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Proceedings of the Boyal Society 
the tubules and the fluid inside by which a certain amount of the 
water passes again into the blood, and so leaves the urine in a more 
concentrated state than it was when it first passed from the glome- 
rulus into the dilated end of the urine tubes (Ludwig). It is believed, 
however, that the epithelium, which lines the convoluted tubes, per- 
forms certain functions in connection with the secretion of the solid 
constituents. 
The rapidity of the secretion of urine may be said to depend upon 
the following factors: — (1) The relationship which exists between 
the pressure of the blood in the glomerulus of vessels and the urine 
in the capsule of the Malpighian body and in the tubuli uriniferi ; 
(2) the state of the blood pressure in the venous system of the kid- 
ney; (3) the pressure upon the lymphatics; (4) the quality of the blood 
in the artery of the Malpighian tuft ; (5) the state of the walls of the 
artery constituting the Malpighian tuft, and of the capsule itself, 
these being regarded as the filter through which the fluids and soluble 
constituents of the blood have to pass. The influence of vaso-motor 
nerves upon secretion must not, however, be forgotten; not only do 
they exert an influence upon the quantity and quality of the secre- 
tion by dilating or contracting the arterioles, but their influence 
upon the chemical processes, by reason of their communications 
with the secreting cells (Pfliiger), must be remembered ; (6) activity 
of tubular epithelium. 
In the experiments I have endeavoured to imitate the conditions 
found in the Malpighian body of the kidney. I have not been 
able, however, to represent the lymphatic arrangement. 
The apparatus, a drawing of which, kindly executed for me by 
my friend Dr Robert Moffatt, is shown in the following woodcut. It 
consists of a piece of rabbit’s bowel A 2 enclosed in a glass tube B 2 . 
To each end of the bowel a small glass T -tube is attached. One of 
those tubes is connected with a pressure-bottle A, and a mano- 
meter A 1 . The pressure exercised upon the fluid inside the bowel 
by the pressure-bottle and indicated by the manometer A 1 will be 
designated the afferent pressure. The tube attached to the other 
side of the bowel conveys the fluid that passes along the bowel to 
the vessel B, and the pressure exercised upon the fluid which it 
contains, called the efferent resistance, is indicated by the manometer 
B 1 . Through a cork at the right hand side of the large tube, another 
