ALBUMENOID SUBSTANCES. 81 



3. Caseine. 



Caseine is the albuminous matter of milk, where it is present in the 

 proportion of a little more than 40 parts per thousand. It is insoluble 

 in pure water and in neutral saline solutions, but soluble in slightly 

 alkaline liquids. In liquids containing an alkaline phosphate, like milk, 

 it remains in solution notwithstanding the alkaline reaction be neutral- 

 ized (Hoppe-Seyler). It is not affected by a boiling temperature, but 

 coagulates by most of the other agents which have this effect on 

 albumen, namely, by the mineral acids, the metallic salts, and alcohol. 

 It is furthermore thrown down by the organic acids and by magnesium 

 sulphate, both of which are without action on albumen. Its specific 

 rotary power for polarized light is greater than that of any other 

 albuminous matter, amounting to 80. Its most remarkable property 

 is that of coagulating by contact with rennet, or the extract of the calf's 

 stomach. Coagulated caseine forms the albuminous ingredient of 

 cheese, whence its name is derived. When milk is taken as food it is 

 coagulated in a similar way, in the stomach, by the ferment of the 

 gastric juice. Caseine, in its various forms of preparation, is an abun- 

 dant and important nutritious material. 



4. Paraglobuline. 



This substance is a constituent of the blood-plasma, where it exists 

 in the proportion of 22 parts per thousand, being a little less than one- 

 half as abundant as the albumen. After coagulation of the blood 

 it remains, together with the albumen, as an ingredient of the serum. 

 Its name is derived from the fact that it has similar, but not identical, 

 reactions with a substance formerly extracted from the blood-globules, 

 and by Lehmann termed "globuline ;" one of the most prominent of 

 these reactions being its precipitation from dilute blood-serum or saline 

 solutions by a stream of carbonic acid, or by the addition of very dilute 

 acetic acid. Paraglobuline belongs to a group of substances, including 

 the two following, which are insoluble in pure water, but soluble in 

 dilute solutions of sodium chloride, in which they are coagulable by 

 heat. If the serum of the blood, accordingly, be subjected to a boiling 

 temperature, the whole of its albumen and paraglobuline coagulate 

 together ; but if it be diluted with ten times its volume of water, and 

 then treated with a stream of carbonic acid, its paraglobuline will be 

 thrown down, while the albumen remains behind. Paraglobuline is 

 also precipitable from blood-serum by the addition of powdered sodium 

 chloride in excess. 



The physiological relations of paraglobuline with albumen are 

 unknown. The similarity in properties and quantity of the two sub- 

 stances, as ingredients of the blood, make it possible that either one 

 may be a product of metamorphosis from the other ; or they may both 

 have been formed out of some other preceding substance, to serve for 

 different purposes in the act of nutrition. 



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