99 



they are the most obvious, and familiar to us. An important addi- 

 tional source is that formed within the body of the animal by the proc- 

 esses of respiration and dehydration; this is metabolic water. The 

 relation of this source to others and to water-loss has recently been 

 summarized in an important paper by Babcock ('12:87, 88, 89-90, 

 91, 160, 161, 171-172, 174-175, 175-176, 181). The following quo- 

 tations from this paper will serve to give a concise statement of the 

 general principles involved in this important process. He says (pp. 

 87-88) : "There are, however, particular stages in the life history of 

 both plants and animals in which metabolic water is sufficient for all 



purposes for considerable periods of time This is also true 



in the case of hibernating animals that receive no water from external 

 sources for several months, although water is constantly lost through 

 respiration and the various excretions. In addition many varieties of 

 insects such as the clothes moths, the grain weevils, the dry wood bor- 

 ers, etc., are capable of subsisting, during all stages of development, 

 upon air-dried food materials containing less than ten per cent water; 

 in these cases, nearly all of the water required is metabolic. . . . Many 

 organisms also, when deprived of free oxygen, are capable of main- 

 taining for a short time, certain of the respiratory functions, and de- 

 riving energy from food material and from tissues by breaking up the 

 molecular structure into new forms of a lower order. This is known 

 as intramolecular respiration, and like direct respiration, results in the 

 production of both water and carbon dioxide." (Pp. 89-90) : "The 

 substances oxidized by both plants and animals, to supply vital energy, 

 consist of carbohydrates, fats, and proteins. All of these substances 

 contain hydrogen, and their complete oxidation produces a quantity 

 of water equal to nine times the weight of hydrogen present in the orig- 

 inal substances. . . . Most of the the fats yield more than their weight 

 of water, while proteins, when completely oxidized, give from 60 to 



65 per cent of water Animals, however, are unable to utilize 



the final products of protein metabolism which are in most cases 

 poisonous and must be removed from the tissues by excretion in vari- 

 ous forms, the principal of which are urea, uric acid, and am- 

 monia The amount of metabolic water formed by oxidation 



during any period is proportional to the rate of respiration 



(Page 91 ) : "With parasitic plants, and with animals, which derive all 

 of their organic nutrients from chlorophyl producing plants, im- 

 bibed water is not so essential to life ; with these the chief function 

 of imbibed water is to aid in the removal of waste products, the 

 metabolic water being in most cases sufficient for transferring nutri- 

 ents and for replacing the ordinary losses incurred by respiration 

 and evanoration." (Page 160) : "Another and more im- 



