1 88 NUTRITION OF FARM ANIMALS 



in providing the necessary conditions for the chemical and physi- 

 cal activities of the various tissues. 



269. Osmotic pressure. The cells of the various tissues 

 draw their nourishment from the lymph which constitutes their 

 immediate nutritive environment (185) and from which they are 

 separated by cell walls which partake of the nature of semi- 

 permeable membranes. In order to maintain normal condi- 

 tions in the protoplasm of the cells the osmotic pressure of the 

 lymph, and therefore that of the blood from which it is derived, 

 must be maintained approximately constant. The osmotic 

 pressure of the blood is stated to be approximately about 8 

 atmospheres, due largely to the ash ingredients contained in 

 solution. With an adequate supply in the feed the concen- 

 tration of mineral matter in the blood is regulated chiefly by 

 the excretory activity of the kidneys. Thus, in the case of 

 sodium chlorid, for example, it is estimated that the blood of 

 an average man contains approximately 30 grams of this sub- 

 stance, of which hardly half a gram is excreted daily when 

 none is consumed. If, however, salt is added to the diet, the 

 excess is promptly excreted in the course of the next twenty- 

 four hours. What is true of salt in this respect is true also of 

 other diffusible ingredients of the blood. 



270. Ionic concentration. The various salts are contained 

 in the body largely in dilute aqueous solution. In such solu- 

 tions, however, it is believed that salts are largely dissociated 

 into their constituent ions, a dilute solution of common salt, 

 for example, containing in addition to some unchanged NaCl 

 the ions Na and Cl, one of calcium sulphate the ions Ca and 

 SO 4 , etc. Acids are similarly dissociated, yielding hydrogen 

 ions (H 2 SO 4 ^:H + SO 4 ), while alkalies yield OH ions 

 (KOH ^ K + OH) . Some of these ions have been shown to 

 have specific effects on certain cellular activities. For example, 

 a frog muscle kept in 0.7 per cent NaCl solution retains its irrita- 

 bility for one or two days. In a solution of a non-electrolyte, 

 like sugar, asparagin, etc., having the same osmotic pressure, the 

 muscle soon loses its irritability, but if NaCl be added to the 

 solution it regains it. Since a number of other sodium salts 

 produce the same effect, while chlorids of other metals do not, 

 it is apparent that the effect is due to the Na ions. On the 

 other hand, Na ions alone cause long continued rhythmic con- 



