viii THE EXCRETION OF URINE 425 



the neighbourhood of the papillae than in other parts of the 

 kidney substance. The spaces not filled by connective tissue are 

 lined in some places by epithelioid platelets, and form the lymph 

 channels which bring to the epithelial cells of the uriniferous 

 tubules both nutrient materials and the urinary constituents, 

 which, as we shall see, it is their office to expel. 



The renal artery, at the point at which it enters the hilum of 

 the kidney, is surrounded by a plexus of nerve filaments (plexus 

 renalis), which consists partly of medullated fibres, varying con- 

 siderably in size, but principally of non-medullated fibres. The 

 nerves of the renal plexus come chiefly from the great solar plexus, 

 which contains fibres of the vagus, as well as of the greater and 

 lesser splanchnic nerves. Numerous small ganglia are also present 

 in the renal plexus. 



The peripheral relations of the nerve fibres of the kidneys 

 with the branches of the small arteries, and with Bowman's 

 capsule and the convoluted tubules, were worked out in 1893 by 

 Berkley. We shall discuss the importance of this discovery later. 



Besides the vasomotor and the secretory fibres, there are 

 undoubtedly sensory fibres to the kidney, seeing that in certain 

 morbid conditions the kidneys can be the seat of acute pain. 



II. The Secretion of Urine is constant, but it fluctuates con- 

 siderably with different circumstances, even under normal condi- 

 tions. On introducing cannulae into both ureters of an animal 

 (or of man by means of the urethral catheter, as clinically 

 employed) it can be seen that, independent of irregularity and 

 asymmetry of excretion, due to the peristalsis of the ureters, 

 secretion is never parallel in both kidneys, but is greater now in 

 the right, now in the left. Apparently there is a kind of alterna- 

 tion of secretory activity and of blood supply in the two kidneys 

 (Ludwig). This shows that the secretion depends not only upon 

 general central conditions, but also upon local peripheral condi- 

 tions which vary in each kidney at different times. 



Although bilateral nephrectorny is fatal in a short time by 

 causing uraemic intoxication (supra], the excision of one kidney 

 alone is compatible with life under normal conditions, not merely 

 in animals but also in man, on whom it has repeatedly been 

 carried out as a surgical operation. This fact shows that removal of 

 one kidney does not diminish the total secretion, and that the 

 kidney which is left may supplement and compensate the lost 

 function of the other, by increased secretory activity. 



So long as the urinary secretion is going on, i.e. as long as 

 urine continues to trickle from the cannula fixed to the ureter, 

 the blood that leaves the kidney by the efferent vein is not dark 

 like the venous blood returned from the muscles, but bright red, 

 like arterial blood (Cl. Bernard). This shows that the velocity of 

 circulation in the kidney is much greater than in the other organs, 



