CHAPTEK VIII 



THE EXCRETION OF URINE 



CONTENTS. 1. Structure of the kidneys. 2. Mechanism of urinary secretion. 

 Vitalist theory of Bowman ; .mechanical theory of Ludwig. 3. Modification of 

 urinary secretion with variations of normal conditions of circulation in kidneys ; 

 conclusions as to functions of glomeruli. 4. Effect on renal secretion of alterations 

 caused in the blood by diuretics ; criticisms of mechanical theory. 5. Experimental 

 data in favour of vitalist theory ; criticisms. 6. Innervation of kidneys. 7. Modi- 

 fications of epithelial cells of renal tubules during secretory activity and functional 

 rest. 8. Function of ureters. 9. Mechanism of retention of urine. 10. Mechanism 

 of micturition. 11. Innervation of bladder. Bibliography. 



THE Kidneys are the organs that secrete urine ; the Ureters are 

 the canals that conduct the urine to the bladder, from which it is 

 periodically expelled through the urethra. These organs as a 

 whole form the Uropoietic System, the functions of which will be 

 considered in the present chapter. 



I. In order to form any idea of the mechanism by which the 

 chemical constituents of the urine are separated from the blood, it 

 is necessary to start with the structure of the kidney. In no other 

 glandular organ do the structural peculiarities, the relations 

 between circulatory system and excretory ducts, correspond as 

 strictly as in the kidneys with the specific character of the 

 functions. For this reason physiological theories as to the 

 mechanism of urinary secretion first assumed a scientific character 

 after the discovery of the highly characteristic structure of the 

 kidneys, by which they are differentiated from all other glands. 



The following were the most fundamental discoveries in regard 

 to the morphology of the kidneys : Bellini (1661) first described 

 the uriniferous tubules to which the kidney owes its character of a 

 tubular gland. Malpighi (1669) first described the corpuscles 

 that bear his name, and succeeded in injecting them through the 

 arteries; he regarded them as small glands, for which Bellini's 

 ducts form the excretory system. Huschke (1828) first pointed 

 out that in birds and batrachia the uriniferous tubules end in the 

 form of spherical dilatations. Johannes Mliller (1830) shortly 

 after discovered these dilatations in the uriniferous tubules of all 

 vertebrates ; but he did not detect their connection with the blood- 



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