24 PROTEIDS. 



acid, yields a very imperfect precipitate which is finely flocculent, 

 almost granular as compared with the compact and coarsely flocculent 

 precipitate yielded by cow's milk. (3) The casein in human milk is, 

 as already stated, very incompletely precipitated by the addition of 

 acids and can only be completely precipitated by saturation with 

 magnesium sulphate 1 . (4) Casein from human milk is less soluble in 

 water than is that of the cow. 



The primary digestive products ' caseoses ' obtained by the action of pepsin on 

 casein have been described and studied by Cbittenden and Painter 2 . 



CLASS III. Globulins. 



Besides the derived albumins there are a number of native proteids 

 which differ from the albumins in not being soluble in distilled water ; 

 they need for their solution the presence of an appreciable, though it 

 may be a small, quantity of a neutral saline substance such as sodium 

 chloride. Thus they resemble the albuminates in not being soluble 

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

 sodium chloride or other neutral saline solutions 3 . Their general 

 characters may be stated as follows. 



They are insoluble in water, soluble in dilute (1 p.c.) solutions of 

 sodium chloride ; they are also soluble in dilute acids and alkalis, being 

 changed on solution into acid- and alkali-albumin respectively unless 

 the acids and alkalis are exceedingly dilute and their action is not 

 prolonged. The saturation with solid sodium chloride or other neutral 

 salts of their saline solutions, precipitates most members of this class. 



1. Crystallin. (Globulin of the crystalline lens.) 

 This form of globulin is usually regarded as identical with vitellin. 

 It is however convenient to treat it separately inasmuch as it can be 

 prepared in a pure form, whereas vitellin has not as yet been obtained 

 free from lecithin (see below). 



Preparation 41 . Crystalline lenses, in which it occurs to the extent 

 of 24 f 62 p.c., are rubbed up in a mortar with a little fine sand and a 

 few crystals of rock salt; the mass is then extracted with water and 

 filtered. The filtrate contains the crystallin and some serum-albumin. 

 The former is separated from the latter by copious dilution with 

 distilled water and passing a current of carbonic anhydride through 

 the diluted mixture, whereupon the crystallin is precipitated. 



1 Makris, Inaug.-Diss., Strassburg, 1876. See Maly's Ber. Bd. vi. (1876), S. 113. 



2 Stud. Lab. Physiol. Ch. Yale Univ. Vol. n. (1887), p. 156. 



3 But see Nikoljukin (Kussian), Abst. in Maly's Ber. Bd. xvm. (1888), S. 5. 



4 Laptschinsky, Pfluger's Arch. Bd. xm. (1876), S. 631. 



