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. 
(b) That lln- 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 of this paper fell at a time when, led by Ludwig, 
Helmholtz, and du Bois Eeyinond, 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 Warner's " Handwdrterbuch," 1844, Bd. ii. S. 637; " Lehrbuch der Physiologie,' 
Aufl. 2, 1858, Bd. ii. S. 373. 
