TEE EI STORY OF DIETETICS 425 



Liebig in 1840; by Beneke in England in 1851 ; and in this country by 

 John Stanton Gould in 1852 and Atwater in 1886. Since then many 

 such studies have been made and a large amount of information 

 collected. 



A knowledge of the processes of metabolism forms another com- 

 ponent of the foundations of dietetics. 



Probably the earliest real metabolism studies were prosecuted by 

 Sanctorius (1561-1636), who published his results in 1614. Sitting in 

 a chair suspended from steelyards, he observed the changes in weight 

 from eating and from the loss of insensible perspiration. 



Theories of the source of animal heat were presented from the early 

 days of physiology. Van Helmont held that animal heat was generated 

 by fermentation of the blood in the heart. Sylvius believed it to be pro- 

 duced by the supposed effervescence resulting from the mixture of 

 venous blood with acid chyle. Mayow in 1668 anticipated the modern 

 view that the body energy is derived from oxidation in the tissues ; but 

 his views were soon forgotten. Toward the end of the seventeenth cen- 

 tury these chemical theories were largely superseded by the physical view 

 that animal heat is generated by the friction of the blood in the capil- 

 laries. It was not until after the discovery of oxygen that our present 

 conceptions of the part played by oxidation as the source of animal 

 energy were founded by Lavoisier. 



The general outlines of our knowledge of metabolism were formu- 

 lated by Liebig, the details being worked out by numerous subsequent 

 investigators, many of them under his personal influence. In this way, 

 chiefly since 1850, an extensive mass of data on this subject has been 

 accumulated. The earliest metabolism study appears to have been one 

 by Lehmann, made in 1839. The study of the metabolism of nitrogen 

 is a comparatively simple matter, and has been an easy and frequent 

 subject for investigation. 



The determination of the exchanges of carbon and hydrogen is a 

 much more difficult matter, involving the collection of the products of 

 respiration and requiring elaborate apparatus and an amount of labor 

 rarely available. Early respiration experiments with animals were 

 made by Boussingault about 1844, Bidder and Schmidt in 1847-1850, 

 and Eegnault and Eeiset in 1849 ; and with man by Barral in 1847- 

 1848, and Hildesheim in 1856. 



A most extensive and accurate series of investigations was con- 

 ducted by Max von Pettenkofer (1818-1901) and, especially, Carl von 

 Voit (1831-1908), in the Physiological Institute at Munich with the 

 respiration apparatus constructed by Pettenkofer about 1860. Animals 

 and men were made the subjects of extended observation with this 

 apparatus from 1861 to 1867, and principles of fundamental importance 

 were established by these classical researches. 



Other important studies of metabolism were prosecuted by Pniiger 



