HISTORICAL. 385 



( 170) = amido-caproic acid, (c) Serin ( = ? amido-lactic acid) obtained from silk -gelatin. 

 (d) Aspartic acid (amido-succinic acid) ; and (e) Glutamic acid, obtained by the splitting up 

 of proteids ( 170). Other amido-acids are (f) Cystin = amido-lactic acid, in which O is 

 replaced by S ( 268). (g) Taurin ( 177), amido-ethyl-sulphonic acid occurs (except in certain 

 glands) chiefly in combination with cholalic acid, as taurocholic acid in bile. Tyrosin (para- 

 hydro-oxyphenyl-amido-propionic acid), an amido-acid of unknown constitution, occurs along 

 with leucin during pancreatic digestion ( 170), is a decomposition-product of proteids, and 

 occurs plentifully in the urine in acute yellow atrophy of the liver ( 269). 



To the amido-acids are related (a) Kreatin in muscle, brain, blood, urine, regarded as 

 raethyl-uramido-acetic acid (C 4 H 9 N 3 2 ). It has been prepared artificially. When boiled with 

 baryta-water, it takes up H 2 0, and splits into urea and (b) Sarkosin (C 3 H 7 N0 2 ), methyl- 

 amido-acetic acid. When boiled with water, heated with strong acids, in the presence of 

 putrefying substances, kreatin gives off water, and is changed into kreatinin (C 4 H 7 N 3 0). This- 

 strong base can be rechanged by alkalies into kreatin. 



(4) Ammonia Derivatives of Unknown Constitution. Uric acid ( 258) ; allantoin ( 260), 

 is formed by the oxidation of uric acid by means of potassium permanganate ; cyanuric acid 

 in dog's urine ; inosinic acid 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 (p. 439); hypoxanthin or sarkin 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 putrefaction, by gastric 

 and pancreatic digestion, and by dilute acids {Salomon, H. Krause, Chittenden) ; xanthin is pre- 

 pared 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). [Adenin (C 5 H 5 N 5 ), dis- 

 covered by Kossel in the pancreas, yeast, and tea-leaves, has also been isolated from the spleen, 

 lymphatic glands, and kidney ; it appears to be present in all highly cellular animal and vege- 

 table tissues. Like the allied bases xanthin and guanin, it is a derivative of the nuclein of 

 the nuclei.] 



Aromatic Substances. 



1. Monatomic phenols (a) Phenol (hydroxyl of benzol) in the intestine ( 184). Phenyl- 

 sulphonic acid in urine ( 262). (b) Kresol, in the form of orthokresol and parakresol, united 

 with sulphonic acid, occur in urine ( 262). 2. Diatomic phenols (a) pyrokatechin united 

 with sulphonic acid in urine ( 262). 3. Aromatic oxyacids (a) Hydroparacumaric acid ; (b) 

 Paraoxyphenylacetic acid in urine ( 262). 4. Indol and skatol in the intestine ( 184), con- 

 joined with sulphonic acid in urine ( 262). 



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 concoction, the heat so formed 

 being distributed to all parts of the body by means of the blood, while the respiration is re- 

 garded 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 decomposi- 

 tion-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 (p. 356). 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. 



2 B 



