KIDNEY AND SKIN AS EXCRETORY ORGANS. 859 



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 Pellacani* 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 

 happens that we have sometimes a strong desire to micturate when 

 the bladder contains but little urine, for example, under emotional 

 excitement. In such cases if the micturition is prevented, probably 

 by the action of the external sphincter, the bladder may sub- 

 sequently relax and the sensation of fullness and desire to micturate 

 pass away until the urine accumulates in sufficient quantity, or the 

 pressure is again raised by some circumstance which causes a reflex 

 contraction of the bladder. 



Nervous Mechanism. According to Langley and Anderson,f the 

 bladder in cats, dogs, and rabbits receives motor fibers from two 

 sources: (1) From the lumbar nerves, the fibers passing out in the 

 second to the fifth lumbar nerves and reaching the bladder through 

 the sympathetic chain and the inferior mesenteric ganglion and 

 the hypogastric nerves and plexus (Fig. 287). Stimulation of 

 these nerves causes a comparatively feeble contraction of the blad- 

 der followed by an inhibition. (2) From the sacral spinal nerves, 

 the fibers originating in the second and third sacral spinal nerves, 

 or in the rabbit in the third and fourth, and taking their course 



* "Archives italiennes de biologie," 1, 1882. 

 f "Journal of Physiology," 19, 71, 1895. 



