The continuous excretion of liquids from the kidneys has given rise to the necessity for a temporary storage sac which may be emptied at suitable intervals. There are three general types of urinary sacs, or bladders, namely, tubal, cloacal, and allantoic.
Tubal bladders, which are present in most fishes from the ganoids on, are formed by the widening or enlargement of the urinary ducts. In many fishes two independent bladders may form, vesica duplex (Fig. 368A), one near the end of each urinary duct, with the two ducts afterwards uniting into a common passage-way of exit; or the two may run together into a common bilobed bladder vesica bicornis (Fig. 368b), as in Lepidosteus and some other ganoids; or finally, the two excretory ducts may first join and then expand into a single bladder, vesica simplex (Fig. 368c), as for example, in the pike, Esox. In all of these cases the urinary ducts enter at one end of the bladder, while the exit is made at the opposite pole. The tubal bladder, which is frequently larger in the female than in the male, is most common in certain teleost fishes like the Pleuronectidae that have no swim bladder. It is somewhat difficult to account for the presence in fishes of these structures that are rarely absent yet of doubtful utility.
The cloacal bladder occurs in dipnoans, amphibians (Fig. 369), and monotremes. It is a diverticulum of the cloacal wall opposite the point where the urinary ducts, with which it has no direct connection, enter. It is located dorsally in lungfishes and ventrally in amphibians. In the perennibranchiate amphibians it is considerably elongated, but rounded and broadened in frogs and toads. Frequently it is bilobed while in some urodeles, for example Salamandra, Triton, and Eurycea, the lobes are prolonged into hornlike processes.
Cloacal bladders are filled by the closure of the outer cloacal sphincter and the backing up of the urinary secretion into them.
The allantoic bladder, according to most embryologists, arises from the enlargement of the proximal or basal end of the embryonic allantoic stalk. It is characteristic of mammals and of such reptiles as turtles and certain lizards that have a bladder. In the case of other amniotes, like snakes, crocodiles, some lizards, and birds, the whole allantois degenerates without developing a bladder. In mammals that part of the allantoic stalk left within the body wall, when the umbilical cord connected with the placenta is severed at birth, enlarges to form the bladder, and also the urachus, or vesicoumbilical ligament which anchors the bladder to the inner body wall at the umbilicus. Thus the proximal end of the allantois stalk enlarges into the hollow sac of the bladder while the distal part within the body wall undergoes quite a different fate in being transformed into a solid ligament utilized as a guy rope to support the bladder.
The occurrence of an occasional urachal cyst in man, with urine escaping through the umbilicus by a fistula, apparently demonstrates the embryonic derivation of the urachus and bladder from the common origin of the allantoic stalk.
Arey says, however, “Contrary to earlier views, the allantois contributes nothing to the bladder or urachus.” According to his view the mammalian bladder is a derivative of the embryonic cloaca. Future studies on this question will be welcome.
The mammalian bladder is lined with mucous membrane and coated on the outside with peritoneum. It has a highly muscular wall abundantly supplied with nerves and blood vessels, the involuntary muscle fibers being diverted from their originally regular, longitudinal and circular arrangement so that they interweave like felt in many directions. Upon contraction the cavity of the bladder becomes smaller, therefore, in all dimensions like a leaking toy balloon, rather than collapsing like a hot-water bag from which the water has been emptied.
The exit from the bladder is by way of a single duct, the urethra, the entrance to which is kept closed by a muscular sphincter, except periodically, during micturition, when, upon the relaxation of the sphincter, the urine is expelled by the contraction of the muscular walls of the bladder.
There is considerable variation in the location of the points where the ureters enter the bladder. Only rarely, as in Lepus and Hyrax, do they come in at the opposite pole from the urethral exit as is the case among fishes. In most mammals they enter low down near the urethra by an oblique passage through the wall of the bladder (Fig. 370). This arrangement makes the backflow of urine into the ureters difficult, particularly when the bladder is full, because the pressure from distention tends to close the bore of the ureters.
Both allantoic and cloacal bladders may be attached to the ventral body wall by a remnant of the ventral mesentery, the ventral ligament of the bladder, continuous with the visceral peritoneum covering this organ.