INTRODUCTION. 5 



certain sense be regarded as a simplification of the functional processes. 

 The more localized the separate functions are, and the more they pertain 

 to certain organs only, the greater becomes the possibility of our succeed- 

 ing in studying them* by themselves and explaining them. In this sense 

 the most suitable objects for our study are those highly complicated beings, 

 the vertebrates. 



In order to show the value of the results obtained by physiological- 

 chemical investigation, a few examples may be cited briefly. We will 

 disregard here that part of the field which concerns itself with the knowl- 

 edge of the separate components of our food and the substances which go 

 to form our tissues. In such cases it is evident that the physiological 

 chemist will make use of the same methods for determining the value of 

 his work as does the pure chemist. He will, furthermore, proceed in pre- 

 cisely the same way as we have described for a chemical investigation, and 

 will thus establish the proof for the constitution of a definite compound. 

 Here physiological chemistry is constantly receiving much aid from the 

 field of pure chemistry, and in fact this part of the work has been 

 developed by trained chemists. 



We will choose here, as an example, a question which has been asked 

 repeatedly, namely this: What becomes of a definite compound after it 

 is introduced into the animal organism, in what way is it broken down, 

 and in what form is it finally excreted? We will use for our illustration 

 a very important discovery, rich in results, which we owe to Wohler. 

 This scientist was interested to learn what became of benzoic acid after it 

 was introduced into the intestine. He was unable to detect it in the 

 urine, nor could he find there any substance which he recognized as 

 one of its lower derivatives. On the other hand, he did find in the urine 

 another acid, hippuric acid, which is closely related to benzoic acid. This 

 is formed, as we shall see later on, by the combination of benzoic acid with 

 glycocoll, the latter substance being formed by the hydrolysis of albumin. 

 Hippuric acid on being boiled with strong mineral acids or alkalies is 

 decomposed into these two constituents. Wohler, by establishing the 

 fact that the benzoic acid which he introduced into the organism left it in 

 the form of hippuric acid, proved for the first time that syntheses may take 

 place in animal organisms. In this way the path was broken and the ob- 

 struction to the development of physiological chemistry which had existed 

 for years was removed. Up to that time it was regarded as an established 

 fact that only plant cells were capable of accomplishing synthetical work, 

 while those of the animal organism could only effect decomposition. 

 This first observation of Wohler was particularly fruitful, and inspired 

 a great deal of similar work, so that to-day we are perfectly justified in 

 ascribing complicated syntheses to the action of the animal cells. If we 

 attempt to subject Wohler's method of proof to close critical analysis, we 



