128 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



water. If the thawing is slowly accomplished the water has time to dif- 

 fuse and restore the normal condition of imbibition. In this way is to 

 be explained the fact, which may be frequently observed in cold spring 

 and autumn mornings, that of the parts of plants which are frozen those 

 which are exposed to the direct rays of the sun are killed, while those 

 which are protected from the sun's heat thaw out gradually and regain 

 their vitality. So, also, red blood-corpuscles may be frozen and gradu- 

 ally thawed without being destroyed, but if rapidly thawed are dissolved. 

 On this fact, undoubtedly, rests the practical point that frozen animal 

 parts must not be rapidly warmed, but have their circulation only gradu- 

 ally restored ; hence, the common practice of rubbing with snow. 



2. SODIUM CHLORIDE (NaCl). Of the saline constituents of cells 

 sodium chloride is the most widely distributed, and is present in larger 

 amount than any other salt in all the tissues of the animal body, with 

 the exception of the bones, teeth, red blood-corpuscles and striated 

 muscle-cell. It is especialty worthy of notice that the amount of sodium 

 chloride in most organs, especially in the blood, is almost constant and 

 is independent of the amount of this salt contained in the food. Its 

 distribution in the body is also remarkable. In the blood-plasma it is 

 abundant, while it is almost absent from the blood-corpuscles which are 

 suspended in the plasma. It is abundant in chyle, lymph, saliva, gastric 

 juice, mucus, and pus, and is present in only small quantity in muscle- 

 juice and many glands. Sodium chloride is present in the form of a 

 solution in water, and in the removal of the fluids from the semi-solid 

 tissues by pressure the greater part of the salt is taken away with the 

 water. The relative proportion of sodium and potassium chlorides in 

 different parts of the animal body is about as follows : 



QUANTITY OF SODIUM AND POTASSIUM CHLOBIDES IN 1000 PARTS IN THE 



All the sodium chloride found in the animal body has entered it 

 from without. It leaves the body in the urine and excrement, perspira- 

 tion, nasal and buccal mucus. By far the greater part is eliminated 

 through the urine, though the total amount eliminated falls short of that 

 taken in the food. A certain amount of the sodium chloride taken in as 



