276 VETERINARY PHYSIOLOGY 



cells shed from the mucous membranes and from the skin 

 and its coverings. Material is used up in the production 

 of enzymes and of internal secretions (p. 588). The new 

 nitrogenous matter required for the replacement of this 

 used-up material and also for formation of new tissue in 

 growth must be supplied to the tissues in the form of 

 amino-acids. 



The various proteins differ in their amino-acid build-up 

 (p. 1 6), and hence they are of different value in the growth 

 and repair of the body. Their relative value has chiefly been 

 investigated by feeding young white rats on a basal diet 

 consisting of protein-free dried milk, since this is known 

 to be an adequate diet when suitable proteins, such as the 

 milk proteins, caseinogen and lactalbumin, are added to it. 

 Various proteins or amino-acids may be added to this, and 

 the effect upon the rate of growth and the duration of life 

 determined. 



In this way it has been shown that some of the amino- 

 acids, certainly glycin, can be formed from others, since 

 caseinogen, which contains no glycin, has proved adequate 

 for normal growth. 



Other amino-acids cannot be formed in the body, 

 or cannot be formed in sufficient amounts for adequate 

 nutrition. Some proteins contain all the necessary amino- 

 acids, some do not. It has been found that among those 

 which are adequate to maintain normal growth when given 

 in sufficient amount are — 



Animal Origin. Vegetable Origin. 



Casein (milk). Edestin (hemp seed). 



Lactalbumin (milk). Glutelin (maize). 



Ovalbumin (hen's egg). Glutenin (wheat). 



Ovovitellin (hen's egg). Glycin (soy bean). 



While among those which are inadequate are — 



Legumelin (soy bean). Hordein (barley). 



Gliadin (wheat). Zein (maize). 



Legumin (pea). Gelatin. 



The reason for this failure is the absence of essential 

 amino-acids. 



Gelatin lacks tyrosin and tryptophan. 



