during rest ; and this may 

 be raised by severe exercise to 400 mgr. or more hourly. Hence the 

 appearance of lactic acid in increased amount in the blood and urine 

 may be taken as evidence that the supply of oxygen to the muscles 

 is either absolutely or relatively deficient. Owing to the passage of 

 lactic acid into the circulating blood, the muscles themselves do not 

 become acid in reaction even after severe exercise, though it is said 

 that the muscles of animals hunted to death are acid. 



It is probable that part of the lactic acid is not oxidised, but is 

 synthesised again by the muscles into the carbohydrate compound by 

 the decomposition of which it was originally formed. 



The chemical changes taking place in contracting muscle are not 

 confined to the process just described, and the oxidation of carbo- 

 hydrate, fat, and protein is also increased during muscular activity. 



Heat Production in Muscle. During muscular contraction heat 

 is produced, and the contraction of a large number of muscles, such 

 as occurs during muscular exercise, may be sufficient to raise the 

 temperature of the whole body one or two degrees. The heat produced 

 in a small isolated muscle during a single contraction cannot be 

 measured by a thermometer, but is usually determined by means of 

 a thermopile, which consists of a junction between two metals, the 

 metals being connected with a galvanometer. When the junction is 

 heated, an electrical current is set up and passes through the 

 galvanometer. * In the more recent forms of thermopile the metals 

 used are copper and an alloy called constantan, and the thermopile 

 may consist of a large number of such junctions, which are connected 

 with a string galvanometer. A muscle, such as the frog's sartorius, is 

 placed in contact with these junctions, and can be made to contract 

 by sending a current through electrodes placed one at each end of the 

 muscle. The muscle is fixed at one end and attached at the other, 

 either to a recording lever or to a spring, whereby the tension on the 

 muscle can be varied or the changes in contractile stress occurring 

 during its isometric contraction can be studied. Any production of 

 heat in the muscle gives rise to a current through the thermopile, and 



