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Its lower fibres, arching over the spermatic funiculus, assist in forming, laterally, the anterior wall of the inguinal canal; medially, by means of the falx inguinalis, it helps to form the posterior wall of the canal.
Its lowest fibres are continued into the cremaster muscle, which is prolonged along the spermatic cord through the inguinal canal.
M. Cremaster.-The cremaster muscle forms an investment for the testis and spermatic funiculus deep to the external spermatic fascia. In the female it is more largely represented by fascia than muscular fibres, and constitutes the cremasteric fascia. It may be said to have an origin from the inferior edge of the obliquus internus and the
FIG. 426. THE LEFT INGUINAL CANAL. STRUCTURES SEEN ON REFLECTION OF THE OBLIQUUS EXTERNUS. adjacent part of the inguinal ligament. Its fibres form loops over the spermatic funiculus and testis, the highest fibres getting an insertion into the pubic tubercle.
M. Transversus Abdominis.-The transversus abdominis muscle arises (1) from the deep surface of the costal cartilages of the lower six ribs, interdigitating with the origins of the diaphragm; (2) from the lumbo-dorsal fascia; (3) from the anterior half of the medial lip of the iliac crest; and (4) from the lateral third of the inguinal ligament.
The muscular fibres run, for the most part, horizontally forwards, and end in an aponeurosis which has a twofold insertion. (1) After forming (along with the aponeurosis of the obliquus internus) the posterior layer of the sheath of the rectus, the aponeurosis is attached to the xiphoid process and linea alba. (2) The inferior fibres of the muscle arising from the inguinal ligament are joined by the inferior part of the obliquus internus to form the larger part of the falx
aponeurotica inguinalis (O.T. conjoined tendon), which passes anterior to the inferior part of the rectus muscle, to be inserted into the crest and tubercle of the pubis and the ilio-pectineal line.
The transversus muscle is separated by the lower intercostal nerves from the obliquus internus muscle, and is lined on its deep surface by the fascia transversalis. Its inferior border forms a concave edge, separated from the inguinal ligament by a lunular interval in which the fascia transversalis appears, and through which the spermatic funiculus emerges at the abdominal inguinal ring, under cover of the obliquus internus muscle and the aponeurosis of the obliquus externus.
The anterior muscles of the abdominal wall include the pyramidalis and rectus abdominis, enveloped by the sheath of the rectus, on either side of the linea alba. M. Pyramidalis Abdominis. The pyramidalis abdominis is a small triangular muscle arising from the pubic crest, anterior to the rectus muscle (Fig. 428, p. 482).
It is directed obliquely upwards, to be inserted, for a variable distance, into the linea alba. The muscle is often absent.
M. Rectus Abdominis.-The rectus abdominis muscle is broad and straplike, and arises, by a medial and a lateral head, from the symphysis and crest of the pubis (Fig. 428, p. 482).
The muscle expands as it passes upwards, and is inserted, from medial to lateral side, into the anterior surface of the xiphoid process (Fig. 428, p. 482), and into the superficial surface of the seventh, sixth, and fifth costal cartilages. On its anterior
surface, but not extending through the entire substance of the muscle, are three or more transverse tendinous intersections (inscriptiones tendineæ), adherent to the sheath of the muscle; the lowest opposite the umbilicus, and the highest about the level of the xiphoid process. The medial border of the muscle lies alongside the linea alba; its lateral border is convex, and corresponds to the linea semilunaris. The muscle is pierced by the terminal branches of the lower thoracic nerves.
FIG. 428.-DEEP DISSECTION OF THE ABDOMINAL WALL. THE RECTUS MUSCLE AND ITS SHEATH.
