. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Outgo Apparatus 439 goes 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 con- tributes nothing
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Outgo Apparatus 439 goes 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 con- tributes nothing to the bladder or ; 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 pas- sage through the wall of the bladder (Fig. 370). This arrangement makes the backflow of urine into
Size: 1335px × 1872px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
