Abstract
The normal muscular anatomy of the urinary bladder of the horse is reviewed. The basic pattern is an outer longitudinal and inner circular layer of smooth muscle, except on the dorsal surface where the circular layer is external. This arrangement permits, according to the principles of mechanics, preferential bulging of the dorsal surface. At the time of birth the urachus is functionally closed, or nearly so. During parturition external pressure on the distended bladder may cause dorsal rupture because the urachus is closed, the penile urethra of the male is compressed in the pelvic canal, and the dorsal surface of the bladder is anatomically a weak point. Rupture of the dorsal surface was produced experimentally by simulating the in vivo conditions in vitro.