SECRETION. 251 



salts (acid sodium and acid calcium phosphate), and an average specific gravity 

 of 1017 to 1020. The quantity formed in twenty-four hours is about 1200 to 

 1700 cubic centimeters. In the very young the amount of urine formed is 

 proportionately much greater than in the adult. The normal urine contains 

 about 3.4 to 4 per cent, of solid matter, of which over half is organic mate- 

 rial. Among the important organic constituents of the urine are the follow- 

 ing : urea, uric acid, hippuric acid, xanthin, hypoxanthin, guanin, creatinin 

 and aromatic oxy- acids (para-oxyphenyl propionic acid and para-oxyphenyl 

 acetic acid, as simple salts or combined with sulphuric acid) ; phenol, paracre- 

 sol, pyrocatechin and hydrochinon, these four substances being combined with 

 sulphuric or glycuronic acid; iudican or indoxyl sulphuric acid; skatol sul- 

 phuric acid ; oxalic acid ; sulphocyanides, etc. These and other organic con- 

 stituents occurring under certain conditions of health or disease in various 

 animals, are of the greatest importance in enabling us to follow the metab- 

 olism of the body. Something as to their origin and significance will be 

 found in the section on Nutrition, while their purely chemical relations 

 are described in the section on Chemistry. 



Among the inorganic constituents of the urine may be mentioned sodium 

 chloride, sulphates, phosphates of the alkalies and alkaline earths, nitrates, and 

 carbon dioxide gas partly in solution and partly as carbonate. In this sect inn 

 we are concerned only with the general mechanism of the secretion of urine, 

 and in this connection have to consider mainly the water and soluble inorganic 

 salts and the typical nitrogenous excreta, namely, urea and uric acid. 



The Secretion of Urine. — The kidueys receive a rich supply of nerve- 

 fibres, but most histologists have been unable to trace any connection between 

 these fibres and the epithelial cells of the kidney tubules. Berkley ' has, how- 

 ever, described nerve-fibres passing through the basement membrane and 

 ending between the secretory cells. 



The majority of purely physiological experiments upon direct stimulation of 

 the nerves going to the kidney are adverse to the theory of secretory fibres, the 

 marked effects obtained in these experiments being all explicable by the changes 

 produced in the blood-flow through the organ. Two general theories of urinary 

 secretion have been proposed. Ludwig held that the urine is formed by the 

 simple phvsical processes of filtration and diffusion. In the glomeruli the 

 conditions are most favorable to filtration, and he supposed that in these struc- 

 tures water filtered through from the blood, carrying with it not only the in- 

 organic salts, but also the specific elements (urea) of the secretion. There was 

 thus formed at the beginning of the uriniferous tubules a complete bul diluted 

 urine, and m the subsequent passage of this Liquid along the convoluted tubes 

 it became concentrated by diffusion with the lymph surrounding the outside 

 of the tubules. S<> far as the latter part of this theory is concerned it has 

 not been supported by actual experiments; recent histological work (see below) 

 seems to indicate that the epithelial cells of the convoluted tubules have a 



1 The Johns Hopkins Hospital Bulletin, vol. iv., No. 28, p. 1. 



