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MANUAL OF PHYSIOLOGY. 



nerve channels passing to and from this centre. The centres 

 may be said to be composed of functionally distinct parts a re- 

 taining and evacuating part. The retaining centre causes the 

 sphincter muscle to contract. The evacuating centre can excite 

 the detrusor to action while the sphincter is relaxed by the inhi- 

 bition of its exciting centre. One set of nerve channels commu- 

 nicates between these centres and the urinary organs, and the 



Diagram of the Nervous Mechanism of Micturition. B. Bladder. M. Abdominal 

 muscles, c. Cerebral centres. R. Represents impulses which pass from the bladder to 

 the centre in the spinal cord, whence tonic impulses are reflected and pass along T to 

 sphincter, which retains the urine. When the bladder is distended, impulses pass to the 

 brain by 1, and when we will, the tonus of the spinal centre stimulating the sphincter 

 is checked, and the abdominal muscles are made by 2 to force some urine into neck of 

 bladder, whence impulses pass by 3 to inhibit the sphincter centre and excite the de- 

 trusor through 4. 



other between the cord centres and the cerebral hemispheres. 

 That which connects the special lumbar centres with the bladder, 

 contains motor (efferent) fibres of two kinds, going to the antag- 

 onistic muscles, the sphincter vesicse, and the detrusor urinse re- 

 spectively, and sensory (afferent) fibres of different kinds ; those 

 going from the bladder to the nerve cells in the cord which 



