THE KIDNEYS AND THE URINE. 77 



by cell-power, but the water and saline elements are given off here 

 by the blood by a process of simple filtration. The amount of 

 fluid which passes here is governed by the blood-pressure in the 

 arteries of the kidney and by the fluidity of the blood. 



(2) The epithelium of the uriniferous tubules has secreting 

 function, and is able to separate from the blood foreign substances 

 (e.g. indigo-carmine) and eject them into the tubules, and to man- 

 ufacture from material taken from the blood new substances not 

 found there (e.g. urea and pigments). 



What is the course of the urine after leaving the kidney ? 



The urine collected in the tubules of the kidney passes into 

 the pelvis, and is carried to the urinary bladder in irregular quan- 

 tities by the ureter. The ureters simply act as ducts, and do 

 not store up urine, nor do they usually actively eject it into the 

 bladder. As a few drops of urine collect in the pelvis of the kid- 

 ney, they run into the bladder, the action of the two kidneys not 

 being in alternation nor absolutely regular in point of time. Re- 

 gurgitation from the bladder is prevented by the oblique course 

 of the ureter through the muscular wall of the bladder. 



Describe the act of micturition. 



When the bladder is filled the act of emptying it is called mictu- 

 rition. It is a voluntary act, aided by the involuntary reflex con- 

 traction of the muscular coat of the organ itself. The voluntary 

 muscles involved are those of respiration the diaphragm and the 

 abdominal muscles. So far as micturition is involuntary, it is a 

 reflex depending upon a centre in the lumbar spinal cord. 



What are the physical characteristics of normal urine ? 



It is a clear, amber-colored fluid of slightly acid reaction. It 

 may develop a flocculent precipitate of a light cloud of mucus upon 

 standing. It has a characteristic odor and a salty-bitter taste. 



What is the normal specific gravity of the urine ? 



About 1020, but under conditions of health it may vary from 

 1010 to 1030. or even beyond these limits. 



What conditions affect the acidity of the urine ? 



The acidity of the urine is due to the presence of acid sodium 

 phosphate. There is no free acid present, as is shown by the fact 

 that no precipitate is formed upon the addition of sodium hyposul- 

 phite. The degree of acidity varies, being less during active diges- 



