THE ABDOMEN 617 



Anterior Part. — ^The fibres of this part become somewhat detached 

 and form the muscle of Houston. They are inserted partly into 

 the outer surface of the fibrous sheath of the corpus cavernosimi 

 penis in front of the ischio-cavernosus, and partly by means of a 

 tendinous expansion into the fascial investment of the penis beneath 

 which the dorsal vein of the penis lies. 



Posterior Part. — ^The fibres of this part arch outwards and back- 

 wards to be inserted into the antero-inferior layer of the triangular 

 ligament neeir its base. 



Nerve-supply. — ^The deep division of the perineal branch of the 

 pudic nerve. 



Action. — (i) The chief part of the muscle, acting with its fellow, 

 compresses the bulb. These portions of the t^vo muscles, therefore, 

 come into play at the end of the act of micturition, when they 

 expel the last drops of urine from this part of the urethra. A 

 further action is to contribute to the maintenance of erection of 

 the penis by compressing the veins of the bulb. (2) The anterior 

 part compresses the dorsal vein of the penis, and so contributes to 

 the maintenance of erection. (3) The posterior part tends to fix the 

 bulb and maintain it in position. 



The chief f)ortions of the bulbo-cavernosi muscles completely 

 surround the bulb, and may be regarded as forming a sphincter 

 muscle to it. 



Compressor or Constrictor Urethral. — ^This muscle is also called 

 the muscle of Guthrie. It extends transversely from the inner 

 border of the ischio-pubic ramus of one side to the inner border 

 of the opposite ischio-pubic ramus, lying in each case behind the 

 attachment of the antero-inferior layer of the triangular liga- 

 ment of the urethra. At the middle line the muscle divides into 

 two layers, which, passing above and below the membranous part 

 of the urethra, form a sphincter muscle to it, the lower layer also 

 ensheathing the glands of Cowper. The posterior fibres of the 

 muscle, which lie close to the base of the triangular ligament, are 

 known as the transversa s perinaei profundus (Henle). The highest 

 fibres are connected on either side with the pelvic surface of the 

 descending pubic ramus at its upper part, and they join each other 

 below the membranous portion of the urethra. These highest 

 fibres, which are oblique in direction, constitute the levatores 

 urethrae muscles, or muscles of Wilson. 



Nerve-supply. — ^The dorsal nerve of the penis. 



Action. — (i) To constrict the membranous part of the urethra. 

 The muscle comes into play at the end of micturition, and it assists 

 the bulbo-cavemosus in emptying the urethral canal. (2) To con- 

 tribute to the maintenance of erection of the penis by compressing 

 the plexus of veins, in the muscle, which receives its tributaries from 

 the corpora cavernosa and bulb. (3) To compress Cowper's glands, 

 and so aid in the expulsion of their secretion. 



The compressor urethrae muscle lies between the two layers of 

 the triangular ligament of the urethra. Close to its attachment to 



