186 ELEMENTARY PHYSIOLOGY less. 



communication with the exterior by means of the cavity 

 of the tubule ; and, further, that in each vessel of the 

 glomerulus a thin stream of blood constantly flows, only 

 separated from the cavity of the tubule by the capillary 

 wall and the very delicate membrane covering the glomer- 

 ulus. The Malpighian capsule may, in fact, be regarded 

 as a funnel, and the membranous walls of the glomerulus 

 as a piece of very delicate but peculiar filtering-paper, into 

 which the blood is poured. 



And indeed we have reason to think that a great deal 

 of the water of urine together with certain of the con- 

 stituents (the inorganic .salts) is thus as it were filtered off 

 by the Malpighian capsules. But it nmst be remembered 

 that the process is after all very different from actual 

 filtering through paper ; for filter paper will let everything 

 pass through that is really dissolved as well as bodies so 

 small as blood-corpuscles, whereas the glomerulus, while 

 letting some things through, refuses to admit others, e.g. 

 the proteins of the plasma, even though they are in a 

 state of solution. 



Speaking of the process, with this caution, as one of 

 filtrati(m, it is obvious that tlie more full the glomerulus 

 is of blood the more rapid will be the escape of urine. 

 Hence we find that when blood flows freely to the kidney 

 the urine is secreted freely, but that when the blood 

 supply to tlie kidney is scanty the urine also is scanty. 

 When the renal nerves going to the kidney are cut, the 

 branches of the renal artery dilate, much blood goes into 

 the kiJney, the I)lood-pressure is rai.sed in the glomeruli, 

 and the flow of urine is copious. If the same nerves 

 be stimulated, the arterial tubes are narrowed and con- 

 stricted, less blood goes to tlie kidney, blood-pressure is 

 reduced, and the flow of urine is scanty or may be stopped 

 altogether. 



We can now explain, in part at all events, how it is that 

 the activity of the kidney is influenced by the state of the 

 skin. The quantity of blood in the body, being about the 

 same at all times, if a large quantity goes to the skin, as 



