UREA NOT A MEASURE OF WORK. 923 



others, as to the increase in the urea excreted during full exercise, 

 and with a full diet, they might, at first, appear to harmonize with the 

 view, that the chemical energy developed by the oxidation of the al- 

 buminoid food, is the source of all the mechanical work performed in 

 the body. The production of the urea is not supposed, by itself, to 

 develop the energy required ; but this substance is an index to the 

 quantity of albuminoid substance oxidated, in the body, into urea, 

 carbonic acid, water, and sulphuric acid. This urea can be easily 

 separated and weighed ; but the carbonic acid and water, derived from 

 the partial oxidation of the albuminoid food, mixing with the much 

 larger quantities derived from the carbhydrates and hydrocarbons, com- 

 pletely escape measurement. 



But the theory of Liebio; (1842-51), as to the special source of the motor 

 power of the system, thus illustrated by arguments and calculations, derived 

 from an advanced state of knowledge concerning the relations between chemi- 

 cal action, heat, and motion, is opposed by many authorities, especially by 

 Mayer, Traube, Bonders, Heidenhain, more recently by Pick and Wislicenus, 

 and, in England, by Lawes and Gilbert, and by Frankland. The experiments 

 of Lehmann, Ed. Smith, Voit, Bisehoff, Speck, and Dr. Parkes, have assisted 

 much to elucidate this subject. 



It was long ago maintained by Mayow, of Bath (1681), that, for the occur- 

 rence of muscular action, combustible material, fat, must be conveyed by the 

 blood to the muscles, together with some principle derived from the air in res- 

 piration. According to Mayer, of Bonn (1845), an early observer in the field 

 of quantitative research as regards heat and its relations to other forms of 

 force, a muscle is not the material by the chemical change of which, mechanical 

 work is produced ; but an apparatus by means of which, the transformation of 

 force is accomplished. If the former were true, he argues that the heart would 

 be completely oxidized, in doing its own work, in 8 days. He believes the 

 capillaries of the muscle to be the seat of the actual changes, and the blood to 

 be the fuel consumed. Traube also has distinctly taught that the substances, 

 by the burning of which, force is generated in the muscles, are not the albumi- 

 noid constituents of those tissues, but non-nitrogenous substances, either fats 

 or carbhydrates. Bonders and Heidenhain coincide in these views. 



It has, moreover, been found that the amount of urea excreted is regulated, 

 not so much by the exercise taken, as by the quantity of albuminoid food which 

 is consumed. Hence much of this substance must be formed independently 

 of the metamorphosis of muscle ; and therefore Haughton's estimates of it, as 

 a measure of work, become seriously invalidated. Lastly, much uncertainty 

 prevails, as to the accuracy of the data employed for the calculations of the 

 heat given off in the oxidation of albuminoid food, and, therefore, as to the 

 correctness of the deductions from them, made by Vierordt, Playfair, and others. 

 Thus Lawes and Gilbert (1852) observed, that of two pigs fed, one on lentils, 

 which contain 4 per cent, of nitrogen, and the other on barley, which contains 

 only 2 per cent., the excreta of the former yielded twice as much nitrogen as 

 those of the latter ; from which they infer that the quantity of urea excreted 

 i. e., of albuminoid substance decomposed, is no guide to the amount of work 

 done, the exercise taken having been the same in each case, but that it depends 

 on the quantity of nitrogenous food consumed. They, moreover, conclude that 

 some of the muscular power depends on the oxidation of non-nitrogenous sub- 

 stances. Again, the researches of Edw. Smith, Voit, Lehmann, Bisehoff, and 

 Parkes, indicate that the urea excreted bears no definite relation to the labor 

 performed ; and that in prolonged exercise, the increase of urea is very small. 

 The effects of treadwheel labor serve only to increase the quantitv of urea, by 

 19 grains in 24 hours, as compared with that eliminated in easy labor. (Smith.) 

 In fasting animals, the effects of increased exertion are also very slight as 

 regards the urea, and seem to be regulated by the periods of ingestion of water, 



