IV. THE MUSCLES OF THE ABDOMEN
| Obliquus externus | Transversus abdominus |
| Obliquus internus | Rectus abdominus |
| Pyrimadalis |
The superficial fascia of the abdomen consists, over the greater part of the abdominal wall, of a single layer containing a variable amount of fat; but near the groin the fascia is easily divisible into two layers, between which are found the superficial vessels, nerves, and inguinal lymph-glands.
The superficial layer of the fascia, is thick, areolar in texture, and contains in its meshes a varying quantity of fat. Below, it passes over the inguinal ligament, and is continuous with the superficial fascia of the thigh. In the male this layer is continued over the penis and outer surface of the spermatic cord to the scrotum. As it passes to the scrotum it changes its characteristics becoming thin, destitute of adipose tissue, and of a pale reddish color: in the scrotum it acquires some involuntary muscular fibers and forms the dartos muscle. From the scrotum it may be traced backwards into continuity with the superficial fascia of the perineum. In the female it is continued from the abdomen into the labia majora.
The Obliquus externus abdominis (fig. 600), situated on the lateral and anterior parts of the abdomen, is the largest and the most superficial of the three flat muscles in this region. It arises, by, eight fleshy slips from the external surfaces and inferior borders of the lower eight ribs: these slips interdigitate with the slips of origin of the Serratus anterior and Latissimus dorsi, and are arranged in an oblique line which runs downwards and backwards, the upper ones being attached close to the cartilages of the corresponding ribs, the lowest to the apex of the cartilage of the last rib, the middle ones to the ribs at some distance from their cartilages. From these attachments the fleshy fibers proceed in various directions. Those from the lower two ribs pass nearly vertically downwards, and are inserted into the anterior half or more of the outer lip of the ventral segment of the iliac crest; the middle and upper fibers, directed downwards and forwards, end in an aponeurosis, opposite a line drawn vertically from the ninth costal cartilage to a little below the level of the umbilicus, and then inclining laterally to the anterior superior iliac spine. None of the fleshy fibers of the muscle extend downwards beyond a line drawn from the anterior superior iliac spine to the umbilicus. The posterior border of the muscle is free.
It is joined with that of the opposite muscle along the median plane, and the aponeuroses of the two muscles cover the front of the abdomen; above and laterally, it is covered by, and gives origin to, the lower fibers of the Pectoralis major; below, its fibers are closely aggregated together, and extend obliquely across from the anterior superior iliac spine to the pubic tubercle and the pectineal line. In the median plane its fibers end in the linea alba (fig. 600), a tendinous raphe which stretches from the xiphoid process to the symphysis pubis.
The complete insertion of the muscle can now be summarized. The highest fibers are inserted into the xiphoid process; the succeeding fibers into the linea alba, the upper border of the pubic symphysis and the adjoining part of the pubic crest, the pubic tubercle and the adjoining part of the pectineal line; and the lowest fibers into the anterior superior iliac spine and the anterior two-thirds of the outer lip of the ventral segment of the iliac crest.
Nerve-supply.-The External oblique muscle is supplied by the anterior primary rasni of the lower thoracic nerves.
Actions.–When the thorax and pelvis are fixed the External oblique muscles compress the abdominal viscera and thus assist in expelling the feces from the rectum, the urine from the bladder, the fetus from the uterus, and the contents of the stomach in vomiting. If the pelvis and vertebral column be fixed the muscles depress and compress the lower part of the thorax, assisting in expiration. If the pelvis alone be fixed, the trunk is bent forwards when both muscles act; if the muscle of one side act, the trunk is bent towards that side and the front of the abdomen turned towards the opposite side. If the thorax be fixed, the muscles, acting together, draw the front of the pelvis upwards and help to flex the lumbar part of the vertebral column.
Certain parts of the aponeurosis of the External oblique muscle require more detailed consideration.
The superficial inguinal ring (subcutaneous inguinal ring) (figs. 601, 602) is an interval in the aponeurosis, just above and lateral to the crest of the pubis. The aperture is somewhat triangular in form, and its direction is oblique, corresponding with the course of the fibers of the aponeurosis. It measures from base to apex about 2.5 cm., and across the base about 1.25 cm. Its base is formed by the crest of the pubis, and its sides by the margins of the opening in the aponeurosis, which are called the crura of the, ring; above, the crura are connected by a series of curved intercrural fibers. The inferior crus of the ring is the stronger, and is .formed by that portion of the inguinal ligament which is inserted into the pubic tubercle; it is curved so as to form a kind of groove, upon which, in the male, the spermatic cord rests. The superior crus is a thin, flat band, the fibers of which are attached to the front of the symphysis pubis, and interlace with the fibers of the opposite superior crus.
