430 ANIMAL PHYSIOLOGY. 
in man at all events, and partly from the pressure within the 
tubules of the kidneys themselves, 2. The evacuation of urine 
may take place independently of the will (see 8), and is a reflex 
(5) act. 3. Micturition may be initiated by the will, which is 
usually the case, when by the action of the abdominal muscles 
a little urine is squeezed into the urethra, upon which afferent 
impulses set up contractions of the bladder by acting on the 
detrusor center of the cord and at the same time inhibit the 
center presiding over the sphincter (if such there be), permit- 
ting of its relaxation. 4. Emotions seem to interfere with the 
ordinary control of the brain-centers over those in the spinal 
cord. 5. It may be assumed that the normal tone of the 
sphincter of the bladder is maintained reflexly by the spinal 
cord. The unwonted muscular contraction associated with an 
obstruction to the outflow of urine may be in part of nervous 
origin, but is also, in all probability, owing in some degree to 
the muscle-cells resuming an independent contractility, due to 
what we recognize as the principle of reversion. The same is 
seen in the heart, ureters, and similar structures. 
Pathological— There may be incontinence of urine from pa- 
ralysis, the cerebral centers being unable to control those in 
the spinal cord. Dribbling of urine may be due to retention in 
the first instance, the tone of the sphincter being finally over- 
come, owing to increase of pressure within the bladder. Over- 
distention of the bladder may arise in consequence of lack of 
tone in the muscular walls, though this is rare. Strangury is 
due to excessive action of the walls of the bladder and the 
sphincter, brought about refiexly, when the organ is unduly 
irritable, as in inflammation, after, the abuse of certain drugs 
(cantharides), etc. 7 
Comparative-——In man the last drops of urine are expelled by 
the action of the bulbo-cavernosus muscle and perhaps some 
others. In the dog and many other animals the regulated and 
voluntary use of this muscle, marked in a high degree, produces 
that interrupted flow so characteristic of the micturition of 
these animals. 
Summary.—Urine is in man a fluid of specific gravity 1015 
to 1020, acid in reaction, pale yellow in color, and containing 
certain salts, pigments, and nitrogenous bodies. The chief of 
the latter is urea, which is excreted daily to the extent of about 
one ounce (500 grains). 
The kidneys and skin especially supplement one another, 
and normally great activity of the one implies lessened ac- 
