No. 582] SIGNIFICANCE OF INTERNAL CONDITIONS 343 



any food which contains the requisite amino acids. The 

 power of synthesis of new amino acids in the body is 

 limited. Glycocoll, for example (amino-acetic acid), may 

 be synthesized in the body. More protein is needed as 

 food if its content in certain of the amino acids is low 

 than if its content of these necessary acids is high. 34 But 

 under all the varying conditions, within wide limits, the 

 animal may maintain itself in nitrogen equilibrium, 

 neither storing up any nitrogen in its flesh nor losing any 

 from its tissues. The excess of any particular amino 

 acid above that necessary for tissue growth or repair is 

 eliminated from the body. Voit's famous 35 kilo dog 

 was able to maintain nitrogen equilibrium when fed on 

 amounts of protein varying from 480 grams of meat a 

 day as a lower limit up to 2,500 grams a day. 



The quantities of fluid, fat and protein in the blood, 

 while undergoing some changes with the varying condi- 

 tions of nutrition, starvation and fasting, remain close to 

 a standard concentration as long as life lasts. Variations 

 of great magnitude are incompatible with the prolonged 

 life of the organism. 



7. Epidermis and Other Protective Mechanisms 

 In addition to absorption, the limiting membranes of 

 the body have other and important functions. The en- 

 trance of many harmful foreign substances into the or- 

 ganism is prevented by the protecting epidermis, the al- 

 veolar mucous membrane, and the intestinal mucosa, al- 

 ready noted. 



But, however important the exclusion of harmful sub- 

 stances may be, the protection of the organism through 

 the retention of valuable material is also of importance 

 and must be provided for. 



The escape of useful substances is prevented by the 

 skin, the liver, the alimentary tract, the lungs and the kid- 

 neys. The splitting products of the hemoglobin arising 

 from the death of the red blood cells are separated by 



