16 TEXT-BOOK OF PHYSIOLOGY 



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. 



(a) Caseinogen. This is the principal protein of milk, in which it 

 exists in association with calcium in a form known as calcium- 

 caseinogenate. It is precipitated by acetic acid and by magnesium 

 sulphate. It is coagulated by rennin, though the nature of the process 

 is not very clear. It was formerly taught that under the action of 

 rennin, an enzyme of the gastric mucous membrane, caseinogen was 

 separated into a solid portion, casein or tyrein, and a soluble portion. 

 The cleavage action of rennin thus indicated has not been verified by 

 subsequent investigations. It is more in accordance with the facts to 

 assume that the process is a double one and that the action of rennin is 

 to change the caseinogen to a soluble form, termed paracasein, after 

 which the lime salts present react with the paracasein in such a 

 manner as to cause it to assume the solid condition. Calcium 

 phosphate seems to be the natural alkali necessary to this process, for 

 if it be removed by dialysis, or precipitated by the addition of potassium 

 oxalate, the casein will not be curdled by rennin, but it will neverthe- 



