The walls of the bladder consist of involuntary muscular fibre which expels the urine by its contraction. Around the neck of the bladder is a band of involuntary muscular fibre, the sphincter vesicae, which by its contraction closes the orifice and prevents the escape of urine. The sphincter vesicae receives its motor supply through the third, fourth, and fifth sacral nerves.

The nerve-centre for the movements of the bladder is situated in the spinal cord opposite the fifth lumbar vertebra in dogs, and the seventh in rabbits. This centre is able to regulate the retention and discharge of the urine by the bladder even when the spinal cord is divided between it and the brain, but the activity of the centre under normal conditions is modified by the brain, so that we may consider that there is a cerebral as well as a spinal centre for the bladder. The spinal centre may be set in action either reflexly, or by stimuli passing down from it to the brain. The cerebral centre may be set in action either reflexly or voluntarily.

Usually when the pressure of the urine within the bladder is increased beyond a certain limit depending not only on the quantity of the water, but on the state of the contraction of the bladder itself, the neck of the bladder becomes slightly dilated, and a drop of urine exuding acts as a stimulus to the sensory nerves of the urethra, and thus calls reflexly into action the centre in the spinal cord by which at the same time the sphincter vesicae is inhibited, and the detrusor urinae stimulated. Reflex action may also be induced by stimulation of other nerves, as for example by the application of a wet sponge to the anus or perinaeum. The cerebral centre is usually called into action by the sensation of the bladder being full. It may be called into action voluntarily, although there is little urine in the bladder; and also may be excited by emotion, such as fear.

It may be also excited reflexly through the sense of hearing. Boerhaave was accustomed, when patients found difficulty in passing water, to make an attendant pour water from a height into a basin in the patient's hearing. The splashing thus occasioned induced the patient to pass water, and a similar effect, as is well known, is produced on horses by whistling. Nervous agitation has often the contrary effect of producing retention of water. When it is desirable for a person to pass water - e.g. when a specimen of urine is wanted for examination - it is advisable to put him in a room by himself and turn on a tap within his hearing. The removal of the restraint exercised by the presence of another person, along with the stimulant action of the sound of falling water, rarely fails to produce the desired effect. Even the recollection of the sound of falling water will tend to cause evacuation of the bladder, and when there is difficulty in passing water the patient may sometimes obtain relief by thinking of a waterfall. Washing the hands in cold water also tends reflexly to cause evacuation of urine, and the effect of a wet sponge to the perinaeum has already been mentioned.

Vesical sedatives are substances which lessen the irritability of the bladder, and thus remove pain, and lessen the desire to urinate. This desire may be excited not only by the presence of urine in the bladder, but by the irritation of calculi, or inflammation of the mucous membrane of the bladder itself. When calculi are a source of irritation, carbonate of calcium taken internally seems to lessen the irritability. In cystitis the irritation is diminished by the use of very hot water externally, in a bidet or hip-bath. The irritability of the nerves may be diminished by opium, belladonna, and hyoscyamus, and by drinking freely of warm water, either alone or in the form of an infusion or decoction of some mucilaginous substance, e.g. linseed-tea or barley-water.

In chronic inflammation the irritation may be diminished by astringents such as buchu, uva ursi, pareira brava, and alche-milla. Vesical tonics are substances which increase the contractile power of the muscular fibres in the bladder. They are therefore useful in two different conditions, for by strengthening the detrusor urinae they prevent retention, and by strengthening the sphincter vesicae they prevent incontinence.

Some of these remedies appear to act by increasing the stimulating power of the urine, so that the sphincter vesicae is consequently more firmly contracted; of this class is cantharides. Others appear to alter the direction of reflex action; such are the passing of a bougie through the urethra once or twice a day, or the application of an injection of nitrate of silver, ten to thirty grains to the ounce, to the neck of the bladder. Others act on the nerve-centres and apparently are useful sometimes by lessening the reflex susceptibility from the bladder, so that the detrusor urinae is less called into action; at other times by increasing the susceptibility of the nerve-centre, so that the sphincter vesicae is more firmly contracted - of the latter class is strychnine; to the former belongs bromide of potassium, which must be given at night. Belladonna, which is one of the most useful remedies in incontinence of urine, acts upon the nerve-centres, but whether it acts in the same way as strychnine or as bromide of potassium, it is difficult to say. It is quite possible that it lessens the sensibility of the bladder to changes of pressure within it in somewhat the same way as it lessens the sensibility of the heart to changes in blood-pressure (p. 298).