ADDITIONS AND NOTES TO VOL. II. 549 



which influence this secretion and regulate the variations in its 

 amount. The science of physiology more especially feels the want 

 of those chemical investigations, which might elucidate the rela- 

 tion of the character and composition of the blood to the secreted 

 urine, although we are not deficient in isolated facts confirming 

 the proposition which had been a priori advanced, that the consti- 

 tution as well as the amount of the urine must depend upon the 

 existing constitution of the blood. In the meanwhile it cannot be 

 overlooked, that the chemical character of the blood cannot be 

 the exclusive cause of all or any of the modifications in the urine, 

 but that the mechanism of this secretion, as well as the condition 

 of the nervous system, must be included amongst the immediate 

 agents of the secretion of the urinary matters from the blood, and 

 therefore must control the amount of the secretion. Whilst it is 

 only recently that the view has been generally admitted, that 

 the most essential constituents of the urine exist preformed in 

 the blood, it has even been attempted to refer the process of the 

 secretion to purely dynamical relations, depending upon the 

 nervous system. No sooner was the fundamental law of endos- 

 mosis established, than it was supposed that the transmission of the 

 urinary constituents into the " tubuli uriniferi" might be referred 

 to this process ; but a more thorough investigation of the laws of 

 endosmosis sufficiently demonstrated that endosmosis alone was 

 insufficient to afford an explanation of the mechanical processes 

 involved in the secretion of urine. Ludwig* made the first suc- 

 cessful attempt to establish a theory for the mechanical part of 

 the process of the urinary excretion in the kidneys. Whichever 

 view one may incline to in reference to the terminations of the 

 urinary canals, it must be admitted that the principal part of the 

 secretion from the blood is effected in those singular coils of 

 vessels, the Malpighian bodies. It would appear, however, from 

 the measurements of most histologists, that the capillaries leading 

 from the Malpighian bodies are of a smaller diameter than the 

 vessels constituting the bodies themselves ; hence it follows from 

 the laws of hydraulics that there must be a greater pressure 

 against the walls of the latter, by which means, according to 

 Ludwig, the water passes through them, and the true urinary con- 

 stituents are introduced into the :i tubuli uriniferi." The collected 

 urinary fluid, which is so rich in water, is further impelled through 

 the "canalicula contorta" by the fluid which is subsequently exuded 

 from the blood ; here, however, these small vessels are surrounded 

 * Handworterb. d. Physiol. Bd. 2, S. G37-G-10. 



