DEEP MUSCLES OF URETHRA. 397 



Perforating the fore part of the ligament, about one inch below the 

 symphysis pubis, is the canal of the urethra, F ; but the margin of the 

 opening giving passage to that tube is blended with the tissue of the cor- 

 pus spongiosum urethras. About midway between the preceding opening 

 and the symphysis pubis is the aperture for the dorsal vein of the penis ; 

 and external to this, near the bone on each side, the terminal parts of the 

 pudic artery and nerve to the penis (b and 3) perforate the ligament by 

 separate apertures. 



Between the layers of the ligament are contained the membranous part 

 of the urethra, with its muscles, vessels, and glands ; and the bloodvessels 

 and nerves of the penis. 



Dissection. The muscles between the layers of the ligament will be 

 reached by cutting through with care, on the left side, the superficial 

 stratum near its attachment to the bone, and raising and turning inwards 

 that piece of membrane. By a little cautious dissection, and the removal 

 of some veins, the following objects will come into viw with the under- 

 mentioned position : 



Parts between the layers of the ligament. Near the base is a narrow 

 transverse muscle, H, which is directed to the bulb of the urethra. 

 Higher up, and crossing inwards from behind the pubic arch, is the fasci- 

 culus of fibres of the constrictor urethra muscle, I, which surrounds the 

 membranous part of the urethra. And below th'e urethra are the glands 

 of Cowper. Beneath the bone are the pudic vessels (a) and nerve, the 

 former giving its branch to the bulb, and the latter being deeper in posi- 

 tion ; and below the pubes is the sub-pubic ligament. Deeper than all, 

 the student will recognize the posterior layer of the ligament, continuous 

 with the recto-vesical fascia, which separates those parts from the cavity 

 of the pelvis. 



MUSCLES. The two muscles connected with the membranous part of 

 the urethra are, deep transverse, and constrictor of the urethral passage. 



The DEEP TRANSVERSE MUSCLE (fig. 131, H) (elevator urethra Santo- 

 rini) is a thin flat band on a level with the base of the triangular ligament. 

 It arises externally from the pubic arch of the innominate bone, and is 

 directed below the tip of the bulb and the membranous part of the urethra 

 to the middle line, where it joins the muscle of the opposite side, and is 

 inserted into the central point of the perinaeum. 



The muscle conceals Cowper's gland, and is frequently placed over the 

 artery of the bulb. The transverse muscle is not always separate from 

 the following. 



Action. Like the superficial muscle it will fix the central point of the 

 perinaeum. 



The COVSTRICTOR MUSCLE (fig. 131, r ) (constrictor isthmi urethralis) 

 incloses the membranous part of the urethra, and consists of transverse 

 fibres above and below that tube. 



The muscle arises by aponeurotic fibres from the pubic arch above the 

 preceding, and from the posterior layer of the triangular ligament, but 

 this attachment is not evident unless it has been dissected from behind. 

 The fibres pass inwards, and separate near the urethra into two layers 

 (fig. 132), of which one (c) passes over, the other (d) under that canal ; 

 in the middle line they unite (sometimes by tendon) with the like parts 

 of the muscle of the opposite side. 



It may be considered a single muscle extending across the perineum 



