170 TOPOGRAPHICAL ANATOMY OF THE 



by the two ureteral folds and a transverse line joining the two ureteral 

 orifices. The trigone is not quite an equilateral triangle, for its base 

 (between the openings of the ureters) is about 3 cm., and its sides about 

 4 cm. in length. The apex of the triangle is at the internal urethral 

 orifice (orificium urethruj internum). 



Structure of the bladder. — From a naked-eye examination, it 

 is evident that the wall of the bladder is composed of the four layers 

 customarily found in connection with hollow viscera of a like nature. 

 The peritoneum provides a .serous tunic that does not cover the whole 

 of the organ. The greater part of the dorsal surface is covered, but a 

 smaller amount of the ventral surface has a serous investment. As 

 already noted, reflections of peritoneum from mid-ventral and lateral 

 lines produce the three ligaments by which the bladder is kept in 

 position. 



The fibres composing the muscular tunic are easily demonstrated in 

 the distended fresh organ, in which they will be observed to run in 

 different directions, with an ill-defined arrangement in strata. The 

 superficial stratum contains fibres mainly longitudinally disposed, and 

 most clearly defined on the dorsal and ventral surfaces. For the most 

 part, circular fibres form a middle stratum, while the deepest fibres are 

 oblique and longitudinal in direction. Around the neck of the bladder 

 the circular and oblique fibres form a sphincter muscle (sphincter 

 vesica;). Muscular fibres pass from the wall of the bladder for a short 

 distance into the middle and lateral peritoneal folds. 



A vascular submucous tela unites the muscular tunic and the 

 mucous lining in which lymph nodules occur. 



The male urethra (Urethra virilis). — The male urethra, extending 

 from the neck of the urinary bladder to the glans penis, may be divided 

 into pelvic and extra-pelvic parts. The extra-pelvic part is contained 

 in the penis. The pelvic part (pars pelvina), now under consideration, 

 is about 10-12 cm. in length, and runs along the floor of the pelvis to 

 the sciatic arch. The initial portion of it, associated with the prostate, 

 is not sharply marked off from the neck of the bladder. 



The prostatic portion of the urethra, which communicates with the 

 bladder by the internal urethral orifice (orificium urethrae internum) 

 is narrow ; but beyond the prostate the lumen of the tube is greater. 

 After the bulbo-urethral glands have been passed, the lumen again 

 diminishes as the root of the penis is approached. This part of the 

 urethra is known as the isthmus (isthmus urethra^). 



Along the dorsal wall of the prostatic portion of the urethra runs a 

 fold of mucous membrane, the urethral crest (crista urethralis), con- 



