THE PROTEINS 43 



In general terms globulins are more readily salted out than 

 albumins ; they may therefore be precipitated and thus separated 

 from the albumins by saturation with such salts as sodium chloride, 

 or, better, magnesium sulphate, or by half saturation with ammo- 

 nium sulphate. 



The typical globulins are also insoluble in water, and so may be 

 precipitated by removing the salt which keeps them in solution. 

 This may be accomplished by dialysis (see p. 38). 



Their temperature of heat coagulation varies considerably. The 

 following are the commoner globulins : fibrinogen and serum 

 globulin in blood ; egg-globulin in white of egg, myosinogen in 

 muscle, and crystallin in the crystalline lens. We must also include 

 under the same heading certain proteins which are the result of 

 ferment coagulation on globulins, such as fibrin (see Blood) and 

 myosin (see Muscle). 



The most striking and real distinction between globulins and 

 albumins is that the latter on hydrolysis yield no glycine, whereas 

 the globulins do. 



5. The Sclero-proteins 



These substances form a heterogeneous group of substances, 

 which are frequently termed albuminoids. The prefix sclero- indicates 

 the skeletal origin and often insoluble nature of the members of the 

 group. The principal proteins under this head are the following : 



1. Collagen, the substance of which the white fibres of connective 

 tissue are composed. Some observers regard it as the anhydride of 

 gelatin. 



2. Ossein. This is the same substance derived from bone. 1 



3. Gelatin. This substance is produced by boiling collagen with 

 water. It possesses the peculiar property of setting into a jelly when 

 a solution made with hot water cools. On digestion it is like ordinary 

 proteins converted into peptone-like substances and is readily 



1 In round numbers the solid matter in bone contains two thirds inorganic or 

 earthy matter, and one third organic or animal matter. The inorganic constituents 

 are calcium phosphate (84 per cent, of the ash), calcium carbonate (13 per cent.), 

 and smaller quantities of calcium chloride, calcium fluoride, and magnesium 

 phosphate. The organic constituents are ossein (this is the most abundant , 

 elastin from the membranes lining the Haversian canals, lacunae, and canaliculi, 

 and other proteins and nuclein from the bone corpuscles. There is also a small 

 quantity of fat even after removal of all the marrow. Dentine is like bone chemi- 

 cally, but the proportion of earthy matter is rather greater. Enamel is the hardest 

 tissue in the body ; the mineral matter is like that found in bone and dentine ; 

 but the organic matter is so small in quantity as to be practically non-existent 

 (Tomes). Enamel is epiblastic, not mesoblastic, like bone and dentine. 



