554 CHEMISTRY OF THE PROTEIDS CHAP. 



The only exception to this rule is the most readily soluble of all the 

 albuminoids, namely collagen. Collagen on being boiled for a short 

 time gives rise to glue (glutin or gelatine), which possesses the remark- 

 able property of being fluid when heated, and of 'setting' into a 

 firm jelly on being cooled ; if this collagen is split up into its disso- 

 ciation-products it no longer ' sets/ and this is analogous to the 

 change in solubility which natural albumins undergo on becoming de- 

 naturalised. It is also not possible to speak of the molecular weights 

 when dealing with solid bodies (Cohnheim). 



Generally speaking, albuminoids are sharply marked off from the 

 rest of the albuminous bodies ; they only show certain transitions to 

 the mucins, such as the chondro-mucoid, but the resemblance is not a 

 chemical one, but rather depends on both substances being combined 

 in the animal body for a common function. To separate off the 

 albuminoids from a number of non-albuminous supporting structures 

 found amongst the lower animals is much more difficult. Chitin we 

 know not to be an albuminous compound, but to consist essentially of 

 a nitrogenous carbohydrate, a derivative of glucosamin, as has been 

 shown by Ledderhose, 1 but where to place ' hyaline ' and compounds 

 related to it is not so easy to decide, and amongst lower animals 

 Krukenberg 2 has described a number of substances, to which he assigns 

 a place intermediate between albumins and carbohydrates. Whether 

 we are dealing here with mixtures of albuminoids and chitin, or with 

 a splitting off of carbohydrates from glyco-proteids, or with genuine 

 transition-compounds, it is impossible to say. Amyloid, which Ham- 

 marsten 3 classifies amongst the glyco-proteids, belongs rather to the 

 albuminoids, if we judge it by its whole habitat, its firmness, and its 

 very great insolubility. 



The typical albuminoids are represented by gelatine, keratin, 

 elastin, fibroin, spongin, conchiolin. In addition to these a number 

 of bodies exist which belong anatomically to the supporting structures 

 or connective tissues, but which differ from the true connective tissues 

 in not yielding gelatine, in not resembling keratin or elastin, and 

 also in offering great resistance to the digestive enzymes. They 

 have been discovered by Morner in the refractive media of the 

 eye ; by Hammarsten and his pupils in muscle and in other situations, 

 and are in the meantime called ' albumoids ' ; in this book they are 

 described collectively in a special chapter. Siegfried's reticulin belongs 



1 G. Ledderhose, Zeitschr. f. physiol Ghem. 2. 213 (1878). 



2 F. C. W. Krukenberg, partly abstracted in the Grundzilge einer vergleichenden 

 Physiologic der tierischen Geriistsubstanzen, Heidelberg, 1885. 



3 0. Hammarsten, Lehrbuch der physiol. Ckem., 4. Aufl. 189, p. 47. 



