16 TEXT-BOOK OF PHYSIOLOGY 



it presents itself as a flocculent substance, insoluble in water, soluble 

 in dilute acids and alkalies, and coagulating at 75C. 



(b) Fibrinogen. This protein is found in blood-plasma in association 

 with serum-globulin and serum-albumin. It is also present in 

 lymph-tissue fluids and in pathologic transudates. It can be ob- 

 tained from blood-plasma which has been previously treated with 

 magnesium sulphate on the addition of a saturated solution of sodium 

 chlorid. It is soluble in dilute acids and alkalies, and coagulates 

 at 56C. 



(c) Paramyosinogen or Myosin. This protein is a constituent of the 

 muscle-plasma from which it can be precipitated by a temperature 

 of 4 7C. 



(d) Myosinogen or Myogen. This protein is the chief constituent of 

 the muscle-plasma and is of great nutritive value. During the 

 living condition it is liquid, but after death it readily undergoes a 

 chemic change and contributes to the formation of an insoluble 

 protein known as myogen fibrin. It is soluble in dilute hydrochloric 

 acid and dilute alkalies. It coagulates at 56C. 



(e) Crystallin or Globulin. This is obtained by passing a stream 

 of CO 2 through a watery extract of the crystalline lens. 



SCLERO -PROTEINS (ALBUMINOIDS). 



The sclero-proteins constitute a group of substances similar to the pro- 

 teins in many respects, though differing from them in others. When ob- 

 tained from the tissues, in which they form an organic basis, they are found 

 to be amorphous, colloid, and when decomposed yield products similar 

 to those of the true proteins. The principal members of this group are as 

 follows : 



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

 substances, found respectively in the white fibrous connective tissue 

 and in bone. When the tendons of muscles, the ligaments, or de- 

 calcified bone are boiled for several hours, the collagen and ossein 

 are converted into soluble gelatin, which, when the solution cools, 

 becomes solid. 



(b) Chondrigen. This is supposed to be the organic basis of the more 

 permanent cartilages. When the latter are boiled, they yield a 

 substance which gelatinizes on cooling, and to which the name 

 chondrin has been given. Chondrin, however, is not a pure gelatin, 

 but has associated with it a compound protein known as chrondro- 

 mucoid. 



(c) Elastin is the name given to the substance composing the fibers of 

 the yellow, elastic connective tissue. 



(d) Keratin is the substance found in all horny and epidermic tissues, 

 such as hairs, nails, scales, etc. It differs from most proteins in con- 

 taining a high percentage of sulphur. 



PHOSPHO-PROTEINS. 



The two members of this group are distinguished by yielding on decom- 

 position a protein which contains phosphorus. It was formerly regarded 

 as a nuclein. 



