SECRETION. 191 



periphery presents a compact border which iu some cases shows a cilia-like 

 striation. These cells have the general appearance of active secretory struc- 

 tures, and recent theories of urinary secretion attribute this importance to them. 



Composition of Urine. The chemical composition of the urine is very 

 complex, as we should expect it to be when we remember that it contains most 

 of the end-products of the varied metabolism of the body, its importance in 

 this respect being greater than the other excretory organs such as the lungs, skin, 

 and intestine. The secretion is a yellowish liquid which in carnivorous ani- 

 mals and in man has normally an acid reaction, owing to the presence of acid 

 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 material, 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, guauin, 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, pyrocatechiu and hydrochinon, these four substances being combined with 

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

 phuric acid ; oxalic acid ; sulphocyauides, 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 section 

 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 kidneys 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. Berkeley 1 has, how- 

 ever, recently discovered 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 physical processes of filtration and diffusion. In the glomeruli the 

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

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