The superficial inguinal ring gives passage to the spermatic cord and ilioinguinal nerve in the male, and to the round ligament of the uterus and the ilio-inguinal nerve in the female; it is much larger in men than in women, on account of the size of the spermatic cord.
The intercrural fibers are curved, tendinous fibers which arch across the lower part of the aponeurosis of the External oblique muscle, describing curves with. the convexities downwards. They have received their name from the fact that they stretch across between the two crura of the superficial inguinal ring. They are much thicker and stronger at the inferior crus, where they are connected to the inguinal ligament, than superiorly, where they are inserted into the linea alba. The intercrural fibers increase the strength of the lower part of the aponeurosis, and tend to prevent the divergence of the crura from each other; they are more strongly developed in the male than in the female. As they pass across the superficial inguinal ring they are connected together by delicate fibrous tissue, forming what is called the external spermatic fascia, (intercrural fascia). This fascia is continued down as a tubular prolongation around the spermatic cord and testis, and forms the outermost of the coverings which enclose them. The superficial inguinal ring is seen as a distinct aperture only after the continuity between this fascia and the aponeurosis of the External oblique muscle has been severed.
The inguinal ligament (figs. 602, 603) is the lower border of the aponeurosis of the External oblique muscle, and stretches from the anterior superior iliac spine to the pubic tubercle. Its general direction is convex downwards towards the thigh, where it is continuous with the fascia lata. Its lateral one-half is rounded, and oblique in direction; its medial one-half gradually widens at its attachment to the pubis, is more horizontal in direction, and supports the spermatic cord.
The pectineal part of the inguinal ligament (lacuna, ligament) (fig. 603) is that portion of the aponeurosis of the External oblique muscle which is reflected backwards and laterally from the medial part of the inguinal ligament, and is attached to the medial end of the pectineal line. It is of a triangular form, and is almost horizontal in direction when the body is in the erect posture; it is a little larger in the male than in the female, and measures about 2 cm. from base to apex. Its base, directed laterally, is concave and thin, and forms the medial boundary of the femoral ring; its apex corresponds to the pubic tubercle. Its posterior margin is attached to the pectineal line; and is continuous with the pectineal fascia; its anterior margin is continuous with the inguinal ligament. Its surfaces are directed upwards and downwards.
The reflected part of the inguinal ligament (figs. 602, 607) is a triangular shaped layer of tendinous fibers; it is formed by an expansion from the pectineal part of the inguinal ligament and the inferior crus of the superficial inguinal ring. It passes medially behind the spermatic cord, expands into a triangular band behind the superior crus of the superficial inguinal ring and in front of the conjoint tendon (inguinal aponeurotic falx) : its fibers interlace with those of the opposite ligament at the linea alba.
The pectineal ligament (ligament of Cooper).-This is a strong fibrous band, which was first described by Sir Astley Cooper. It extends laterally from the base of the pectineal part of the inguinal ligament (fig. 603) along the pectineal line, to which it is attached. It is strengthened by the pectineal fascia, and by a lateral expansion from the lower attachment of the linea alba (adminiculum lineve albae).
Nerve-supply.-The Internal oblique muscle is supplied by the anterior primary rami of the lower thoracic and first lumbar nerves.
The Cremaster (figs. 604, 606) is a thin muscular layer, composed of a number of fasciculi which arise from the middle of the inguinal ligament, where its fibers are continuous with those of the Internal oblique and also occasionally with the Transversus. It passes along the lateral side of the spermatic cord, and descends with it through the superficial inguinal ring upon the anterior and lateral surfaces of the cord, where it forms a series of loops which differ in thickness and length. At the upper part of the cord the loops are short, but they become successively longer; the longest reaching as far as the tunica vaginalis, into which a few are inserted. These loops are united together by areolar tissue; and form a thin covering over the cord and testis, named the cremasteric fascia. The fibers ascend along the medial and posterior surfaces of the cord, and are inserted by a small pointed tendon into the tubercle and crest of the pubis and into the front of the sheath of the Rectus abdominis.
