THE PROTEINS 105 



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 polypep tides which present considerable resist- 

 ance 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 constituent 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 solu- 

 tion. 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 supporting 

 network of adenoid tissue, and has also been described in the spleen, the mucous mem- 

 brane 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. 



The keratins are produced by the modification of epithelial cells and 

 form the horny layer of the skin as well as the main substance of hairs, 

 wool, nails, hoofs, horns, and feathers. They are distinguished by their 

 insolubility in water, dilute acids or alkalies, and in the higher animals 

 pass through the alimentary canal unchanged. Although differing in their 

 elementary composition, according to the tissue from which they are pre- 

 pared, they are all distinguished by the very large amount of sulphur present 

 in their molecule. The greater part of this sulphur is in the form of cystine, 

 of which as much as 10 per cent, can be extracted from keratin. They also 

 yield, on acid hydrolysis, tyrosine in larger quantities than is the case with 

 the ordinary proteins. 



N euro keratin, which forms the basis of the neuroglial framework of the 

 central nervous system, must be grouped by its general behaviour as well as 

 by its origin with the keratins. It resembles the other members of this class 

 in its insolubility and in its high content in sulphur. It is extracted from 

 nervous tissues by boiling these with alcohol and ether and then submitting 



