2 LECTURE I. 



interesting biological discoveries upon a firm foundation; yet, neverthe- 

 less, the investigations of the physiological chemist are obliged to halt for 

 the present before the individual cell. Here for a time is our boundary, 

 but that it will not prove insurmountable is shown by recent investigations. 



Physiological chemistry seeks to accomplish still another result. We 

 desire to know not merely the manner in which each separate substance 

 passes through the body, and how it is broken down, but we also wish to 

 know its relations to the other compounds which are likewise introduced 

 into the body. This is true especially for our organic food-stuffs. We 

 wish to know whether they can mutually replace and supplement one 

 another, and whether it is possible for a representative of one class of 

 foods to exercise a function usually assumed by another. 



Beyond these limits, physiological chemistry sets itself a number of 

 other tasks which are now only just beginning to be attacked. As little 

 as we are content with the discovery of the anatomical structure of a 

 definite organism, but seek to understand clearly its ontogenesis and 

 phylogenesis, just so little should we be satisfied to trace in the case of a 

 single individual, or in only one species, all of the processes which may 

 be referred back to chemical decompositions. Comparative physiological- 

 chemical research is called upon to explain clearly certain important 

 processes which now appear enigmatical to us. On the other hand, com- 

 parative chemical investigation concerning the nature of the bodies and 

 the metabolism of individuals belonging to different animal species, will 

 give a new support for purely morphological research. The gradual 

 evolution in the entire animal kingdom takes place step by step parallel 

 to an ever more delicate and ever more marked specialization of the single 

 organs. In many cases where the histology of an organ permits some 

 doubt to arise as to where it belongs, the determination of its function 

 often gives a clear decision. 



More and more the investigation of physiological-chemical processes in 

 the animal organism extends beyond its more narrow field. For a long 

 time it has been evident that there is no sharp boundary between the 

 animal and vegetable kingdoms. More and more it becomes recognized 

 that the line once drawn was an entirely artificial one. We now know 

 that countless processes take place in plant and animal organisms which 

 are common to both. The more this uniformity has found expression, 

 the more sharply defined have become the differences in the metabolism 

 of the different members of each kingdom. Everywhere there are tran- 

 sition stages, and nowhere do we meet with sudden changes. To-day there 

 is no longer any doubt but that a complete understanding of the pro- 

 cesses transpiring within the animal organism is only possible when atten- 

 tion is paid as well to those changes taking place in plant organisms. We 

 desire to know, moreover, the source of each article of food, how it has been 