Action-The Cremaster is an involuntary muscle which pulls up the testis. The Transversus abdominis (fig. 605), so called from the direction of its fibers, is the most internal of the flat muscles of the abdomen, being situated deep to the Internal oblique. It arises by fleshy fibers from the lateral one-third of the inguinal ligament, from the anterior two-thirds of the inner lip of the ventral segment of the iliac crest, from the lumbar fascia as it extends between the iliac crest and the twelfth rib, and from the inner surfaces of the cartilages of the lower six ribs, interdigitating with the Diaphragm (fig. 597). The muscle ends in an aponeurosis, the lower fibers of which curve downwards and medially, and are inserted, together with those of the aponeurosis of the Internal oblique muscle, into the crest and pectineal line of the pubis, forming the conjoint tendon. The rest of the aponeurosis passes horizontally to the median plane, and is inserted into the linea alba: its upper three-forths lie behind the Rectus abdominis and blend with the posterior lamella of the aponeurosis of the Internal oblique muscle : its lower one-fourth is in front of the Rectus. The upper muscular fibers of the Transversus abdominis are continued medially behind the Rectus abdominis and the posterior lamella of the aponeurosis of the Internal oblique, and near the xiphoid process; reach to within 2 or 3 cm. of the linea alba. The muscular fibers of the Transversus abdominis run into the aponeurosis along a line which is concave medially (fig. 605), the aponeurosis being widest opposite the origin of the muscle from the lumbar fascia.
Nerve-supply.-The Transverses abdominis is supplied by the anterior primary rami of the lower thoracic and first lumbar nerves.
Actions.-The Transversi abdominis almost completely encircle the abdominal cavity; in action they compress the abdominal contents.
The Rectus abdominis (fig. 609) is a long flat muscle, broader above than below, which extends along the whole length of the front of the abdomen, and is separated from its fellow of the opposite side by the linea alba. It arises by two tendons; the lateral and larger is attached to the crest of the pubis, the medial interlaces with its fellow of the opposite side and is connected with the ligamentous fibers covering the front of the symphysis pubis. The muscle is inserted by three slips of unequal size into the cartilages of the fifth, sixth, and seventh ribs; the most lateral fibers are usually inserted into the anterior extremity of the fifth rib; the most medial are occasionally connected with the costoxiphoid ligaments and the side of the xiphoid process.
The Rectus abdominis is enclosed in a sheath (figs. 604, 605, 610) formed by the aponeuroses of the Obliqui and Transversus, which are arranged as follows At the lateral margin of the Rectus, the aponeurosis of the Internal oblique divides into two lamely, one of which passes in front of the Rectus, blending with the aponeurosis of the External oblique, the other, behind it, blending with the aponeurosis of the Transversus, and these, joining again at the medial border of the Rectus, reach the linea alba. This arrangement of the aponeuroses exists from the costal margin to midway between the umbilicus and symphysis pubis, where the posterior wall of the sheath ends in a curved margin, named the arcuate line (linea semicircularis) (fig. 600), the concavity of which is directed downwards. As already stated the muscular fibers of the upper part of the Transversus abdominis are continued behind the corresponding part of the Rectos abdominis to within 2 or 3 cm, of the linea alba, (figs. 605, 609). Below the level of the arcuate line the aponeuroses of all three muscles pass in front of the Rectos; those of the Transversus and Internal oblique are intimately fused together, but the aponeurosis of the External oblique is bound to them merely by loose connective tissue except in and near the median plane behind, the Rectos is separated from the peritoneum by the transversalis fascia (fig. 611). Since the aponeuroses of the Internal oblique and Transversus only reach as high as the costal margin it follows that above this level the sheath of the Rectus is deficient posteriorly, the muscle resting directly on the cartilages of the ribs; the front of this part of the Rectus is covered merely by the aponeurosis of the External oblique muscle.
Actions.—-The Rectos abdominis acting from above elevates the front of the pelvis; acting from below it depresses the thorax, and in continued action flexes the vertebral column. The two muscles are also powerful compressors of the abdominal viscera.
The Pyramidalis (fig. 609) is a triangular muscle, placed at the lower part of the abdomen, in front of the Rectos abdominis and within the sheath of that muscle. It arises by tendinous fibers from the front of the pubis and from the ligamentous fibers in front of the symphysis; the fleshy portion of the muscle passes upwards, diminishing in size as it ascends, and ends in a pointed extremity which is inserted into the linea alba midway between the umbilicus and pubis, but may extend to a higher level. This muscle may be larger on one side than on the other, or may be wanting on one or both sides.
Besides the Rectus and Pyramidalis, the sheath of the Rectus contains the superior and inferior epigastric arteries, and the terminal portions of the lower intercostal nerves.
Nerve-supply.-The Pyramidalis is supplied by the twelfth thoracic nerve.
Action.-The Pyramidalis is a tensor of the linea alba.
The linea alba (figs. 600, 610) is a tendinous raphe stretching between the xiphoid process and the symphysis pubis. It is placed between the medial borders of the Recti, and is formed by the interlacement of the fibers of the aponeuroses of the Obliqui and Transversi. It is narrow below, corresponding to the linear interval existing between the Recti; but broader above, where these muscles diverge from each other. Its lower end has a, double attachment–its superficial fibers passing in front of the medial heads of the Recti to the front of the symphysis pubis, while its deeper fibers form a triangular lamella, attached behind the Recti to the posterior surface of the crest of the pubis, and named the adminiculum linea albae. The linea alba presents apertures for the passage of vessels and nerves; in the fetus the umbilicus transmits the umbilical vessels, but it is closed a few days after birth.
