MICTURITION 373 



the first lumbar nerves (Fig. 130). On the other hand, when 

 the mucous membrane of the internal trigone is irritated by 

 the presence of a vesical calculus, referred pain is experi- 

 enced in the perineum and along the penis, i.e., in the areas 

 supplied by the terminal branches of the second and third 

 sacral nerves (Fig. 130). 



The Act of Micturition is partly automatic and partly 

 under voluntary control. As the bladder fills, the inhibitory 

 nerves are stimulated and the muscular walls become relaxed, 

 with the exception of the circular fibres which surround the 

 internal urethral orifice, and they, on the other hand, become 

 tonically contracted. As the intra-vesical pressure increases, 

 the afferent nerves of the bladder are stimulated and the 

 micturition reflex is brought into play. This reflex is con- 

 trolled by a centre in the hypogastric sympathetic plexus 

 (p. 1 88). So long as the connexions between this centre and 

 the cerebral cortex are intact, the subject becomes conscious 

 of the desire to micturate, and he is able to control it. If, 

 however, the connexions are interfered with, as in fracture 

 dislocations in the mid or upper thoracic region, the act of 

 micturition becomes entirely automatic (p. 48). 



A lesion of the lower thoracic or upper lumbar region of 

 the spinal medulla may lead to paresis of the bladder. In 

 this condition the viscus is able to distend, but is unable to 

 evacuate its contents. Unless the possibility of this con- 

 dition is recognised, the bladder will become enormously 

 distended, and, eventually, the urine will commence to dribble 

 away. As the nervous mechanism regains control after a short 

 time in some cases, it is important that the damage caused by 

 overstretching of the walls of the bladder should be anticipated 

 and prevented by the periodical passage of a catheter. 



Frequency of micturition, with or without straining efforts, is 

 a common symptom in disturbances of the urinary tract. It is 

 necessary to remember, however, that this symptom is the out- 

 come of the presence of a "focus of irritation " (p. 195) in the 

 lumbar region of the spinal medulla, because, although such 



