This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The male urethra is variable in length, as it can be stretched, therefore only average measurements can be given. Its length is 20 cm. (8 in.) in the adult, 8 to 10 cm. at 5 years, and 10 to 12 cm. at puberty. Of this, 3 cm. (1 1/4 in.) is prostatic, 0.5 cm. ( 2/5 in.) being in the bladder wall; 1 cm. ( 2/5 in. ) is membranous; 16 cm. (a little over 6 in.) is penile. Its calibre varies, being narrowest at the meatus and next narrowest at the membranous portion. The internal meatus is a little larger than the membranous portion. The meatus admits a No. 24 French sound (often larger), the membranous portion a 26 to 28. The prostatic portion is the largest, admitting a No. 32 sound. The bulbous is almost or quite as large, admitting a 30 to 32. Therefore a sound which passes the meatus should find no further obstruction. The fossa naviculars just beyond the meatus is larger than the urethra beyond (Fig. 473). Distensibility. - The meatus and membranous portions are the least distensible. The former is fibrous in character and will not stretch. In the membranous portion the support of the superficial and deep layers of the triangular ligament prevent dilatation. The prostatic is the most dilatable portion and the bulbous urethra next. Relations. - The internal urethral meatus lies about 6.25 cm. (2 1/2 in.) from the surface just behind the middle of the symphysis, if the body is in a vertical position. The membranous portion pierces the triangular ligament, 2.5 cm. (1 in.) or a little less below the subpubic ligament. The lowest portion is just in front of the triangular ligament. The urethra then rises slightly, 0.5 cm. (1/5 in.), and finally drops to the meatus. The subpubic curve of the urethra has a radius of about 5 cm. (2 in.) and urethral instruments are made with approximately this curvature, though they vary much. The membranous urethra can be palpated at the apex of the prostate by the finger in the rectum. Structure. - The urethra is composed of an external layer of erectile tissue covering a muscular layer which is continuous with that of the prostate and bladder. Beneath the muscular layer is the submucous, rich in blood-vessels, on which is laid the mucous layer. This latter is covered with flat, pavement epithelium in the fossa navicularis, and columnar epithelium beyond.
Fig. 473. - The male urethra.
The urethra contains small mucous glands opening on its surface - glands of Littre - and small pockets or recesses, called the lacunae of Morgagni, into which the glands of Littre sometimes empty. The lacunae are mostly in three rows on the roof of the penile portion and open forward toward the meatus. A large one - lacuna magna - opens in the posterior portion of the roof of the fossa navicularis, a couple of centimetres behind the meatus. The glands of Cowper open into the bulbous urethra just in front of the triangular ligament. The racemose glands of the prostate open into the sides of the floor of the prostatic urethra, and the ejaculatory ducts open near the middle line just in front of the urethral crest (verumontanum), with the utricle (prostatic sinus) between.
The mucous walls of the urethra are normally in contact, making a vertical slit at the external meatus, a transverse one in the penile portion, horseshoe shape in the prostate, and again transverse just before the bladder is reached.