4 PHYSIOLOGICAL CHEMISTRY. 



some syntheses are constantly taking place which are com- 

 monly overlooked because of the much greater importance of 

 the oxidation reactions. In recent years low forms of animal 

 life have been found which contain chlorophyll grains and 

 which are able to produce oxygen in presence of sunlight. In 

 some of these cases the chlorophyll may be present in a sym- 

 biont organism, but in others it appears to be diffuse, and 

 therefore brings the animal structure containing it into close 

 relation with vegetable cells. 



In the essential phenomena of life plants and animals have, 

 then, much in common; it is only when we follow them into 

 details that the characteristic differences appear. 



From the nature of the materials entering into the struc- 

 ture of plants and animals it follows that the discussions of 

 physiological chemistry are, in the main, but special cases 

 of the general field of organic chemistry. The important 

 nutrients, the carbohydrates, the fats and the protein sub- 

 stances, are all organic and the products appearing as stages 

 in their metabolism are also organic. Therefore much which 

 the student has met with in his study of organic chemistry 

 may be found repeated in his work in physiological chemistry. 

 Inasmuch as many of the relations to be now traced out are 

 quantitative, it is highly important, also, that the student 

 should bring to the work before him a good knowledge of the 

 principles of volumetric analysis, as these will be applied fre- 

 quently in what is to follow. 



Historical. To trace the beginnings of Physiological 

 Chemistry we are not obliged to go far back in the develop- 

 ment of science. With the old medical chemistry of the 

 so-called iatro school it has nothing in common, and in fact 

 is in no sense a development of that science of the sixteenth 

 and seventeenth centuries. Before the days of Lavoisier, it is 

 true, some little advance had been made in the study of bodies 

 of animal or vegetable origin, but without a rational theory 

 of chemical combination the isolated facts established led to 

 little of real value. With the nature of respiration explained, 



