THE PHYSIOLOGICAL PROBLEMS OF ToDay 499 
d-glacose, d-mannose, and (-galactose are fermentable, while 
their stereoisomeres are not fermentable. But the influence 
of the geometrical configuration goes farther. Voit has 
suggested and Cremer has demonstrated that there is a far- 
reaching parallelism between the fermentability and assimila- 
tion of carbohydrates. Higher animals as well as yeast cells 
are able to form glycogen from such carbohydrates as are 
fermentable by yeast. The further development of these 
stereochemical relations and their extension to proteids and 
nucleins is another of the problems of physiology which will 
contribute to the main problem, the analysis of the constitution 
of living matter. I believe that the influence of stereochem- 
istry will be more or less directly felt in many branches of 
physiology, in questions of heredity, as well as in the theory 
of space-sensations, as E. Mach has already intimated. 
In living organisms chemical energy is frequently trans- 
formed into osmotic energy. Van’t Hoff’s theory of osmotic 
pressure permits an application of the law of conservation of 
energy to a class of phenomena to which this law was hith- 
erto inapplicable, namely, the phenomena of growth, func- 
tional adaptation, secretion, absorption, and even pathological 
processes such as cedema. The physiologists who thought 
that the blood-pressure determined secretion could not under- 
stand why secretion took place under a higher pressure than 
the blood-pressure. Comparative physiology shows that 
secretion does not depend upon circulation, and the theory 
of osmotic pressure indicates that the osmotic pressure in the 
cells is more than twenty times as high as the blood-pressure. 
The work of secretion is done by osmotic pressure and not 
by blood-pressure. A prominent physiological chemist has 
become a vitalist because he could not explain why the secre- 
tions differ from the blood from which he thinks they are 
formed. He overlooks among others the fact that the pro- 
toplasm possesses the quality of semi-permeability, which 
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