514 HISTORICAL. 



(4.) Ammonia Derivatives of Unknown Constitution. iTHc add, 



allantoin (see Urine) is formed by the oxidation of uric acid by means of 

 potassium permanganate; cyanuric add in dog's urine; inosinic add in muscle; 

 guanin in traces in the liver and pancreas, in guano, the excrements of spiders, in 

 the skin of amphibia and reptiles, in the silver sheen of many fishes (A. Ewald and 

 Krukenberg); by oxidation it yields urea; hypoxanthin or sarTcin occurs along with 

 xanthin in many organs and in urine. Kossel prepared hypoxanthin from nuclein 

 by prolonged boiling of the latter. It may be obtained from fibrin by putrefac- 

 tion, by gastric and pancreatic digestion, and by dilute acids (Salomon, H. Krause, 

 Chittenden); xanthin is prepared by oxidation from hypoxanthin. It occurs very 

 rarely in the form of a urinary calculus. Paraxanthin in urine, and a similar body 

 carnin in flesh ( 233). 



Aromatic Substances. 



1. Monatomic phenols () Phenol (hydroxyl of benzol) in the intestine 

 (p. 376). Phenylsulphuric acid in urine. (6) Kresol in the form of orthokresol 

 and parolcresol, united with sulphuric acid, occur in urine. 2- Diatomic phenols 

 (a) Benzkatechin united with sulphuric acid in urine. 3. Aromatic oxyacids 

 (a) Hydroparacumaric acid; (&) Paraoxyphenylacetic add in urine. 4. Indol 

 and skatol in the intestine (p. 376), conjoined with sulphuric acid in urine. 



253. Historical. 



According to Aristotle, the organism requires food for three purposes for 

 growth, for the production of heat, and to compensate for the loss of the bodily 

 excreta. The formation of heat takes place in the heart by a process of concoc- 

 tion, the heat so formed being distributed to all parts of the body by means of the 

 blood, while the respiration is regarded as an act whereby the body is cooled. 

 Galen accepted this view in a somewhat modified form; according to him, the 

 metabolic processes may be compared to the processes going on in a lamp; the blood 

 represents the oil; the heart, the wick; the lungs, the fanning apparatus. 

 According to the view of the iatrochemical school (van Helmont), the metabolic 

 processes of the body are fermentations, whereby the food is mixed with the juices 

 of the body. Since the middle of the seventeenth century (Boyle), the knowledge 

 of the metabolic processes has followed the development of chemistry. A. v. 

 Haller regarded heat as due to chemical processes the food continually supplying 

 the waste which is excreted from the body. After the discovery of oxygen (1774, 

 by Priestley and Scheele), Lavoisier formulated the theory of combustion in the 

 lungs, whereby carbonic acid and water were formed. Mitscherlich compared the 

 decomposition-processes in the living body with putrefactive processes. Magendie 

 was the first to emphasise the difference between nitrogenous and non-nitrogenous 

 foods, and he showed that the latter alone were not able to support life. Even 

 gelatin alone is not sufficient for this purpose. 



The greatest advance in the theory of nutrition was made by J. v. Liebig, who 

 laid the foundation of our present knowledge of this subject. According to Liebig, 

 foods may be divided into two classes, viz., the "plastic," suitable for the 

 construction of the organism, and the ' ' respiratory " for the maintenance of the 

 temperature ; to the former class he referred the albuminates or proteids, to the 

 latter, the non-nitrogenous carbohydrates and fats. 



Amongst recent observers, the Munich School, as represented by v. Bischoff, 

 v. Pettenkofer and v. Voit, has done most to give us an exact knowledge of this 

 department of physiology. 



