346 MUSCULAR MECHANISMS OF URINARY TRACT. 



circular fibres at the neck of the bladder. The main events in reflex 

 micturition may be summarised as follows : — 



1. Accumulation of urine. 2. Slow rise of intravesical pressure (to 15 cms. 

 water). 3. Rhythmical contractions of bladder, increasing in force with in- 

 creasing tension. 4. Afferent impulses ascending from bladder to lumbo-sacral 

 cord. 5. Reflex discharge of impulses by hypogastric and pelvic nerves, causing 

 contraction of the whole bladder wall, and a rise of intravesical tension to 20 

 or 30 cms. water. 6. The resistance at the neck of the bladder is overcome, 

 the beginning of the urethra is straightened out, and the adhesion of the walls 

 of the empty tube overcome. 7. Discharge of urine and emptying of bladder. 

 8. Contraction of the unstriated and striated muscles surrounding the urethra 

 and emptying this canal. 



At 5 there will be in nearly all cases a reinforcing contraction of the 

 abdominal muscles. 



Voluntary control of micturition. — In the adult, the processes of retention 

 and evacuation of urine are modified and controlled by voluntary effort. The 

 normal action of the sphincter mechanism may be aided by the contraction of 

 the perineal muscles, which keep the urethra closed. The reflex process of 

 micturition may be set in motion by the contraction of the abdominal muscles, 

 by which the pressure in the bladder is increased and the resistance to the 

 passage of urine into the urethra overcome. It is possible, too, that the in- 

 dividual has a certain extent of voluntary power over the unstriated muscles 

 of the bladder, and that the contraction of the muscular wall may be augmented 

 by impulses proceeding from the cortex to the micturition centre in the bladder 

 cord. In this case, the initiation of the act would be analogous to that in 

 which micturition is caused by stimulation of some sensory nerve, since in both 

 cases the centre in the lumbar cord must first be influenced by the stimulus. 



This view is favoured by the fact that contraction of the detrusor urinae 

 can be evoked by stimulation of the crura cerebri, or of the medulla oblongata 

 or anterior columns of the cord (Budge, 1 Mosso 2 ), and that this effect may be 

 observed even after exclusion of contraction of the abdominal muscles by opening 

 the abdomen or by the administration of curari. Bechterew and Mislawsky 3 

 obtained contraction of the bladder on stimulating the sigmoid gyrus in dogs. 



The series of striated muscles which surround the urethra are under the 

 control of the will, so that micturition can be stopped at any time by their 

 forcible contraction. According to some authors, however, there is direct 

 voluntary control over the unstriated fibres at the neck of the bladder, the 

 so-called " internal sphincter," which, as I have already mentioned, does not 

 exist as a distinct muscle. According to Reyfisch, 4 the " internal sphincter" 

 may be inhibited or set into contraction directly by voluntary impulses. 



Thus, if a catheter be pushed up the human urethra until its opening lies 

 in the prostatic part of the urethra just outside the bladder, no urine flows, 

 unless micturition be inaugurated voluntarily by the person experimented on. 

 When micturition is proceeding, the flow can be instantly checked at the will 

 of the person experimented on. Reyfisch considers that the voluntary muscles 

 surrounding the urethra are in this case powerless to stop the stream, since the 

 closure of the canal is prevented by the presence of the rigid catheter. He 

 concludes, then, that the stoppage can only be accounted for by a voluntary 

 contraction of the internal sphincter. It seems to me, however, possible that 

 a lifting up of the base of the bladder and the prostate by the anterior fibres 

 of the levator ani might be responsible for the sudden stoppage in the urinary 

 flow observed in Reyfisch's experiments. 



1 Arch.f. d. gcs. Physiol., Bonn, 1869, Bd. ii. S. 511 ; Firchow's Arehiv, 1865, Bd. xv. 

 S. 115 ; Ztschr. f. rat. Med., 1864, Bd. xix. S. 434. 



2 Arch, ftal.de biol, Turin, 1882. 



3 Neurol. CeritralbL, Leipzig, Bd. vii. 4 Virchow , s Arehiv, 1897, Bd. cl. S. 111. 



