118 PHYSIOLOGY 



(10) THE ALBUMINOIDS OR SCLERO-PROTEINS. Under this 

 beading are grouped a number of diverse substances which play an 

 important part in building up the framework of the body. Their 

 value as skeletal tissues Eeems to be determined by their insoluble 

 character. On this account it is practically impossible to speak of 

 purifying them. In every case we can simply take the residue of a 

 skeletal tissue which is left after extraction of the soluble constituents. 

 When broken down by the action of strong acids they yield a series 

 of disintegration products which are included among those we have 

 already studied as the disintegration products of proteins. Their 

 difference from the proteins which are employed in metabolism for 

 their nutritive value is caused either by the absence of certain groups 

 common to all the nutritive proteins, by the presence of an excess of one 

 or two groups, or by the presence of certain polvpeptides which 

 present considerable resistance to the action of digestive ferments. 

 This class plays the part in the animal economy which in the vegetable 

 kingdom is filled by the anhydrides of the hexoses and pentoses, e.g. 

 the celluloses, lignin, the pentosanes, &c. Collagen forms the main con- 

 stituent of white fibrous tissue and the ground substance of bone and 

 cartilage. It is insoluble in water, hot or cold, and in trypsin. Under 

 the action of acids or when subjected to prolonged boiling with water, 

 especially under pressure, it is converted into gelatin, which is soluble 

 in hot water, forming a colloidal solution liquid at high temperatures, 

 but setting to a jelly when cold. When subjected to acid hydrolysis it 

 gives a series of amino-acids from which tyrosine and tryptophane are 

 wanting. On this account gelatin does not give any reaction either 

 with Millon's reagent or with glyoxylic acid. On the other hand, there 

 is a preponderance of such groups as glycine and phenylalanine, and 

 it is probable that glycine, phenylalanine, and leucine are joined 

 together, perhaps with other amino-acids, to form a polypeptide 

 which is not attacked by digestive ferments, and therefore determines 

 the resistance of the original collagen molecule to solution. Gelatin 

 is precipitated by tannic acid, but not by acetic acid. It is 

 dissolved with hydrolysis by gastric juice or by pancreatic juice, 

 whereas collagen, its anhydride, is unaffected by the latter. On 

 prolonged boiling in water it is converted into a modification which 

 does not form a jelly on cooling. Under the action of formaldehyde 

 it is converted into an insoluble modification which does not melt on 



warming. 



Reticulin. This name has been applied to the tissue which forms the support- 

 ing network of adenoid tissue, and has also been described in the spleen, the 

 mucous membrane of the intestine, liver, and kidneys. It differs from collagen 

 in resisting digestion by gastric juice, and also in containing phosphorus in organic 

 combination. According to Halliburton there is no essential difference between 



reticulin and collagen. 



