CHEMICAL BASIS OF THE ANIMAL BODY. 871 



When prepared from milk by magnesic sulphate (see below), freed by ether 

 from fats, and dissolved in water, casein possesses a specific rotatory power of 80 

 for yellow light ; in dilute alkaline solutions, of 76 ; in strong alkaline solutions, 

 of 91 ; in dilute hydrochloric acid, of 87. 



Casein has been asserted to occur in muscle, in serous fluids, and in blood-serum 

 (serum-casein). In many cases it has probably been confounded with globulins 

 (see Class III. ) ; but blood-serum and muscle-plasma undoubtedly contain an 

 alkali-albumin in addition to whatever globulin may be present, and the usual 

 doubt exists as to the identity of this with true casein. Its presence maybe shown 

 by adding dilute acetic acid to blood-serum which has been freed from globulin by 

 a current of carbonic anhydride ; a distinct precipitate is thrown down. A sub- 

 stance similar to casein has also been described as existing in unstriated muscle and 

 in the protoplasm of nerve-cells. 



Preparation. Dilute milk with several (10 to 15) times its bulk of water, add 

 dilute acetic acid till a precipitate begins to appear, then pass a current of carbonic 

 anhydride, filter, and wash the precipitate with water, alcohol, and ether ; the com- 

 plete removal of the fat carried down with the casein presents some difficulties. 

 Magnesic sulphate added to saturation also precipitates casein from milk ; the pre- 

 cipitate as thus formed is readily soluble on the addition of water. 



CLASS III. Globulins. 



Besides the native albumins there are a number of native proteids which differ 

 from the albumins in not being soluble in distilled water ; they need for their solu- 

 tion the presence of an appreciable, though it may be a small, quantity of a neutral 

 saline body, such as sodic chloride. Thus they resemble the albuminates in not 

 being soluble in distilled water, but differ from them in being soluble in dilute sodic 

 chloride or other neutral saline solutions. Their general characters may be stated 

 as follows : 



They are insoluble in water, soluble in dilute ( 1 per cent. ) solutions of sodic chlo- 

 ride ; they are also soluble in dilute acids and alkalies, being changed on solution 

 into acid- and alkali-albumin respectively, unless the acids and alkalies are exceed- 

 ingly dilute. The saturation with solid sodic chloride of their solutions in dilute 

 sodic chloride, precipitates most members of this class. 



1. Globulin (CrystalUn). 



If the crystalline lens be rubbed up with fine sand, extracted with water and 

 filtered, the filtrate will be found to contain at least three proteids. On passing a 

 current of carbonic anhydride a copious precipitate occurs; this is globulin. 



The addition of dilute acetic acid to the filtrate from the globulin gives a precipitate of 

 alkali-albumin; 1 and the filtrate from this, if heated, gives a further precipitate, due to serum- 

 albumin. 



In its general reactions globulin corresponds almost exactly with the next mem- 

 bers of this class (paraglobulin and fibrinogen), but has no power to form or pro- 

 mote the formation of fibrin in fluids containing the above-mentioned bodies, and 

 possesses the following special features : 1. According to Lehmann, its oxygenated 

 neutral solutions become cloudy on heating to 73 C., and are coagulated at 93 C. 

 2. It is readily precipitated on the addition of alcohol. According to Hoppe- 

 Seyler, it is not precipitated on saturation with sodic chloride, resembling vitellin 

 in this respect. 



According to Kiihne 2 and Eichwald 3 a globulin with properties identical with those just 

 given may be precipitated from dilute serum by the cautious addition of acetic acid. This body 

 is stated by Weyl* to be the same as paraglobulin (fibrinoplastin),the latter differing from it only 

 by a small admixture of fibrin-ferment. 



2. Paraglobulin (Fibrmoplastm}. 



Preparation. Blood-serum is diluted tenfold with water, and a brisk current of 

 carbonic anhydride is passed through it. The first-formed cloudiness soon becomes 



1 But see also Pfiiiger's Arch., Bd. xiii. (1876), S. 631. 

 2 Lehrb. d. Physiol. Chem., 1868, S. 175. 



3 Beitrnge zur Chem. d. gewebebild. Subst., Berlin, 1873, H. 1. 

 4 Zeitschr. f. Physiol. Chem., Bd. i. (1878), S. 79. 



