The central tendinous point of the perineum is situated in the middle line, equidistant from the anus and the bulb of the urethra. It is a common point of insertion to many muscles ; thus the superficial sphincter of the anus can be fairly traced to this spot, and so can many fibres of the acceleratores urinx, the transversi perinei, the levatores ani, and NVilson's mus cles. Here muscular fibres from opposite sides of the perineum become blended with each other; here, too, those from before and those from behind are intermixed, and some even descend from within the pelvis to identify themselves in this place with others which are subcntaneous.
The superficial perineal artery (the perinea] artery of some anatomists), though not a large vessel, yet takes a lengthened and very regular course. Its origin is from the internal pudic artery at some little distance behind the trans versus perinei muscle, whilst the parent trunk is still under cover of the obturator fascia; it immediately pierces that membrane, and like wise very generally passes through the base of the triangular ligament of the urethra. The artery next curves around the transversus peri nei muscle lying on its superficial surface and running across its fibres. The vessel then in clines forwards and inwards through the trian gular interval between the accelerator urinw and the erector penis muscles, until at length it reaches the scrotum, and assuming the name of " the arteria septi," it terminates in a free anastomosis with the other arteries supplying the envelopes of the testicles. The superficial perineal artery lies deep posteriorly, but itg position becomes very superficial as it ap.i proaches the scrotum; its branches are distri buted freely to the muscles and integuments, and they likewise anastomose internally with' branches from the corresponding artery of the opposite side, and externall/ with superficial branches from the thigh. lIvo veins accom pany this artery; they are frequently dilated and tortuous in front, and in certain diseased conditions of the testicle or its coverings they sometimes form a complicated net-work in the scrotum.
The inferior or superficial division of the pudic nerve (" the perineal nerve" of many authors) follows pretty nearly the course of these vessels; like the artery, it is destined for the scrotum, where it terminates by several long and very fine branches, after supplying in its progress numerous twigs to the transversus perinei, accelerator urinre, levator ani, and erector penis muscles. One very remarkable branch of this nerve (the bulbo-urethral of Cruveilhier) may be traced fairly into the bulb, whilst another (the external perinea! of the same anatomist) runs superficially along the lateral part of the region, reinforcing and anastomosing with the perineal cutaneous branch of the lesser sciatic nerve.
The transversalis perinei artery is of small size; it springs from the internal pudic, a little anterior to the source of the superficial perinea] artery, but these two vessels not unfrequently arise by a single common trunk. The trans versalis perinei artery passes through the obtu rator fascia, and often perforates the base of the triangular ligament of the urethra ; it quickly becomes superficial, applying itself to the cuta neous surface, and running usually near the posterior edge of the transversus perinei mus cle, it thus g,ains the central tendinous point of the perineum, where it anastomoses with the inferior hemorrhoidals from behind, the transversalis and superficialis perinei arteries from the opposite side, and svith the neigh bouring superficial perinea]. This vessel is accompanied by two veins and by one or more branches of the superficial division of the pudic nerve.
The accelerator urinz2 muscle (the ejaculator seminis of some anatomists, the bulbo-caver nosus of others) extends from the central ten dinous point of the perineum forwards along the urethra, being identified with the correspon ding muscle of the opposite side in a raplie which occupies the middle line; and so inti mate is this connection that both might be conveniently described together as a single muscle. The fibres of the accelerator urinm spring from the side of the raphe, and pass from thence outwards and upwards upon the urethra. The anterior fibres incline very ob liquely forwards and outwards to arrive at the surface of the corpus cavernosum penis, where they terminate. In consequence of this disposition the acceleratores urinze muscles are separated from each other in front by a Y-shaped interval in which the urethra appears. The succeeding fibres, after embracing the urethra laterally, are lost on a short horizontal tendon, which likewise receives the correspond ing fibres of the opposite muscle. This tendon is placed above the urethra, beneath the junction of the crura penis, and anterior to the triangular ligament of the urethra. The posterior fibres incline outwards more than the others, and are inserted into the superficial surface of the triangular ligament of the urethra, many of them extending nearly to the crus penis. The accelerator uritm muscle lies on the bulb and the neighbouring portion of the corpus spongi osuin. In conjunction with its fellow it con stitutes a fleshy sheath, all but perfect, for the urethra. Its action is to compress the canal, and at the same time to draw forwards the bulb. It is employed in expelling the semen and the last drops of urine from the sinus of the bulb; and by contracting spasmodically it may even arrest the progress of a catheter, an occurrence explained by the manner in which many of its fibres completely surround the passage.