CHLORIDE OF SODIUM. 71 



temporary. After being taken in with the food and drink, it is 

 associated with other principles in the fluids and solids, passing 

 from the intestine to the blood, and from the blood to the tissues 

 and secretions. It afterward makes its exit from the body, from 

 which it is discharged by four different passages, viz., in a liquid 

 form with the urine and the feces, and in a gaseous form with the 

 breath and the perspiration. Of all the water which is expelled in 

 this way, about 48 per cent, is discharged with the urine and feces, 1 

 and about 52 per cent, by the lungs and skin. The researches of 

 Lavoisier and Seguin, Yalentin, and others, show that from a pound 

 and a half to two pounds is discharged daily by the skin, a little 

 over one pound by exhalation from the lungs, and a little over two 

 pounds by the urine. Both the absolute and relative amount dis- 

 charged, both in a liquid and gaseous form, varies according to 

 circumstances. There is particularly a compensating action in this 

 respect between the kidneys and the skin, so that when the cutane- 

 ous perspiration is very abundant the urine is less so, and vice versa. 

 The quantity of water exhaled from the lungs varies also with the 

 state of the pulmonary circulation, and with the temperature and 

 dryness of the atmosphere. The water is not discharged at any 

 time in a state of purity, but is mingled in the urine and feces with 

 saline substances which it holds in solution, and in the cutaneous 

 and pulmonary exhalations with animal vapors and odoriferous 

 substances of various kinds. In the perspiration it is also mingled 

 with saline substances, which it leaves behind on evaporation. 



2. CHLORIDE OF SODIUM. This substance is found, like water, 

 throughout the different tissues and fluids of the body. The only 

 exception to this is perhaps the enamel of the teeth, where it has 

 not yet been discovered. Its presence is important in the body, as 

 regulating the phenomena of endosmosis and exosmosis in different 

 parts of the frame. For we know that a solution of common salt 

 passes through animal membranes much less readily than pure 

 water ; and tissues which have been desiccated will absorb pure 

 water more abundantly than a saline solution. It must not be sup- 

 posed, however, that the presence or absence of chloride of sodium, 

 or its varying quantity in the animal fluids, is the only condition 

 which regulates their transudation through the animal membranes. 

 The manner in which endosmosis and exosmosis take place in the 



1 Op. cit., vol. ii. pp. 143 and 145. 



