THE PHYSIOLOGY OF MICTURITION 1209 



THE FILLING OF THE BLADDER 



Under normal circumstances the sphincters at the neck of the bladder are 

 in a state of tonic contraction, presenting a resistance to the emptying of 

 this organ which will vary according to their degree of contraction. Thus it 

 requires a considerably greater pressure in the bladder to overcome the 

 resistance of the sphincters during life than after death of the animal. In 

 some cases after death they may permit the passage of urine when the 

 pressure of the bladder is only about 20 mm. water, whereas in the normal 

 animal the pressure has as a rule to be at least 160 mm. of water before any 

 escape takes place. The urine therefore as it is secreted must accumulate 

 and distend the bladder. The bladder wall reacts to a distending force 

 in the manner which is characteristic of all muscular tissue, especially 

 unstriated. An extending force applied to an unstriated muscle fibre has 

 a double effect. In the first place, if the stretching force is applied very 

 slowly, a considerable increase in length of the muscle may occur with the 



U.B. 



AAMAAMAUAAAAAAMA^ 



FIG. 653. Tracings of rhythmic contractions of urinary bladder. 

 (SHERRINGTON.) 



application of a very small amount of force. If however the force be 

 applied more rapidly, the sudden increase of tension acts as a direct excitant 

 to the muscle, causing it to enter into contraction, which may be tonic or 

 rhythmic. The effect of the entry of urine into the empty bladder on the 

 tension in this organ will depend therefore on the rapidity with which the 

 kidneys are secreting. Under normal circumstances micturition occurs 

 in man when the intravesical pressure has risen to about 150 mm. water. 

 Under these conditions the bladder will contain between 230 and 250 c.c. 

 of urine. If however the secretion of urine has occurred very rapidly, the 

 same pressure may be attained with a much smaller bladder content, and if 

 the bladder be artificially distended by the injection of fluid through a 

 catheter, 50 c.c. of fluid may suffice to raise the pressure to this level. As the 

 urine is slowly secreted, the bladder wall at first gives to the incoming fluid, 

 so that a considerable amount can be stored without any marked rise of pres- 

 sure. Later on the pressure begins to rise more rapidly, and finally attains 

 a pressure of between 120 and 150 mm. water. At this point the second 

 effect of the stretching of the muscular wall makes its appearance. A mano- 

 meter connected with the bladder shows a series of rhythmic contractions 

 of the muscular wall (Fig. 553), each lasting about a minute, at first slight 



