26 PHYSIOLOGICAL CHEMISTRY 



creted by way of the bile into the intestine. Iron is also ex- 

 creted in the urine, the amount being no greater than 10 or 11 

 mg. per liter. 



Iron is detected by precipitation as "Prussian Blue" or fer- 

 ric ferrocyanide or by the formation of ferric thiocyanate which 

 is red in color. 



Chlorine Iodine, Fluorine, etc. Chlorides are present in 

 traces in all the tissues and fluids; the blood contains about 0.4% 

 sodium chloride, the gastric juice about 0.4% hydrochloric acid. 

 The necessity of adding sodium chloride to the diet has already 

 been referred to. Chlorides are excreted in the urine in 

 amounts varying with the amount in the food, 10-12 gms. 

 daily being an average figure. Chlorine may be detected by 

 precipitation as silver chloride. Iodine is interesting chiefly in 

 connection with its occurrence in the thyroid gland. This gland, 

 by means of a compound which it produces and pours out into 

 the blood stream, has far reaching influence on the chemical 

 reactions going on in the body. This important substance is an 

 iodine compound. If an iodide gets into the blood stream, the 

 salivary glands have the power of excreting it. After taking a 

 capsule containing potassium iodide, iodine may be demonstrated 

 in the saliva. Fluorine is found in the bones and teeth. 



Other elements may occur either regularly or accidentally 

 in the body tissues or fluids, but usually only in traces. 



Water. Water makes up about % of the body weight of 

 mammals, and a much larger part in some lower animals. All 

 tissues contain it, even the enamel of the teeth: blood, lymph, 

 the digestive juices, urine ; etc., are %o-%o water; the organs 

 and softer tissues about %. Water serves a variety of func- 

 tions. It is the circulating medium for transporting food and 

 waste material to and from the cells, it holds various sub- 

 stances in solution and makes ionization possible, and is a 

 medium for the excretion of waste. It also distributes the heat 

 of the body, and by evaporation from the skin is instrumental 

 in regulating body temperature. An animal may survive for 

 many days without nourishment, but if water also is withheld, 

 death follows in a few days' time. 



