Although closely invested by the dense tissue of the prostate and its capsule, this is nevertheless the most dilatable part of the whole urethra, and will readily admit the introduction of the fore-finger. The levatores prostatce muscles, together with the pubo prostatic ligaments, support it, and attach it to the pubis, and it is compressed by the levatores ani. The general direction of this division of the canal is obliquely downwards ; it pre sents a slight concavity (the prostatic sinus) at its floor, where it is traversed by the caput gallinaginis, which, running from behind for wards, divides the sinus into two equal parts. In the prostatic sinus the ducts of the prostate open, assuming a crescentic arrangement around the base of the caput gallinaginis, whilst the vasa ejaculatoria terminate usually on the side of the latter body.
The description of the urethral orifice of the bladder belongs to the anatomy of this viscus, and to the article BLADDER the reader is referred ; but it is requisite here slightly to allude to it, as it bears materially upon the general direction of this part of the urethra. When viewed from before backwards, the opening will be generally found somewhat raised, so that the floor of the urethra forms a slight depression; and this depression is materially increased in hypertrophy of the prostate, a circumstance always to be remem bered in the introduction of the catheter in such cases.
In infancy the direction of the prostatic part is very different from what it is in after life. In consequence of the bladder at this early period being situated higher up, that is, more in the abdomen, the urethra at this part rises more vertically, and thus forms an angle with the membranous portion ; hence, as in the aged, the necessity, in passing a sound or ca theter, to depress the handle to a considerable extent, and thus to raise the point of the in strument. As the prostate becomes evolved, the prostatic part of the urethra gradually de scends, until its general direction is more con tinuous with that of the membranous part. I shall defer the description of the caput gal linaginis until I come to that of the general surface of the urethra.
The connection of the urethra to the pro state is very close, the ducts of the gland passing directly into it ; it is therefore im possible to raise it t'rom its attachment without division of the prostatic ducts. When a ca theter is introduced, the course of the canal invested by the prostate can be indistinctly traced by the finger introduced into the rectum.
The membranous portion (pars muscularis, isthmus urethra) commences from the anterior part of the prostate, and extends beneath the arch of the pubis as far as the bulb : it is included between the prostatic and spongy portions, and is covered slightly at its anterior and under part by the bulb, so that it is really shorter below than above. It makes a
slight curve, the concavity facing upwards. The concavity is at the distance of nearly an inch from the interpubic substance ; the con vexity looks towards the perinmum. Be tween it and the pubis, and just beneath the pubic arch, the dorsal veins of the penis run.
Proceeding from the anterior extremity of the prostate, the membranous portion of the urethra traverses the triangular which splits into two lamellar : one passing backwards over the prostate, is continuous with the capsule of the gland, the other ad vances forwards over the bulb, and blends with the tendinous investment of the spongy body. The opening through which the ure thra passes is round. The membranous por tion forms the segment of a circle, whose radius is, according to Krause, 2f lines. This part of the canal has been termed membran ous, from the idea that it represented a simple membrane when deprived of its surrounding muscular structure, and that it was wholly destitute of any investment of the spongy body. This, however, is not correct ; there is a thin layer of vascular tissue, continuous in front with the spongy body, and closely sur rounding the mucous membrane of this part of the urethra, between it and its muscular layers. The vessels of this extension of the spongy body pass backwards, to terminate in the plexus surrounding the neck of the blad der : this is mixed up with elastic tissue, and constitutes a truly erectile tissue. An exten sion of the same structure enters into the for mation of the caput gallinaginis.
Between the layers of the triangular liga ment, and in close connection with the mem branous part of the urethra, are found three sets of muscular fibres (museulus urethralis): they consist of the two pairs of muscles de scribed, one by Wilson and the other by Guthrie ; and the circular fibres of Santorini, which closely surround the urethra. The anteprostate, or Cowper's glands, covered by the inferior stratum of the compressor urethra?, are placed beneath this part, and the arteries of the bulb are in close approximation to it, running beneath and on either side of it. The floor of the membranous portion is traversed longitudinally in the median line by the pointed end of the caput gallinaginis.