If a catheter be passed into the bladder, the membranous portion can be distinguished by the finger introduced into the rectum and drawn forwards, there being only a small quan tity of cellular membrane interposed between the under part of its muscular investment and the intestine.
Boyer estimated the length of the mem branous portion at about 1 inch ; Ducamp from 9 to 10 lines ; Lisfranc found it in twelve subjects to vary from 7 to 11 lines. In these its anterior diameter varied from 31 to 41 lines, its posterior from 4f to 5 lines, and just behind the bulb it measured at least 11 line less than in any other part. It is universally admitted that the point of junction between the membranous and spongy portions is the narrowest part of the urethra, with the excep tion of the meatus.
The membranous part of the urethra is sur rounded by muscular fibres, which have been variously described by different anatomical writers. Thus Santorini, as early as 1724, in his Observationes Anatomicce, pointed out some transverse fibres as encircling the urethra at this part: he terms them " the elevator or eja culator urethrm," and describes them as being inserted into the lower part of the urethra. Mr. Wilson, in the year 1808, gave a descrip tion of two muscles surrounding the mem branous part of the urethra, the origin of which is from a tendon attached to the pos terior part of the symphysis pubis, a little above its lower border: the muscle thus arising from a single origin then descends, and divides into two portions, which, reaching the mem branous portion of the urethra, spread them selves out by its side, and are implanted into a common tendon below it. The muscle is termed by Wilson the compressor urethrae. In 1834, Mr. Guthrie, in his lectures at the Royal College of Surgeons, demonstrated another series of muscular fibres, as sur rounding the membranous portion of the urethra, and of which he considers Wilson's muscle as a part. That portion described by Guthrie, and now known by his name, arises by a thin tendon on either side, from the ra mus of the ischium, and, passing transversely, splits into two portions, one above and the other below the urethra; the two muscles are connected together above by a menial tendon, which, passing forwards, is inserted in part into the upper part of the urethra, whilst another portion, passing backwards, is im planted into the upper surface of the front of the prostate. The under portion of the muscle
is also connected with its fellow by a similar mesian tendon, which, advancing forwards, goes to the central tendon of the perinmum, and, sending a slip backwards, is inserted into the under part of the prostate.
The muscles are included between the two layers of the triangular ligament; and the effect of this muscular apparatus must be to support and compress this part of the urethra, and to prevent any retrogression of the seminal fluid in the venereal act.
The urethra now enters the corpus spongi °sun', by which it is surrounded, until its ter mination at the extremity of the glans penis. The spongy body surrounds the urethra equally, except at the bulb and at the glands : in the former situation there is a much thicker layer below than above, which gives to the bulb an appearance of great dilatation, whilst at the glans there is but very little of this structure at the under part. The bulb and the glans, both dilatations of the spongy body, bear a tolerably constant relation to each other.
This part of the canal is called the spongy portion : it has already been stated that its length varies in different subjects, and accord ing to the condition of the penis, being 4 or 5 inches in the flaccid state, and 6 or 8 inches in the erect condition of the organ. It com mences at the termination of the membranous portion, and is lodged in a groove between and beneath the corpora cavernosa penis, forming, with its investing corpus spongiosum, a convexity, projecting beyond the general circumference of the penis.
The direction of the spongy portion is curved ; it passes upwards in front of the lower part of the symphysis pubis as high as the suspended part of the penis ; and then taking the direction of this organ, it bends downwards, thus forming an abrupt curve : by simple traction of the penis forwards and upwards, it can be rendered perfectly straight.