The erector penis muscle (called sometimes " the compressor penis," or " the ischio-caver nosus,") is placed obliquely along the lateral margin of the genito-urinary division of the perineum, where it partially envelopes and conceals from view the corresponding crus penis. It is elongated, broader in the centre than at the extremities, and curved so as to embrace the crus on which it is moulded. The erector penis springs by a narrow tendi nous attachment from the inner surface of the tuber ischii, and from the extremity of the great sciatic ligament beneath the transversus perinei muscle; the fleshy fibres soon succeed, and after continuing in an oblique direction up wards, forwards, and inwards, they end in a fibrous expansion which inclines outwards and forwards to terminate by two processes on the surface of the corpus cavernosum penis. Anatomists are not agreed on the action of this muscle: it may serve to draw down and to compress the crus penis, and in that manner to influence the circulation therein, but it can have no direct concern in causing the erection of the organ.
The transversus perinei muscle (" the ischio perineal " of some anatomists) passes from the tuber ischii to the central tendinous point of the perineum ; in this course the muscle in clines forwards and slightly downwards, so that its direction is not exactly transverse. It is attached externally to the inside of the tuber ischii, above the origin of the erector penis and the crus penis; and internally it is confounded with the several muscles already specified as reaching the central tendinous point. The transversus perinei is often of a triangular shape, the base at the ischium, and the apex at the central tendinous point of the perineum ; it is mostly fleshy, except at its insertion, which aponeurotic. This muscle is exceedingly uncertain as regards its developement, being sometimes replaced by a few scattered fibres derived apparently from the levator ani, and occasionally reinforced by a second muscle, termed the transversus perinei alter. The transversus perinei alter, when present, lies anterior and superior to the other, and extends from the ramus of the ischium to the bulb, where it becomes confounded with the accele rator urin. The transversus perinei is related by its superficial surface to the superficial peri nea] fascia, the superficial perineal and the transverse perineal vessels and nerves, the inser tion of the superficial sphincter ani, and the origin of the erector pems and the crus penis. Its deep relations are the levator ani and Wil son's muscles, together with the triangular ligament of the urethra. The transversus peri nei contributes to the strength of the perineum by raising and fixing the central tendinous point ; it also assists the levator ani in raising and supporting the rectuna and the pelvic vis cera. The transversi perinei muscles have been by some described as a single digastric muscle, semilunar in shape, and with the concave mar gin directed backwards and upwards towards the gut ; the result of the simultaneous action of these two bellies would be to raise and compress the intestine in front, and thus to assist in completing the process of defcation.
The triangular spaces are situated one at either side of the urethral prominence; each is bounded internally by the accelerator urin and the urethra, externally by the erector penis a»d the crus penis, posteriorly by the trans versus perinxi, which constitutes the base of the triangle, whilst the apex is in front where the crus penis and the urethra unite. These spaces are srnall in the natural condition, but when carefully dissected they become very distinct ; the superficial perineal vessels and nerves tmverse them from base to apex ; by separating the accelerator minx from the erector penis the anatomist obtains a view of the triangular ligament of the urethra between these muscles, and he may also form some estimate of its thickness and strength by the touch.
When the mtiscles and other structures be longing to the same layer of parts have been renioved, the crura penis along the sides of the region, the urethra in the centre, and the triangular ligament of the urethra stretching across the arch of the pubis, are brought fairly into view ; still further back, in the middle line, the recto-urethral triangular space may be partially seen, and also some fibres of the levator ani muscle descending to their insertion from behind the triangular ligament.
It is unnecessary in this article to describe the crura penis minutely. Each crus adheres to the rami of the ischium and pubis, becoming gradually thicker and larger as it approaches the symphysis, and at length the two crura unite to form the body of the penis; the lateral margins of the triangular ligament of the ure thm, and the great pudic vessels and nerves in the last part of their course, are overlapped by the crum as they ascend.
The anatomy of the urethra in respect to catheterism has received the fullest considera tion already, but the position of the bulb may be now ag-ain studied with advantage. This body lies in front of the triangular ligament of the urethra, and projects backwards and downwards towards the rectum ; it is situated about one inch from the anus, and scarcely more than half that distance from the anterior wall of the intestine ; yet the narrow interval between the bulb and the rectum constitutes a portion of the recto-urethral triangular space, through which the early incisions in the bila teml operation of lithotomy are carried. The bulb is retained in its position by a thin expan sion derived from the anterior layer of the tri angular ligament, and continuous with the membrane which invests the corpus spongio sum urethrx.