SECRETION. 463 



in the blood, the disease is termed uraemia. The toxic substance is 

 probably not urea, but some other organic body. The usual cause 

 of uremia is Bright's disease. Uric acid in excess is supposed to be 

 the cause of rheumatism and gout. 



Influence of the Nerves Upon the Secretion of Urine. As has 

 been elsewhere stated, the nerves of the kidneys are derived from 

 the renal plexus and are composed of both medullated and non- 

 medullated fibers with nerve-cells. These are both vasodilator and 

 vasoconstrictor in function. As yet, no true secretory nerves are 

 known, so that it is by the influence of the vasomotor nerves dis- 

 tributed along the course of the renal vessels that variations in the 

 amount of urine secreted occur. Thus, the amount of urine secreted 

 depends upon the pressure of the blood circulating through the 

 capillaries. 



Frequent and small urinations, under mental apprehension, 

 show a very probable nervous influence upon the excretion of the 

 urine. Polyuria and the peculiar aspect of the urines of hysteria 

 are also known; whether these peculiarities are dependent upon 

 direct nervous influence upon the secretion is not known. Ludwig 

 believes that the cause lies in the increased pressure in the renal 

 arteries from spastic contraction of other vascular regions. 



Injury by puncture of the vasomotor center in the floor of tKe 

 fourth ventricle likewise is followed by polyuria, accompanied by 

 haematuria and albuminuria. By this experiment it is demonstrated 

 that variations in urinary secretion are, for the most part, very inti- 

 mately concerned with vasomotor innervation. 



If, while the renal vasomotors are paralyzed, the majority of the 

 vasomotor nerves of the entire body be also paralyzed (as by section 

 of the medulla), there follows a general dilatation of the arterioles 

 and capillaries of the body. This causes such a decided fall in the 

 blood-pressure that the amount of urine secreted is much diminished 

 or entirely absent. 



However, secretion is not suspended by removal of the brain, 

 nor destruction of the spinal cord below the cervical portion, pro- 

 vided that the medulla is intact and with it the respiration and cir- 

 culation. (Krimer.) 



Urinary Excretory Apparatus. After the urine has been se- 

 creted by the kidneys, it must needs be carried away from the body, 

 so that the economy may not suffer from resorption of contained 

 toxic principles, as well as not to interfere with the renal action by 



