856 A MANUAL OF ANATOMY 



surrounded externally by the inner circular muscular fibres of the 

 prostate, and is covered internally by transitional epithelium. 



The membranous portion of the urethra succeeds to the pros- 

 tatic portion, and extends from the apex of the prostate gland to 

 the commencement of the spongy portion. It is contained, for the 

 most part, between the two layers of the triangular ligament, but 

 it also extends for a short distance (J inch) beyond the antero- 

 inferior layer of that structure. It is the shortest and narrowest 

 part of the canal, with the exception of the meatus urinarius. Its 

 length is | inch along the anterior wall, and -| inch along the pos- 

 terior, the difference being due to the fact that the membranous 

 urethra passes into the spongy part in a slanting manner at a point 

 I inch in front of the posterior extremity of the bulb. The back 

 part of the bulb projects backwards for ^ inch over the posterior 

 wall of the membranous urethra, and is here lying in front of the 

 antero-inferior layer of the triangular ligament. It is in this 

 situation where a false passage is liable to be made in catheteriza- 

 tion, partly because the walls are here very thin, and parti}' by 

 reason of the backward extension of the posterior extremity of 

 the bulb. The diameter of the membranous urethra is ~ inch. It 

 lies about i inch behind and below the subpubic ligament, and its 

 direction is downwards and slightly forwards. It describes a gentle 

 curve, the concavity of which looks forwards and upwards towards 

 the lower part of the symphysis pubis. As the membranous urethra 

 passes through the postero-superior layer of the triangular ligament, 

 which is formed by the parietal pelvic fascia, the fascia is prolonged 

 upwards to form part of the capsule of the prostate gland. More- 

 over, as it pierces the antero-inferior layer of the triangular liga- 

 ment about I inch below the symphysis pubis, it carries with it a 

 prolongation from the margins of the urethral opening, which forms 

 a fascial investment for the bulb. Between the two layers of the 

 triangular ligament the membranous urethra is surrounded by the 

 fibres of the compressor urethrae muscle, and Cowper's glands lie 

 behind it, one on either side of the middle line. 



Structure. — External to the mucous membrane there is a layer 

 of erectile tissue, and outside this there is a layer of circularly- 

 disposed plain muscular fibres, which are continuous above with 

 the circular muscular fibres of the prostate around the prostatic 

 urethra. External to this, again, there are the sphincter fibres of 

 the compressor urethrae. The mucous membrane is covered by 

 columnar epithelium. The membranous portion of the urethra in 

 transverse section presents the appearance of a circular opening, 

 the lumen of which is branched, this being due to the longitudinal 

 folds into which the mucous membrane is thrown. 



The spongy portion of the urethra succeeds to the membranous 

 portion. It is contained within the corpus spongiosum of the 

 penis, and extends from a point \ inch in front of the posterior 

 extremity of the bulb and of the antero-inferior layer of the 

 triangular ligament to the meatus urinarius on the extremity of 



