BLADDER, or lexica Urinal-la, so called to distinguish it from the Gall-Bladder, is""a musculo-membranous bag or pouch, which serves as a temporary reservoir for the urine; it communicates with the kid neys by means of the ureters, and opens externally by means of the urethra.
The urinary apparatus is confined to the red-blooded classes of animals, all of which have kidneys, whilst some orders and genera have no urinary bladder. In quadrupeds the bladder is of a pyriform shape, and is completely surrounded by the peritoneum or serous lining of the abdomen ; and it may be taken as a general rule that it is smaller, stronger, and more muscular in carnivorous than in grami nivorous animals ; in the latter it is almost membranous, and in some of them is particularly large.
In the whole class of birds there is no urinary bladder, and the ureters open into the cloaca, a musculo-membranous bag, which takes the place of the rectum, bladder, and uterus, and serves as a reservoir for the solid excrements, the urine, and eggs. The urine in these animals dilutes the faeces, and deposits the carbonate of lime which constitutes the basis of the shell. The urinary bladder exists in seve ral genera and species of fishes.
In the human subject the urinary bladder is placed in the pelvis or basin immediately behind the symphysis pubis, and before the rectum, or terminal portion of the intestines, in the male ; but it is separated from mt in the female by the uterus and vagina. Its form and relations vary according to the age of the individual. In infancy it is of a pyriform shape, and is contained almost entirely in the abdomen, thus resembling its permanent condition in quadrupeds. At this period it may be considered as consisting of three portions; the narrow tapering part, or neck, the upper rounded portion, or fundus (sometimes called summit), and the intermediate portion, or body ; but as the pelvis expands the bladder gradually subsides into it, and undergoes a remarkable change of form. Thus, in the adult its figure is that of a short ova], compressed at the fore and back part ; its lower surface subsides on the rectum, and expand.
ing forms what is termed by anatomists the has fond of the bladder. This change of form is dependent not only upon the enlargement of the cavity in which the bladder is contained, but also upon the weight of the fluid which it habitually sustains, and thus in advanced age it is more deeply sunk in the pelvis than in the middle periods of life. In the female its transverse diameter is greater than in the male, in consequence of the antero-posterior diameter of the pelvis being encroached upon by the uterus. Its capacity varies in the different periods of life ' • and as a general rule it may be said to increase iu proportion as the individual advances in years, and to be greater in females than iu males. Its capacity is modified in different individuals by their habits and the natural exercise of its functions. It is more particularly changed by disease; thus, from the effects of long-continued irritation it maybe reduced to such a state that it will not contain more than a few drops of urine ; and, on the con trary, when from any cause its contents cannot be duly evacuated, it may be distended so as to con tain many quarts of urine, and occupy a large proportion of the abdomen. Its ordinary capacity may be estimated at a pint and a half.
The neck or constricted portion of the bladder is compared to a truncated cone, longer at the sides and below than above. In infancy, owing to the position of the bladder, its direction is oblique; for a similar reason it is horizontal in the adult ; it differs in structure from the rest of the organ. The neck, which is formed of a somewhat fibrous whitish substance, is the connecting medium between the bladder and the urethra. Its posterior part rests on the rectum; its anterior is surrounded, at least below and at the sides, by the prostate gland, which is peculiar to the male, and is composed of an aggregation of mucous follicles, disposed so as to form three lobes, one on each side of the neck of the bladder, and one below called the middle lobe, which forms a slight projection into the opening of the urethra.