78 THE CHEMICAL CONSTITUENTS OF THE BODY 



About the same products (with the exception of carbohydrates) have been 

 obtained by cleavage of the keratin molecule, as from the typical proteids. 



C. Albumoid, a substance obtained from the cartilage of the trachea, which 

 stands in certain respects between the true proteid and keratin. Another albu- 

 moid occurs in the fibers of the crystalline lens. 



D. Elostin, a yellowish white, amorphous substance containing only a small 

 quantity of sulphur; insoluble in water, alcohol and ether; and attacked by 

 chemical reagents with great difficulty. It occurs in connective tissue and espe- 

 cially in the ligamentum nuchse. 



Among the cleavage products of proteid which have been obtained from 

 elastin, are the guanidin rest, leucin, diamino acids, tyrosin and iridol, but 

 neither glutamic acid nor carbohydrates. 



E. Collagen, a sulphur-containing substance; insoluble in water, salt solu- 

 tions, and very weak acids and bases, but swelling up in less dilute acids; it is 

 the chief constituent of fibrillar connective tissue and occurs in bone (ossein) 

 and other tissues. 



When collagen is boiled for a long time with water, it passes over into 

 gelatin (glutin). 



F. Gelatin, a colorless, amorphous, nondiffusible substance of about the 

 same chemical constitution as collagen. It swells up in cold water and is dis- 

 solved in warm water. On cooling the solution sets into a jellylike mass. 



Neither carbohydrates, cystin, serin, tyrosin, nor indol occurs in the mole- 

 cule of gelatin. Sixteen and six-tenths per cent of the total nitrogen is present 

 as arginin, and not less than 8.4 per cent in the form of glycocoll. 



Gelatin solutions are not coagulated on boiling, and are not precipitated by 

 mineral acids, acetic acid, alum, lead acetate or metallic salts. They are pre- 

 cipitated by tannic acid in the presence of salt, by acetic acid and XaCl in 

 substance, by mercuric chloride in the presence of HC1 and NaCl. 



By prolonged cooking gelatin is transformed into a nongelatinizing modi- 

 fication. It is acted upon by the digestive enzymes yielding gelatin albumoses 

 and peptones. 



G. Reticulin, a constituent of the connective tissue framework, of the lymph 

 glands, of the spleen, of the intestinal mucosa, the liver, kidneys and alveoli of 

 the lungs. 



D. OTHER NITROGENOUS SUBSTANCES 



Among the remaining nitrogenous substances which may be isolated from 

 animal tissues, lecithin should be especially mentioned. It is found in almost 

 all animal and plant cells, especially in the brain, the nerves, the blood cor- 

 puscles and in egg yolk ; it occurs also in almost all animal fluids. 



Lecithin represents an esterlike compound of a base, cholin, oxyethyl- 



O'TT OH 

 trimethyl-ammonium-hydroxide, nTjVfCH ") OH W ^ ^y cer i n 'P nos P nor i c 



acid which is united with two fatty acid radicles into a glyceride. There 

 are different lecithins therefore according to the kind of the fatty acid 

 radicles. The stearic-palmitic acid lecithin to be obtained from egg yolk, 



O.COC 17 H 35 

 ^sHsO- COC ir ,H 31 . (CH 3 ) 3 -^ .--r- takes the form of a crystalline, waxlike mass 



readily soluble in alcohol and ether. With water it swells up forming an opales- 

 cent solution, from which it is precipitated again by means of different salts. 

 Lecithin is found in several cases in loose combination with proteid or 



