KIDNEY AND SKIN AS EXCRETORY ORGANS. 865 



of the abdomen, especially toward the end of the act. As in defeca- 

 tion and vomiting, the contraction of the abdominal muscles, when 

 the glottis is closed so as to keep the diaphragm fixed, serves to in- 

 crease the pressure in the abdominal and pelvic cavities, and thus 

 assists in or completes the emptying of the bladder. It is, 

 however, not an essential part of the act of micturition. The last 

 portions of the urine escaping into the urethra are ejected, in the 

 male, in spurts produced by the rhythmical contractions of the 

 bulbocavernosus muscle. 



The act of micturition as it takes place in man is pictured by 

 Rehfisch* as follows: As the urine accumulates in the bladder the 

 pressure-stimulation of the sensory fibers leads to a reflex stimu- 

 lation of the internal sphincter. Further accumulation by a pres- 

 sure effect on the sensory fibers causes reflex contractions of the 

 muscle of the bladder, and the additional sensory stimuli produced 

 by these contractions spreading upward from the lower center occa- 

 sion the conscious desire to urinate. In the adult at least the 

 urination takes place by a voluntary act, which consists in an inhi- 

 bition of the tonus center in the lumbar cord, whose reflex stimula- 

 tion has up to this point maintained the tonic contraction of the 

 internal sphincter. The effect of this inhibition is to relax the 

 sphincter and to allow the bladder to empty itself by its reflex 

 contraction, aided perhaps by a voluntary contraction of the ab- 

 dominal muscles. During the emptying of the bladder the process 

 can be brought to a stop voluntarily by removing the inhibition, 

 thus allowing the internal sphincter to contract and shut off the 

 flow. Rehfisch considers that the external sphincter (and compres- 

 sor urethrse) play a relatively small role, serving as a reserve mech- 

 anism to aid in the closure of the urethra. 



Mosso and Pellacanif have made experiments upon women in 

 which a catheter was introduced into the bladder and connected 

 with a recording apparatus to measure the volume of the bladder. 

 Their experiments indicate that the sensation of fullness and desire 

 to micturate come from sensory stimulation, in the bladder itself, 

 caused by the pressure of the urine. They point out that the 

 bladder is very sensitive to reflex stimulation; that every psychical 

 act and every sensory stimulus is apt to cause a contraction or in- 

 creased tone of the bladder. The bladder is therefore subject to 

 continual changes in size from reflex stimulation, and the pressure 

 within it will depend not simply on the quantity of urine, but on 

 the condition of tone of its muscles. At a certain pressure the 

 sensory nerves are stimulated and under normal conditions mictu- 

 rition ensues. We may understand, from this point of view, how it 



* Virchow's "Archiv f. path. Anat.," etc., 150, 111, 1897. 

 t "Archives italiennes de biologic," 1, 1882. 

 55 



