THE CHEMISTRY OF THE ANIMAL BODY. 575 



blood-plasma, butter, egg-yolk, and of fat; likewise visual purplt of the retina, which is 

 bleached by light, Solutions of the pure visual purple from rabbits or dogs become clear 

 as water on exposure to light. 1 



Cholesterin. 



Cholesterin, C 27 H 45 OH. — This is found in all animal and vegetable cells and in the 

 milk. 2 It is especially present in nervous tissue and in blood-corpuscles. It is excreted 

 through the bile and through the intestinal wall. 3 In the blood-plasma it is present as 

 an ester combined with oleic and palmitic acids, while in the corpuscle it occurs as simple 

 cholesterin.* It may be prepared by dissolving gall-stones in hot alcohol, from which 

 solution the cholesterin crystallizes on cooling in characteristic plates. It is insoluble in 

 water or acids, but soluble in the biliary salts, ether, and hot alcohol. Tt is probably not 

 absorbed by the intestinal canal. In human feces stercorin appears instead of choles- 

 terin. 5 This stercorin (the koprosterin of Bondzynski) is a dihydrocholesterin, 8 (\, 7 II, ; OH, 

 and is the result of putrefactive change. 7 Cholesterin feels like a fat to the touch, 

 but is in reality a monatomic alcohol. With concentrated sulphuric acid it yields a 

 hydrocarbon, cholesterilin, C 26 H 4! „ coloring the sulphuric acid red (Salkowski's reaction i. 

 Iso-cholesterin, an isomere, is found combined as an ester with fatty acid in wool-fat or 

 lanolin. The physiological importance of cholesterin is unknown. 



The Proteids. 



Consideration of the proteids from a purely chemical standpoint is impos- 

 sible, for their composition is unknown. There exist only the indices of com- 

 position furnished by the products of cleavage and disintegration. Bodies at 

 present classed as individuals may sometimes be shown to be identical, with 

 characterizing impurities. It remains for the chemist to do for the proteid 

 group what Emil Fischer with phenyl-hydrazin has accomplished for the 

 sugars. 



As a characteristic proteid, egg-albumin may be mentioned. Proteid forms 

 (after water) the largest part of the organized cell, and is found in all the 

 fluids of the body except in urine, sweat, and bile. Proteid contains carbon, 

 hydrogen, nitrogen, oxygen, sulphur, sometimes phosphorus and iron. 



General Reactions. — A neutral solution of proteid (with the exception of 

 the peptones and proteoses) is partially precipitated on boiling, and is quite 

 completely precipitated on careful addition of an acid (acetic) to the boiling 

 solution. Proteids are precipitated in the cold by nitric and the other com- 

 mon mineral acids, by metaphosphoric but not by orthophosphoric acid. 

 Metallic salts, such as lead acetate, copper sulphate, and mercuric chloride, 

 precipitate proteid; as do ferro- and ferricyanide of potassium in acetic-acid 

 solution. Further, saturation of acid solutions of proteid with neutral salts 

 (NaCl, Na 2 S0 4 , (NH 4 ) 2 S0 4 ) precipitates them, as docs likewise alcohol in 



1 Kiihne : Zeitschrift fur Biologie, 1895, Bd. 32, S. 26. 

 •Schmidt-Muhlheim: Pfluger's Arehiv, 1883, Bd. 30, S. 384. 

 3 Moraczewski : Zeitschrift fiir physiologische Chemie, 1898, Bd. 25, S. 122. 

 * Hepner: Pfluger'a Arehiv, 1898, Bd. 73, S. 595. 



5 Flint: American Journal of Medical Sciences, 1862. 



6 Bondzynski and Hnmnicke: Zeitschrift fur physiologische Chemie, 1S96. Bd. 22, B. 39(5. 

 : Miiller, P. : Ibid., 1900, Bd. 29, S. 129. 



