106 THE HUMAN BODY . 



abundant water already present in the Body and cannot be iden- 

 tified as can the gaseous carbon dioxid. 



The Chemistry of Muscular Contraction. In addition to the 

 fact just stated, that there is oxidation of fuel substances, with 

 production of carbon dioxid and water, about the only definite 

 chemical process we know to be associated with muscular activity 

 is the production of lactic acid (p. 16). Lactic acid is chemically 

 closely related to dextrose in that one molecule of dextrose can 

 be split into two molecules of lactic acid with no residue. The re- 

 lationship is expressed by the equation C 6 Hi 2 O c =2 C 3 H 6 O 3 , the 

 latter symbol being that of lactic acid. For a long time the lactic 

 acid that appears in active muscles was supposed to be merely a 

 stage in the oxidation of dextrose, and its invariable appearance 

 was taken to mean that dextrose is the only fuel that muscles are 

 able to use. This conception involves the view that before muscles 

 can use fats as fuel the fats must be converted into dextrose. 

 Chemically the conversion of fat into sugar is exceedingly difficult, 

 and it seemed a very remarkable thing that a chemical transforma- 

 tion so hard to bring about in the laboratory should occur con- 

 stantly in the Body. As a result of this difficulty a very celebrated 

 controversy has arisen in Physiology over the question of whether 

 or not fats are converted into sugar in the Body before being utilized. 



Of recent years evidence has been accumulating that the lactic 

 acid which appears in active muscles is not a mere incidental stage 

 in the transformation of dextrose, but an essential feature of the 

 contraction process. In fact the view held at present by many 

 physiologists is that lactic acid is associated in intimate fashion 

 with the mechanical act of shortening, but is not involved at all in 

 the chemical processes by which the energy for the contraction is 

 obtained. In other words, lactic acid is not a fuel substance, and 

 the fact that it can be obtained from sugar by a simple splitting of 

 the molecule does not prove that it is so derived in the muscle. 



The importance of this newer conception is that it releases us 

 from the necessity "of supposing sugar to be the only possible fuel 

 for muscles. The question of whether or not fats are transformed 

 into sugar in the Body, instead of being absolutely fundamental, 

 becomes an interesting problem in Biological Chemistry. 



The Energy Relationships of Contracting Muscle. Muscular 

 Efficiency. Since the muscle is an engine for the conversion of 



