Bladder (urinary) is a hollow bag for the reception of the urine. When moderately distended it has a capacity of about a pint, but under various conditions it is capable of holding far more. Thus, the bladder of a drunkard has been seen so dilated that it held twenty pints, and when completely paralysed there is no limit to its distension. Instances of very large bladders are not infrequent in females, but this is probably the result of over-distension, and not due to the female bladder being naturally larger than that of the male. Indeed, competent authorities state that the reverse is the case.
In all young animals possessing a bladder, it is elongated in form, indicating that originally it was a tube. In an infant it is pear-shaped—the condition which is permanent in quadrupeds; but as man gradually assumes the erect attitude, the lower part expands under the weight of the urine. In the adult, its size, shape, position, and relations to surrounding organs vary according to its state of collapse or distension. When empty it sinks deeply into the pelvis, and appears flattened above and below; when moderately full its form is rounded, but still it remains within the pelvis; when completely distended it projects above the brim of the pelvis, becoming an abdominal organ, and having an ovoid or egg-shaped outline. It now presents a base or larger end directed backwards, an apex or summit—the smaller end—directed upwards and resting against the lower part of the anterior abdominal wall. At the front of the base, the urethra, which leads off the urine, commences abruptly in a thickened portion named the neck. At this point the bladder is fixed to the walls of the pelvis by fibrous bands named true ligaments. In all other directions it is freely movable, although receiving support from surrounding parts such as rectum or vagina, according to the sex, also to a slight extent from blood-vessels and the two ducts from the kidneys (ureters), and lastly from the peritoneum, which, on account of its various reflections from the organ, forms folds known as the false ligaments. The wall of the bladder consists of several layers or coats, in which blood-vessels and nerves ramify. The outer coat is formed of peritoneum, which only partially invests the bladder, being found on its upper and posterior half. The next coat consists of bundles of muscular fibres arranged in various directions. Between this and the lining membrane there is a well-marked layer consisting of areolar tissue, in which the blood-vessels ramify, and named the vascular or submucous coat. The lining membrane (mucous coat) is soft and smooth. It is continuous with the lining membrane of the ureters and that of the urethra. Being loosely attached, it is nearly everywhere thrown into wrinkles, which disappear as the bladder is distended. If an imaginary line be drawn between the openings of the ureters, and from each end of this line others be drawn to the orifice of the urethra at the neck of the bladder, a triangular portion of the base will be mapped out, having its apex pointing forwards, and each side measuring about an inch and a half. Within this area, to which the name trigone is applied, there are never any wrinkles, even when the bladder is empty, because the mucous coat adheres more closely to subjacent parts. This area has a greater number of blood-vessels and nerves than other parts of the bladder, and therefore possesses more acute sensibility. As the urine accumulates in the bladder, its pressure on the trigone gives rise to the feeling of a full bladder and the necessity for micturition. The intense pain produced by stone in the bladder is due to the same cause (see CALCULUS). As the bladder becomes distended, its muscular walls begin to contract periodically upon the fluid contents; so do the muscular fibres which surround the outlet of the bladder—viz. the urethra. As long as the pressure within the bladder is not very high, the latter is able to resist the former, as is seen during sleep; but after a certain stage of distension is reached, the contractions of the bladder wall overcome those of the fibres closing the urethra, and the bladder is emptied. This process occurs naturally in young children, but as age advances the fibres surrounding the outlet of the bladder come under the control of the will, and thus the urine can be voluntarily retained or expelled. The bladder is liable to catarrhal inflammation of the coats, to paralysis, and other affections. Rupture of the bladder by accident is usually fatal. For incontinence of urine and retention of urine, see URINE. There is no urinary bladder in birds (see BIRD).
In general zoological usage, the term bladder is applied (1) to that outgrowth from the dorsal surface of the gut which forms the Air- or Swim-bladder (q.v.) of many fishes, and serves in a few cases as an incipient lung; and (2) to the urinary bladder. It is necessary to note, however, that the latter may have a very different origin in different animals. (a) In the gristly fishes the terminal extremities of the so-called 'Wolfian ducts' coalesce to form a urinary cloaca, which opens into the general cloacal chamber. (b) In the bony fishes the urinary bladder is a dilatation at the exit of the ureter. (c) In amphibia the bladder arises as an outgrowth from the hind-gut, and represents the allantois of higher animals. (d) In reptiles the stalk of the allantois dilates to form a permanent urinary bladder, which is absent in birds. (e) In mammals the stalk of the allantois again dilates to form the bladder.