PELVIS 575 



at the neck of the bladder and ends on the glans penis. 

 Its average length is about eight or nine inches (220-225 mm.). 

 It is customary to divide the canal into three parts, from 

 the different characters of the structures which it traverses. 

 The first part, pars prostatica urethra, is contained within the 

 substance of the prostate gland; the second part, pars 

 membranacea urethra, extends from the prostate to the bulb of 

 the corpus cavernosum urethrae, and is surrounded, between 

 the layers of the uro-genital diaphragm, by the fibres of 

 the sphincter muscle of the membranous urethra ; the third 

 part, pars cavernosa urethra, traverses the entire length of 

 the corpus spongiosum urethrae. 



Pars Prostatica Urethra (Prostatic Portion of the 

 Urethra). This part of the urethra is about one inch and 

 a quarter in length (31 mm.). It is fusiform, being wider in 

 the middle than at either its commencement or its termina- 

 tion. It traverses the prostate in front of its so-called middle 

 lobe, and takes a very nearly vertical course through its 

 substance. It is the widest, and at the same time the most 

 dilatable part of the canal. 



In connection with the posterior wall or floor of the prostatic 

 portion of the urethra there are certain important features 

 to be noted. The mucous membrane along the median plane 

 is raised into a prominent ridge called the crista urethra or 

 urethral crest. This commences a short distance below the 

 orifice of the urethra in the bladder, and extends downwards 

 for about three-quarters of an inch. At first it increases 

 gradually in height, until it forms a prominent eminence, 

 the colliculus seminalis or seminal hillock, then its height 

 -suddenly diminishes, and, finally, the ridge fades away into the 

 membranous part of the canal. On each side of the urethral 

 crest the floor of the urethra is hollowed out into a longi- 

 tudinal depression, termed the prostatic sinus, into which the 

 numerous prostatic ducts open. These may be rendered 

 evident by squeezing the prostate, when fluid will be found 

 to exude into the sinuses through the ducts. A close 

 inspection of the floor of the urethra, above the crista, will 

 reveal the apertures of the ducts of the so-called middle lobe 

 of the prostate. 



Immediately below the seminal hillock the mucous 

 membrane dips backwards and upwards, forming a small cul- 

 de-sac behind the middle lobe of the prostate. This is the 



