PEOTEIDS. 415 



insoluble in water and in neutral salt solutions, are readily dissolved 

 by dilute acids, and also by diluted alkaline solutions, most likely 

 with conversion into alkali-albumins. 



6. Syntonin is the name given to the acid-albumin obtained by 

 digesting myosin with very weak (0.1 to 0.2 per cent.) hydrochloric 

 acid. From the solution thus formed it is precipitated by neutraliz- 

 ing with an alkali; it is soluble in lime-water and in dilute solution 

 of sodium carbonate. Syntonin represents the first stage in the 

 action of gastric juice upon the proteids. It is a pasty, whitish sub- 

 stance, possessing the solubilities mentioned above for acid-albumins. 



c. Alkali-albumins. Obtained by treating the different proteids 

 with alkalies and precipitating the solution by neutralizing with an 

 acid. They resemble acid-albumins in many respects, dissolve very 

 slightly in water, neutral salt solutions, and also in lime-water. 



d. Casein. This proteid, which resembles alkali albumin closely, 

 is found in milk, but in no other fluid or secretion of the body. It 

 is best obtained by diluting milk with about 4 volumes of water and 

 acidulating the mixture with acetic acid until it contains about 0.1 

 per cent, of this acid. Casein can also be precipitated from milk by 

 the addition of sodium chloride or magnesium sulphate. 



Pure casein is a fine, snow-white powder, insoluble in water, 

 soluble in alkalies, carbonates, and phosphates of the alkalies, and in 

 lime-water. From these solutions it is precipitated by excess of 

 neutral salts (sodium chloride) and by dilute acids, an excess of which 

 redissolves them. It is not coagulated by heat, but by rennet, an 

 enzyme found in gastric juice. 



e. Vegetable casein or Legumin occurs in considerable quantities in 

 the seeds of leguminosse, such as peas and beans, which contain 

 nearly 25 per cent. ; it is also found in almonds, various nuts, etc. 

 It shows reactions very similar to milk-casein. 



Class IV. Fibrins. Insoluble in water ; soluble with difficulty 

 in strong acids and alkalies, and undergoing a simultaneous change 

 into members of Class III. Coagulated by heat. 



a. Blood-fibrin does not exist as such in the body, but is produced 

 during the process of clotting of blood, lymph, chyle, and other coag- 

 ulable fluids of the body. The formation is due to the action of the 

 fibrin-ferment (of which but little is known) upon fibrinogen and pos- 

 sibly a second substance, fibrinoplastin. It may be obtained unstained 

 by the red corpuscles (as it is in a common blood-clot) by whipping 

 fresh blood with a bundle of twigs and then washing with water. It 



