PELVIS 577 



The prostate is very liable to become enlarged as old age 

 approaches. When this happens the most important result 

 is the effect exerted on the urethra. If the enlargement is 

 uniform the canal is merely lengthened, but, if excessive 

 growth is localised the enlarged part may impinge upon the 

 urethra and interfere with micturition. When the middle 

 lobe enlarges it projects upwards and forwards, over the orifice 

 of the urethra, and forms a kind of ball valve, which may pre- 

 vent the exit of urine through the internal urethral orifice. 

 It is only in pathological conditions that the so-called 

 middle lobe becomes a distinct and more or less in- 

 dependent part of the organ. During health it is merely 

 marked off from the rest of the prostate by the utricle and by 

 the passage of the ejaculatory ducts through the substance 

 of the gland. 



Pars Membranacea Urethras (Membranous Portion of the 

 Urethra). This is the narrowest and the shortest division of 

 the urethra. It extends from the prostate to the bulb of the 

 penis, curving gently downwards and forwards, behind the 

 lower border of the symphysis pubis, from which it is distant 

 about one inch (25 mm.). Its length is barely three-quarters 

 of an inch, and the concavity of its curve is directed 

 forwards and upwards. Throughout its entire length it is 

 enveloped by the fibres of the sphincter urethne membranaceae 

 (O.T. compressor urethrae) muscle. Towards its termination 

 the bulbo-urethral glands are placed behind it one on each 

 side. 



The membranous part of the urethra has important rela- 

 tions to the uro-genital diaphragm and to the pelvic fascia. 

 As it emerges from the prostate, it pierces the parietal pelvic 

 fascia (i.e. the upper fascia of the uro-genital diaphragm), 

 and the margins of the aperture through which it passes are 

 carried backwards to become continuous with the sheath of 

 the prostate. At its termination it pierces the inferior fascia 

 of the uro-genital diaphragm about an inch below the 

 symphysis. It lies therefore in the interval between these 

 two fasciae. 



The mucous membrane of the membranous part of the 

 urethra is directly surrounded by a thin coat of erectile tissue, 

 and this is embraced by a muscular tunic composed of 

 involuntary fibres arranged circularly. 



Pars Cavernosa Urethrse (Spongy Portion of the Urethra). 



VOL. I 37 



