34 TEXT-BOOK OF PHYSIOLOGY. 



relation to prepared foods and to the digestive process. They are 

 produced when solutions of egg-albumin, serum- albumin, or globulins 

 are subjected to a temperature of 100 C. or to the prolonged action 

 of alcohol. They are insoluble in water, in dilute acids, and in 

 neutral saline solutions. In this same group may be included also 

 those coagulated proteids which are produced by the action of animal 

 ferments on soluble proteids e. g., fibrin, myosin, casein. 



Fibrin. Fibrin is derived from a soluble proteid fibrinogen 

 by the action of a special ferment. It is not present under 

 normal circumstances in the circulating blood, but makes its 

 appearance after the blood is withdrawn from the vessels 

 and at the time of coagulation. It can also be obtained by 

 whipping the blood with a bundle of twigs, on w r hich it accu- 

 mulates. When freed from blood by washing under water, 

 it is seen to consist of bundles of white elastic fibers or threads. 

 It is insoluble in water, in alcohol, and ether. In dilute acids 

 it swells, becomes transparent, and finally is converted into 

 acid-albumin. In dilute alkalies a similar change takes place, 

 but the resulting product is an alkali-albumin. Fibrin pos- 

 sesses the property of decomposing hydrogen dioxid, H 2 O 2 

 i. ., liberating oxygen, which accumulates in the form of 

 bubbles on the fibrin. On incineration fibrin yields an ash 

 which contains calcium phosphate and magnesium phosphate. 



5. PROTEOSES AND PEPTONES. 



During the progress of the digestive process, as it takes place in 

 the stomach and intestines, there is produced by the action of the 

 gastric and pancreatic juices, out of the proteids of the food, a series 

 of new proteids, known as proteoses and peptones. The chemic 

 properties of these substances will be considered in connection with 

 the process of digestion. 



6. ALBUMINOIDS. 



The albuminoids constitute a group of substances similar to the 

 proteids in many respects, though differing from them in others. 

 When obtained from the tissues, in which they form an organic 

 basis, they are found to be amorphous, colloid, and when decom- 

 posed yield products similar to those of the true proteids. The 

 principal members of this group are as follows: 



(a) Mucin. This is the viscid, tenacious constituent of mucus 

 secreted by the epithelial cells of mucous membranes. It is 

 also present in the intercellular substance of the connective 

 tissue. It is readily precipitated by acetic acid. 

 (6) Collagen, Ossein. These are two closely allied, if not identical, 

 substances, found respectively in the white fibrous connec- 



