BLADDER OF URINE (normal anato my).—(Kvon. ovpohxoc, vesica urinaria. Germ. Harnblase. Commonly known as the Bladder.) The urinary, like the biliary appa ratus, consists of four principal organs, each accomplishing a different purpose, yet all con tributing to the same end, namely, the sepa ration from the circulating medium of a consi derable portion of aqueous and saline matter : these are, first, the kidney or kidneys, which are the principal, indeed the sole agents in this function ; secondly, the ureters, the excre tory ducts, whose office it is to convey the fluid secreted, drop by drop, as fast as it is formed, which is by a slow and gradual pro cess, to, thirdly, the urinary bladder, which serves merely as a temporary receptacle for it ; and, fourthly, the it' ethra, or terminating ex cretory tube, whereby this fluid is wholly discharged from the system.
A urinary bladder has not been ascertained to exist in any of the invertebrate division of animals, and in the vertebrate there is a great diversity with respect to it : thus in the class Pisces, this organ is absent in all the osseous family, in most of whom, however, the two ureters unite below, and form a slight heart shaped dilatation which opens externally be hind the anus in common with the sexual organs : this vesicle, though somewhat analogous to, cannot be considered as a perfect reservoir. In most of the cartilaginous fishes it is absent also, as in the ray and shark, in whom the ureters open as in birds into a cloaca, or reser voir common to the renal, sexual, and intes tinal discharges ; in some, however, of this family it is present, as in the cyclopterus or lump-fish, the lophius piscatorius, &c. ; in the latter it is very capacious, and its coats are so thin as to be transparent; it receives the ureters anteriorly, and opens, as is usual in fish, behind the anus, in common with the genital ducts.
In Reptilia, the bladder is present in some, as the Batrachia and Chelonia ; it is absent in all the Ophidia, and in many of the Sauria, as the crocodile, the gecko, and the lizard; while again it exists in many of the same division, as the iguana, chameleon, draco, &c. In the Batrachia, as the frog and the toad, it is situated in front of the rectum or cloaca, into which it opens ; the ureters open into the latter poste riorly, from whence the urine is directed into the bladder by the muscular contraction of the cloaca and of the sphincters of the anus. In the frog its cavity is large, parietes thin, and its fundus divided into two cornua. In the Chelonia, as the tortoise, it is very large, and the ureters open into the urethra anterior to its cervix, the urine must therefore return or reas cend to enter the bladder. In the Ophidia or the Serpent tribe, each ureter dilates inferiorly into a small vesicle, which then opens into the cloaca, and there is no other approximation to a bladder; in such of the Sarnia as this organ exists, it opens into the cloaca.
In Ayes the bladder is always absent ; in the whole of this extensive class, the ureters open into the cloaca, and the urine, which is so earthy as to appear almost solid, is there min gled with the faeces, in common with which it is discharged at short and repeated intervals. In the Ostrich and Cassowary the cloaca is very dilatable, and its muscular structure is so organized as to be enabled to retain within it, and to discharge occasionally a considerable quantity of urine ; hence in these animals a vesica urinaria has been by some erroneously supposed to exist.
In all mammalia this organ exists, and in every member of this class the ureters enter it obliquely at a little distance behind the cervix, with the exception of the ornithoryncus and monotrematous animals generally ; in these the ureters open into the urethra a little beyond or anterior to the cervix of the bladder, so that the urine must return or ascend, in order to enter its cavity; this curious arrangement is similar to that adopted in the cbelonia, and would appear to indicate, as Carus ingeniously suggests, that in these strangely formed animals, in the same manner as in reptiles and in birds, the allantois (the remains of the urachus of ,-which form the bladder in mammalia) arises from the expansion of the rectum or the cloaca, whilst in other quadrupeds it is solely connected to the genital passages. In all mammalia this organ presents a tolerably uniform appearance both as to structure and shape, but great diver sity as to capacity or size ; the latter appears to be in an inverse ratio to its muscularity : hence in Carnivora, the bladder being more muscular, appears smaller in proportion to the size of the animal than in some of the Herbivora, where its coats are thinner, and therefore more dila table ; in others, however, of the latter order, in whom it is very muscular, its capacity is inferior to that of some even of the carnivora : in the Itodentia it is muscular and small, par ticularly if contrasted with the genital appa ratus. In quadrupeds the bladder is usually more covered by the peritoneum, and hence it appears more loose and free in the abdomen than in the human subject ; its figure is usually rounded, pyriform, or oval; and it may be re marked (and the remark will even apply to the human child and embryo) that the younger the animal the more elongated is the bladder, a fact which is indicative of its derivation from, or original continuity with the and allantois.