160 SCIENCE PROGRESS 



chemical identity throughout the group of proteid substances. 

 He held therefore that the proteid of the diet remained to all 

 intents and purposes unaltered, digestive processes notwith- 

 standing, as it slipped from the alimentary canal into its place 

 in the living tissue, taking there a position made vacant by the 

 previous breakdown of the tissue proteid, or leading, in other 

 cases, to growth. While, to Liebig, this was the only fate 

 conceivable for proteid eaten by an animal — to enter the tissues 

 as intact proteid — his views were equally clear as to its sub- 

 sequent destiny. Proteid was the sole source of mechanical 

 energy in the body, and was only broken down — could only be 

 broken down — when such mechanical energy was being liberated. 

 The proteid contained in a muscle could undergo metabolism 

 only when the muscle was actually contracting. Thus Liebig 

 came sharply to distinguish between the part played by proteid 

 and that played by fats and carbohydrates. These could be 

 oxidised at any time, and supplied heat, not mechanical energy, 

 for the body. Proteid could supply both, but the former only 

 after being broken down in the course of tissue activity. 



I am tempted to quote two paragraphs from Liebig's Lectures 

 on Animal Chemistry, published in 1842, not only to show how 

 clearly defined were his views on the points referred to, but to 

 give a base from which to measure our present divergence of 

 view. Referring to the proofs of identity in the percentage 

 composition of vegetable and animal proteids, he says : " How 

 beautifully and admirably simple, with the aid of these dis- 

 coveries, appears the process of nutrition in animals, the 

 formation of their organs, in which vitality chiefly resides ! 

 Those vegetable principles which, in animals, are used to form 

 blood, contain the chief constituents of blood — fibrin and 

 albumen — ready formed, as far as regards their composition." 

 And to illustrate his belief in the locking up in the tissues 

 of all proteid consumed, and of its stability therein in the 

 absence of mechanical activity, we have the following — " Man 

 when confined to animal food, respires, like the carnivora, at 

 the expense of the matters produced by the metamorphosis of 

 organised tissues ; and just as the lion, tiger, hyaena, in the 

 cages of a menagerie, are compelled to accelerate the waste of 

 the organised tissues by incessant motion, in order to furnish 

 the matter necessary for respiration, so, the savage, for the very 

 same object, is forced to make the most laborious exertions, 



