EXCRETION AND THE EXCRETORY ORGANS 473 



the epithelial cells of the tubules are stimulated to secrete, and 

 urine rich in urea is formed ; but in these circumstances it cannot 

 come from the Malpighian bodies. It would seem then that urea 

 is a special stimulant to some cells of the tubules, and that an 

 excess of it in the blood can stir them up to its elimination along 

 with some water, quite independently of any formation of tran- 

 sudation urine. 



The Relation of Renal Blood-Flow to the Secretion of Urine. 

 The kidneys have probably a richer blood supply than any other 

 organs of the Body. It has been estimated that under proper 

 circumstances their own weight of blood may flow through them 

 each minute. This rich blood supply is, of course, an adaptation 

 to secure the withdrawal of waste substances from the blood at a 

 rapid rate. From the structure of the glomeruli and the fact that 

 most of the water of the urine is derived from them it is a priori 

 probable that anything tending to increase the pressure of blood 

 in them will increase the bulk of urine secreted, and anything 

 diminishing that pressure will decrease the urine. This is con- 

 firmed by experiment. The kidney is supplied with both vaso- 

 constrictor and vasodilator nerves which reach it mainly through 

 the solar plexus. When the spinal cord is cut in the neck region 

 of a dog the kidney vessels as well as those of the rest of its Body 

 dilate and blood-pressure everywhere is very low. Under these 

 circumstances the secretion of urine is suppressed. If the lower 

 end of the cut cord be stimulated the vessels all over the Body of 

 the animal contract, and blood-pressure everywhere becomes very 

 high. But the kidney vessels being constricted with the rest allow 

 very little blood to enter the glomeruli in spite of the high aortic 

 pressure, and little or no urine is secreted. If, however, the vaso- 

 constrictor nerves of the kidney be cut before the stimulation of 

 the cord, we get a dilatation of the kidney vessels with a constric- 

 tion of vessels elsewhere, and abundant blood flows through the 

 glomeruli under high pressure : the whole kidney swells and abun- 

 dant urine is formed. When the skin vessels contract on exposure 

 to cold, more blood flows through internal organs, the kidneys 

 included, and the blood-pressure in these is if anj^thing increased, 

 the expansion of internal arteries not at the most more than 

 counterbalancing the constriction of the cutaneous. Hence the 

 greater secretion of urine in cold weather. 



