THE PHYSIOLOGY OF MICTURITION 1293 



send white rami communicantes to the lateral chain of the sympathetic, 

 and thence to the collateral ganglia, which are grouped round the 

 inferior mesenteric artery to form the inferior mesenteric ganglion. 

 Most of the fibres end in this collection of ganglion-cells, and a new relay 

 of axons passes by two main trunks, the hypogastric nerves, into the 

 pelvis on each side of the rectum and ends in a plexus, the hypogastric 

 plexus, at the base of the bladder. From this plexus fibres pass to the 

 bladder wall. The pelvic visceral nerves are derived from the second 

 and third sacral nerves. They make no connection with the sym- 

 pathetic system, but pass directly to the hypogastric plexus and are 

 carried with branches of this plexus to the neck of the bladder. The 

 fibres do not run directly from the spinal cord to their ending in the 

 bladder wall, but make connection with cells situated peripherally, 

 partly in the hypogastric plexus, but chiefly in the walls of the bladder 

 itself. Both sets of fibres supply also the rectum and the colon, and 

 carry efferent impulses to the bladder. Afferent impulses from the 

 bladder travel chiefly in the pelvic visceral nerves. 



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 

 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 circum- 

 stances 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 