Vagina M. Recti Abdominis.-The sheath of the rectus muscle is derived from the aponeuroses of the lateral muscles of the abdominal wall, which, after enclosing the muscle, give rise, in the median plane, to the linea alba. At the linea semilunaris along the lateral border of the rectus muscle, the aponeurosis of the obliquus internus splits into anterior and posterior layers. The anterior layer, joined by the aponeurosis of the obliquus externus, passes in front of the rectus, and constitutes the anterior lamina of the sheath. The posterior layer, joined by
the aponeurosis of the transversus muscle, passes behind the rectus, and constitutes the posterior lamina of its sheath. This arrangement obtains in the superior threefourths of the abdominal wall. Below the level of the iliac crest the sheath of the muscle is deficient posteriorly, and a crescentic border, the linea semicircularis (semilunar fold of Douglas), marks the inferior limit of the posterior lamina. In consequence, the rectus in the lower fourth of the abdominal wall rests directly upon the fascia transversalis. Close examination, however, usually reveals a thin layer behind the muscle in continuity with the fold of Douglas, and merging below with the fascia transversalis. In this region the rectus is covered anteriorly by the falx aponeurotica inguinalis of the obliquus internus and transversus, and by the aponeurosis of the obliquus externus, which gradually separates from the subjacent aponeurosis. The superior part of the rectus, lying on the chest wall, is only covered anteriorly by a single layer of aponeurosis derived from the obliquus externus, which in this situation is giving origin to the pectoralis major muscle.
Canalis Inguinalis. --Inguinal canal. The spermatic funiculus in the male, and the round ligament in the female, in their passage through the inferior part of the abdominal wall, pass through the inguinal canal, which is bounded by these abdominal muscles. The canal begins at the abdominal Dinguinal ring, placed half an inch above Ethe inguinal ligament, and midway between the anterior superior iliac a spine and the symphysis pubis. It ends at the subcutaneous inguinal ring, placed above the tubercle and crest of the pubis. The anterior wall of the canal is formed by the aponeurosis of the obliquus externus, and in its lateral
part by the muscular fibres of the FIG. 429.-THE SHEATH OF THE RECTUS ABDOMINIS obliquus internus; the posterior wall of the canal is formed by the fascia (I.) transversalis, and in its medial part by the falx aponeurotica inguinalis; while A, the floor of the canal is formed by the inguinal ligament, and in its medial part by the lacunar ligament. The spermatic funiculus, piercing the transversalis fascia, enters the inguinal canal
In the thoracic wall; (II.) In the superior three
quarters of the abdominal wall; (III.) In the inferior fourth of the abdominal wall.
RECTUS MUSCLE ; B, OBLIQUUS EXTERNUS; C, Dia
PHRAGM; D, OBLIQUUS INTERNUS; E, TRANSVERSUS ABDOMINIS. a, Anterior layer of rectus sheath; b, Fifth costal cartilage; c, Sixth costal cartilage; d, Xiphoid process; e, Posterior layer of rectus sheath; f, Fascia transversalis; g, Peritoneum; h, Linea alba. 1, Inferior epigastric artery
at the abdominal inguinal ring, and is there invested by its first envelope, the infundibuliform or internal spermatic fascia, a sheath of fascia derived from the margins of the ring and continuous with the fascia transversalis. It then passes obliquely medially, downwards, and forwards, and escapes below the inferior border of the obliquus internus muscle, from which it carries off a second investment, partly fascial, partly muscular, the cremaster muscle or cremasteric fascia. Continuing its course, in front of the falx inguinalis, it emerges through the subcutaneous inguinal ring, from the edges of which the intercolumnar fascia is derived, the third or external investment for the funiculus.
Hesselbach's triangle, bounded below by the line of the inguinal ligament, medially by the rectus abdominis muscle, and laterally by the inferior epigastric artery, coursing upwards and medially beneath the fascia transversalis on the medial side of the abdominal inguinal ring, is the site of one form of inguinal hernia.
The spermatic funiculus passes over the base of the triangle, covered over by the aponeurosis of the obliquus externus. Behind the funiculus, and forming the floor of the triangle, is the fascia transversalis partially covered, in the medial portion of the triangle, by the falx inguinalis of the obliquus internus and transversus muscles.
Inguinal Hernia. For an account of the anatomical relations of the inguinal canal to the various forms of inguinal hernia, see the section on " Applied Anatomy."
Nerve-Supply. The nerve-supply of the majority of the foregoing muscles is derived from the anterior rami of the lower six thoracic nerves. The pyramidalis muscle is innervated by the last thoracic nerve. The cremaster muscle receives its supply from the genito-femoral nerve (L. 1. 2.).
Actions. (1) The chief action of these muscles is to retract the abdominal walls. By compressing the contents of the abdomen, they are powerful agents in vomiting, defæcation,