S April, 1907] Elements of Animal Physiology. 235 



haematin contains iron, is rich in derivatives of tryptophane, and is red 

 in colour. We tind haemoglobin in the red corpuscles of the blood, and 

 constituting; about 40 per cent, of their weight. 



INFRAPROTEINS. — When proteins are treated with dilute acid or 

 alkali in the cold, products called acid-albumen and alkali-albumen 

 respectively are formed. These are soluble in the acid or alkali emploved. 

 and do not coagulate on heating. If, for instance, a piece of meat .is 

 placed in dilute hydrochloric acid, a considerable part of the protein is 

 •dissolved out as acid-albumen; this acid extract can be boiled without for- 

 mation of a coaguilum, but, if it be neutralized with alkali, a curdv pre- 

 cipitate at once forms. If alkali be employed to extract the meat^ Lhe 

 solution is rapid. 



PROTEOSES, PEPTONES, and POLYPEPTIDES.— When pro- 

 tein undergoes disintegration- into its component amino-acids, a number of 

 steps can be traced by the loss of one true protein property after another. 

 When coagulation by heat is lost, and when copper sulphate and potash 

 give a rose-pink instead of a violet, the product consists of a mixture of 

 PROTEOSES. When, in a further stage of disintegration, the property of 

 being precipitated bv ammonium sulphate is lost, and the substance can 

 penetrate through parchment paper, the product is termed peptone.^ 

 Below the peptones come the polypeptides, which no longer give anv dis- 

 tinctive colour with copi)er sulphate and potash, and finallv tlie amino-acids 

 themselves. 



SCLERO-PROTEINS (called by some authors ALBUMINOIDS').— 

 These are compounded of amino-acids, but, unlike the previously-men- 

 tioned proteins, contain little or no tyrosin or tryptophane. They are 

 all insoluble in cold water, and are fairly resistant to chemical reagents. 

 They cannot be substituted for protein in food, and are used in the body, 

 as constituents of connective tissue, horn, hoof, &c., on account of their 

 physical properties. 



1. Keratin, the basis of hair, feathers, horn, and the outer layers ot 

 the skin. It is very tenacious and resistant to chemical attacks. It con- 

 tains more cystin, and, therefore, more sulphur, than ordinary protein. 



2. Collagen, generally present in the form of fme fibres or sheets 

 which make up the greater part of connective tissues (see Chapter II.). 

 Collagen is a colourless inelastic substance, insoluble in water, and of re- 

 markable tenacity. When boiled with water, or treated with dilute acids, 

 it swells up and changes into gelatine. 



3. Chondrogen, closely allied to collagen, and forming the basis of 

 cartilage (gristle). 



4. Ela'stin, also closely allied to collagen, but differing in its physical 

 properties. It is a highly elastic substance in the sense that indiarubber is 

 elastic, and forms the basis of elastic fibres, such as are found in the 

 Ugamentum mucha, &c. 



VEGETABLE PROTEINS.— These, which occur widely distributed 

 throughout all plants, and which are of special imjiortance in connexion 

 v.ith the question of fodders, are partly globulins, partly albumens, and 

 pjartly proteins which cannot well be classified. Some of them possess 

 the remarkable property of being soluble in alcohol. Generally speaking, 

 they are insoluble in water, and require much more salt for their solution 

 than do the animal globulins. 



1 The article on the market called Witte's Peptone contains vsry little true peptone ; it is chiefly 

 a mixture of proteoses. 