The inguinal canal contains the spermatic cord and the ilio-inguinal nerve in the male, and the round ligament of the uterus and the ilio-inguinal nerve in the female, It is an oblique canal about 4 cm. long, slanting downwards and medially, and placed parallel with and a little above, the inguinal ligament; it extends from the deep (abdominal) inguinal ring to the superficial (subcutaneous) inguinal ring. It is bounded in front throughout its whole length by the skin, superficial fascia, and aponeurosis of the External oblique, and in its lateral one-third by the fleshy fibers of origin of the Internal oblique muscle; behind, by the reflected part: of the inguinal ligament, the conjoint tendon, the transversalis fascia, the extraperitoneal connective tissue, and the peritoneum; above, by the arched fibers of the Internal oblique and Transversus abdominis below, by the union of the transversalis fascia with the inguinal ligament, and at its medial end by the pectineal part of the inguinal ligament (lacunar ligament).
The presence of the canal weakens the lower part of the anterior abdominal wall; but the weakness thus produced is compensated for partly by the obliquity of the canal and partly by the arrangement of the constituent parts of its walls. Owing to the oblique direction of the canal the two inguinal rings do not lie opposite to one another, and increases in the intra-abdominal pressure exercise their effect not only at the deep inguinal ring but also on the posterior wall of the canal so as to approximate it to the anterior wall. The posterior wall of the canal is strengthened by the conjoint tendon and the reflected part of the inguinal ligament precisely opposite to the superficial inguinal ring, and the fleshy fibers of the Internal oblique muscle take part in the formation of the anterior wall, where it lies opposite to the deep inguinal ring.
The extraperitoneal connective tissue.-Between the peritoneum and the inner surface of the general layer of the fascia which lines the interior of the abdominal and pelvic cavities, there is a considerable amount of connective tissue, termed the extraperitoneal or subperitoneal connective tissue. For descriptive purposes it may be divided into a parietal and a visceral portion.
The parietal portion lines the cavity, and varies in quantity in different situations. It is especially abundant on the posterior wall of the abdomen, and particularly around the kidneys, where it contains much fat. It is scanty on the anterolateral wall of the abdomen, except in the pubic region and above the iliac crest; there is a considerable amount in the pelvis.
The visceral portion follows the course of the branches of the abdominal aorta between the layers of the mesenteries and other folds of peritoneum which connect the viscera to the abdominal and pelvic walls.
2. THE POSTERIOR MUSCLES OF THE ABDOMEN
| Psoas major | Iliacus. (Iliopsoas) |
| Psoas minor | Quadratus lumborum |
The Psoas major, the Psoas minor, and the Iliacus, with the fascia covering there, are described with the muscles of the lower limb.
The fascia covering the Quadratus lumborum is the anterior layer of the lumbar fascia, It is attached, medially, to the anterior aspects of the transverse processes of the lumbar vertebrae; below, to the iliolumbar ligament; above, to the apex and lower border of the last rib. The upper margin of this fascia, which extends from the transverse process of the first lumbar vertebra to the apex and lower border of the last rib, constitutes the lateral arcuate ligament. Laterally, the fascia blends with that part of the lumbar fascia which gives origin to the transversus abdominis (fig. 594).
The Quadratus lumborum (figs. 595, 643) is irregularly quadrilateral in shape, and broader below than above. It arises by aponeurotic fibers from the iliolumbar ligament and the adjacent portion of the iliac crest for about 5 cm., and is inserted into the medial one-half of the lower border of the last rib, and by four small tendons into the apices of the transverse processes of the upper four lumbar vertebrae. Occasionally a second portion of this muscle is found in front of the preceding; it arises from the upper borders of the transverse processes of the lower three or four lumbar vertebrae, and is inserted into the lower margin and the lower part of the anterior surface of the last rib.
In front of the Quadratus lumborum are the colon, the kidney, the Psoas major et minor, and the Diaphragm; the subcostal, iliohypogastric, and ilio-inguinal nerves lie in front of the fascia which covers the muscle brat are bound down to it by the continuation medially of the transversalis fascia.
Nerve-supply.-The Quadratus lumborum is supplied by the anterior primary rami of the twelfth thoracic and upper three or four lumbar nerve.
Actions.–The Quadratus lumborum draws down the last rib; and acts as a muscle of inspiration by helping to fix the origin of the Diaphragm. If the thorax and vertebral column are fixed, it may act upon the pelvis raising it towards its own side where only one muscle is put in action; and when both muscles act together they help to extend the lumbar part of the vertebral column.
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