BENAL SECKETION. 649 



such circumstances, is a purely reflex action, and may be carried on 

 without the assistance of the will. 



The state of contraction of the vesical muscular fibres, as was 

 found to be the case as regards the rectum, is governed by a spinal centre 

 located in the lumbar portion of the spinal cord. When the spinal cord 

 is divided in the dorsal region in a dog, after the shock of the operation 

 has passed off the bladder may fill with urine, and, when distended, 

 empties itself in a perfectly normal manner. 



The distention of the bladder starts sensory impulses, which are 

 conducted to the spinal cord through the posterior roots of the third, 

 fourth, and fifth sacral nerves. The centre of micturition, which in dogs 

 is situated opposite the fifth and in rabbits opposite the seventh lumbar 



c 



FIG. 270. DIAGRAM OF THE NERVOUS MECHANISM OF MICTURITION. ( Yeo.) 



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 the 

 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 the neck of the bladder, whence impulses pass by 3 to 

 inhibit the sphincter centre and excite the detrusor through 4. 



vertebra, is then called into play, and the muscular fibres of the sphinc- 

 ter of the bladder relax, while contractions of the longitudinal fibres, or 

 the so-called detrusor urinae muscle, are called forth. 



The contraction of this muscle serves to contract the capacity of 

 the bladder in all directions, its contents are thus forced out through 

 the relaxed sphincter muscle through the urethras, and urination is ter- 

 minated by the rhythmical contraction of the bulbo cavernosus, or 

 ejaculator nrinae muscle. 



Ordinarily the emptying of the bladder is assisted by the co- 

 operation of the abdominal muscles in the same way as their contraction 

 aids in defsecation. A deep inspiration is made as the bladder becomes 



