640 THE SECRETION OF URINE. 



(3) The capillary network surrounding the uriniferous tubes is 

 the counterpart of that investing the tubes of the testis, 

 allowance being made for the difference in the capacity of 

 these canals in the two glands. 



(6) That the Malpighian bodies differ from the secreting parts of 

 true glands. 



(1) The Malpighian bodies comprise but a small part of the 



inner surface of each kidney, there being but one to each 

 tortuous tube. 



(2) The epithelium immediately changes its characters as the 



tube expands to embrace the tuft of vessels. 



(3) The blood vessels, instead of being on the deep surface of 



the membrane, " pass through it and form a tuft on its 

 free surface." 



(4) The peculiar arrangement of the vessels in the Malpighian 



tufts is clearly designed to produce a retardation of the 

 blood through them, while the orifice of the tubule is 

 encircled by cilia in active motion directing a current 

 towards the tubule, so tending to remove pressure from 

 the free surface of the vessels and to encourage the escape 

 of their more fluid contents. " Why is so wonderful an 

 apparatus placed at the extremity of each uriniferous 

 tube, if not to furnish water to aid in the separation and 

 solution of the urinous products from the epithelium of 

 the tube ? " 



The appearance ui' this paper fell at a time when, led by Ludwig,. 

 Helmholtz, and du Bois Keyraond, physiologists were endeavouring to, 

 replace the misty " vitalistic " conceptions which had until then prevailed, 

 by an accurate comparison of vital phenomena with their physical or 

 chemical counterparts, and seeking to establish physiology as an exact 

 experimental science on a par with physics. It was impossible, therefore, 

 that the views of Bowman, devoid as they were of experimental founda- 

 tion, should remain unchallenged. 



Theory of Ludwig. In 1844, Ludwig 1 put forward his well-known 

 mechanical theory, for the establishment and testing of which a large 

 volume of work has been done, the greater part under the direction of 

 Ludwig himself. According to this theory, all the energy for the secretion 

 of urine is ultimately derived from the heart-beat. In consequence of 

 the high pressure obtaining in the capillaries of the glomeruli, a fluid 

 is filtered through, containing all the constituents of the urine in very 

 dilute solution. This dilute solution passes down the tubules, and in its 

 passage undergoes changes, in consequence of diffusion between it and 

 the fluid (lymph) surrounding the tubules. Since water will always 

 pass from a dilute to a more concentrated fluid, and since the glomerular 

 filtrate is, according to the theory, poorer in solid constituents than 

 the serum, water will pass from urine to lymph, and the urine will 

 become more concentrated until it acquires the normal characters of 

 urine. 



In this theory of Ludwig there are three distinct propositions to be 

 investigated. These are 



1. That the secretion of water is a purely mechanical process, 



1 Wagner's " Handworterbuch," 1844, Bd. ii. S. 637; " Lehrbuch der Physiologic,' 

 Aufl. 2, 1858, Bd. ii. S. 373. 



