136 COLLOIDS IN BIOLOGY AND MEDICINE 



Commercial dextrins which are mixtures of starch fragments of 

 different size are almost entirely held back by impermeable ultra- 

 filters (10 per cent) (H. BECHHOLD* 4 ). 



Closely related to the starches is inulin, the reserve carbohydrate 

 in dahlia bulbs and the roots of Inula helenium, etc., as well as 

 lichenin, which occurs in many lichens, especially Iceland moss. 

 Unlike the starches, inulin and lichenin are soluble in water without 

 forming a paste and form yellow adsorption compounds with iodin 

 (see p. 135). 



Besides these, a series of starches have been identified, some of 

 which show differences in their final cleavage products, the sugars. 

 As yet they have not been studied colloid-chemically. 



In its biological function, animal starch, glycogen, resembles the 

 plant starches closely, and in its colloid properties stands midway 

 between these and inulin. It swells in cold water and forms with 

 it an opalescent hydrosol. The electric current carries it to the 

 anode (Z. GATIN-GRUSZEWSKA*). With iodin it forms according to 

 its concentration, a brownish yellow to deep red adsorption com- 

 pound. 



The internal friction of glycogen solutions have been studied by 

 F. BOTTAZZI and G. D'ERRICO* as well as by J. FRIEDLANDER.* 



Glycogen is split up by acids and ferments, and according to the 

 degree of hydrolysis we find all sorts of fragments, from the highly 

 colloidal to the easily diffusible grape sugar. E. RAHLMANN*! fol- 

 lowed this process with the ultramicroscope. 



The glucosides must be mentioned in this connection. They are 

 compounds of the aliphatic and the aromatic series with sugars, 

 which may be split into their components by acids or ferments. In 

 the vegetable kingdom they include very active pharmacologic and 

 toxic substances, such as digitalis glucoside, phloridzin and saponins. 

 Recently several glucosides have been discovered in the animal organ- 

 ism, e.g., cerebron in the human brain. Though some glucosides, 

 e.g., amygdalin and myronic acid are unquestionably crystalloids, 

 others, e.g., saponin, are entirely colloidal. Since we know very little 

 of the biological significance of glucosides, it is evident that we do not 

 know what importance may be ascribed to the crystalloid form in 

 one and the colloidal form in the other. 



The gums are carbohydrates which are widely distributed through- 

 out the vegetable kingdom. Some of them play a part, in many 

 respects analogous to that of fibrin in the animal kingdom, since they 

 solidify on issuing from a wound, thus sealing it. Best known of 

 the gums are gum arabic, carraghen and cherry gum, while agar, de- 

 rived from Japanese sea weed, is of especial importance in bacteri- 



