The urine, which is continuously secreted and rhythmically conveyed to the bladder, is only voided at convenient times; therefore special arrangements exist for its retention and expulsion.
The retention of urine in the bladder up to a certain point depends on the elasticity of the parts concerned, the dense elastic tissues around its outlet being able to resist the elastic force exerted upon its contents by the walls of the bladder and the viscera. Thus, where no active muscular forces can possibly come into play, as in the case of the dead subject, or in complete paralysis following destruction of the spinal cord, a considerable amount of urine is retained. But when a certain pressure is arrived at by the gradual accumulation of urine within the bladder, the elasticity of the sphincter and the other tissues around the outlet is overcome by the elasticity of the bladder wall, and the urine slowly dribbles away.
In the normal condition, however, the urine is retained by a muscular mechanism over which we have acquired considerable voluntary control.
This is the sphincter muscle, which, by contracting, helps the elastic power of the tissues around the urethra and retains the urine. The accumulation of urine after a certain time gives the sensation known as a full bladder, but this feeling is not necessarily accompanied by any irresistible call to make water, though it soon produces a desire in that direction. We suppose that the stimulus given to the afferent nerves by filling the bladder reflexly causes a constriction of trie sphincter muscle, so that, in proportion as the pressure within the bladder increases, the resistance to its outflow is also augmented. This does not imply any automatic action of the sphincter vesicae, but merely a constant reflex excitation of that muscle, which secures its contraction and the retention of a considerable amount of urine without the intervention of voluntary influences or attention. When the bladder becomes very 'full, the reflex mechanism may require the assistance of the voluntary centres to augment this power and prevent the urine being evacuated.