330 HAND-BOOK OF PHYSIOLOGY. 



about 70 C, it undergoes coagulation and loses most of its original char- 

 acters. It becomes insoluble both in water and in saline solutions, and 

 although soluble in strong acids and alkalies in boiling, partially decom- 

 poses during the process. They are not soluble in dilute acids or alka- 

 lies, but dissolve freely under the action of the gastric or of the pancre- 

 atic secretion, being converted into peptones. 



VII. Lardacein. Lardacein or amyloid substance is found in cer- 

 tain organs of the body, chiefly in the liver, as a morbid deposit. It is 

 insoluble in water, and in saline solutions. It is unacted upon by the 

 digestive juices. It is colored red by iodine. It is soluble in acids or in 

 alkalies, thus forming acid or alkali albumin. 



(2.) Gelatinous principles include: (1.) Gelatin; (2.) Mucin; (3.) 

 Elastin; (4.) Chondrin; and (5.) Keratin. They are very like the Pro- 

 teid group, but exhibit considerable differences among themselves. 



(1.) Gelatin is produced by boiling fibrous tissue, or by treating bones 

 with acids, whereby their salts are dissolved, leaving the framework of 

 gelatin, which is soluble in hot water. 



It is a yellow, amorphous, transparent body, which does not give any 

 of the proteid reactions if pure, insoluble in cold, but soluble in hot 

 water, forming a jelly on cooling. Its solutions are precipitated by tan- 

 nin, by alcohol and by mercuric chloride. 



(2.) Mucin, contained in mucus. It is a substance of ropy consist- 

 ency. 



Prepared from ox-gall by precipitation with alcohol, and afterward 

 redissolving in water, and reprecipitating with acetic acid. It may be also 

 prepared from diluting mucus with water, filtering, treating the insoluble 

 portion with weak caustic alkali, and precipitating with acetic acid. It 

 is precipitated by alcohol and mineral acids, but dissolved by excess of 

 the latter^dissolved by alkalies. It gives the proteid reaction with Mil- 

 Ion's reagent, but not with cupric sulphate and potassium hydrate. It is 

 not precipitated by mercuric chloride or by tannic acid. It is a colloid 

 substance. 



(3.) Elastin is the basis of elastic tissue; it is soluble only in strong 

 alkalies on boiling; strong sulphuric or nitric acid dissolves it in the cold. 



(4.) Chondrin is contained in the matrix of hyaline cartilage, and 

 may be extracted by boiling with water and precipitating with acetic 

 acid. 



(5.) Keratin is obtained from hair, nails, and dried skin. It contains 

 sulphur, evidently only loosely combined. % 



(3.) Decomposition Nitrogenous products. These are formed by the 

 chemical actions which go on in digestion, secretion, and nutrition. 



Most of the compounds are amides, Avhich are acids in which amidogen 9 

 NH 2 , is substituted for liydroxijl, OH. Amides may also be represented 

 as obtained from the ammonium salts by abstraction of water, or as de- 

 rived from one or more molecules of ammonia, JSTH 3 , by substituting 

 acid radicals for hydrogen. Thus acetamide may be written in any of 

 the following ways: 



