58 



way, in a manner not yet understood by physiological chemists, it is 

 made to form part of this substance urea and in the form of that com- 

 pound it is separated from the body. 



A very pertinent question, and one of the greatest importance is 

 this : What is the special function of nitrogen in the animal economy ? 

 To what purpose is this continuous stream of it which passes through 

 the body? Why are the albuminoids so essential to life, apart alto- 

 gether from their carbon which goes partly to sustain the animal heat ? 

 Liebig's theory is well known and it is probably the one which to-day, 

 in spite of its defects, finds widest acceptance. It is simply this : The 

 conversion of part of the substance of the muscles into urea produces 

 the power which the muscles require in performing movement and 

 work. The nitrogen which is discharged from the body is therefore the 

 equivalent of the transformed fibre, and therefore of the developed 

 power and of the accomplished work. But there have been many 

 objections to this teaching. It has been maintained that the muscles 

 do not form the material by the chemical transformation of which 

 power is produced, but only the apparatus in which the change is 

 effected. Voit showed that, although the supply of albuminoids to an 

 animal might remain unchanged, the mechanical work performed by 

 that animal mieht be increased at pleasure, and that without provoking 

 any increase in the amount of nitrogen discharged. Lawes and Gilbert 

 too proved that this quantity depended entirely upon the 

 nitrogen contents of the food, and therefore that the consumption of the 

 muscle substances was entirely independent of the work accomplished. 

 But since the muscular power must have come from the nourishment 

 some opponents of Liebig's theory have sought its cause in the com- 

 bustion of the non-nitrogenous nutrients, and they feel themselves the 

 more justified in doing this because Smith, Von Pettenkofer and Voit 

 had established beyond doubt the fact that, with the increase of muscular 

 activity, the quantity of carbonic acid exhaled from the lungs increased 

 also. In 1870 Liebig admitted the defects of his first theory and 

 and brought forward a revision or modification of it. He felt himself, 

 however, obliged to admit that even his new explanations were not 

 entirely satisfactory, and declared that the true theory of the origin of 

 muscular power had not yet been discovered and could only be 



