The muscles in this region are, the
Obliquus Externus.
Rectus.
Obliquus Internus.
Pyramidalis.
Transversalis.
Quadratus Lumborum.

To dissect the abdominal muscles, a vertical incision should be made from the ensiform cartilage to the pubes; a second oblique incision should extend from the umbilicus upwards and outwards to the outer surface of the chest, as high a" the lower border of the fifth or sixth rib; and a third, commencing midway between the umbilicus and pubes, should pass transversely outwards to the anterior superior iliac spine, and along the crest of the ilium as far as its posterior third. The three flaps included between thcss incisions should then be reflected from within outwards, in the direction indicated in the figure.

The External Oblique Muscle (Obliquus descendens), so called from the direction of its fibres, is situated on the lateral and anterior aspects of the abdomen; being the largest and the most superficial of the three flat muscles in this region. It is broad, thin, irregularly quadrilateral in form, its muscular portion occupying the sides, its aponeurosis the anterior wall of that cavity. It arises, by eight fleshy digitations, from the external surface and lower borders of the eight inferior ribs; these digitations arc arranged in an oblique line running 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 intermediate ones, to the ribs at some distance from their cartilages. The five superior serrations increase in size from above downwards, and are received between corresponding processes of the Serratus magnus; the three lower ones diminish in size from above downwards, receiving between them corresponding processes from the Latissimus dorsi. From these attachments the fleshy fibres proceed in various directions. Those from the lowest ribs pass nearly vertically downwards, to be inserted into the anterior half of the outer lip of the crest of the ilium; the middle and upper fibres, directed downwards and forwards, terminate in tendinous fibres, which spread out into a broad aponeurosis. This aponeurosis, joined with that of the opposite muscle along the median line, covers the whole of the front of the abdomen: above, it is connected with the lower border of the Pectoralis major; below, its fibres are closely aggregated together, and extend obliquely across from the anterior superior spine of the ilium to the spine of the os pubis and the pectineal line. In the median line, it interlaces with the aponeurosis of the opposite muscle, forming the linea alba, and extends from the ensiform cartilage to the symphysis pubis.

That portion of the aponeurosis which extends between the anterior superior spine of the ilium and the spine of the os pubis is a broad band, folded inwards, and continuous below with the fascia lata; it is called Pouparfs ligament. The portion which is reflected from Pouparfs ligament backwards and inwards into the pectineal line, is called Gimbernats ligament.

In the aponeurosis of the External oblique, immediately above the crest of the os pubis, is a triangular opening, the external abdominal ring, formed by a splitting of the fibres of the aponeurosis in this situation; it serves for the transmission of the spermatic cord in the male, and the round ligament in the female. This opening is directed obliquely upwards and outwards, and corresponds with the course of the fibres of the aponeurosis. It is bounded below by the crest of the os pubis; above, by some curved fibres, which pass across the aponeurosis at the upper angle of the ring so as to increase its strength; and on either side, by the margins of the aponeurocis, which are called the pillars of the ring. Of these, the external, which is, at the same time inferior, from the obliquity of its direction, is inserted into the spine of the os pubis. The internal, or superior pillar,;-being attached to the front of the symphysis pubis, interlaces with the corresponding fibres of the opposite muscle. To the margins of the pillars of the external abdominal ring is attached an exceedingly thin and delicate fascia, which is prolonged down over the external surface of the cord and testis. This has received the name of inter-columnar fascia from its attachment to the pillars of the ring. It has also received the name of external spermatic fascia, from being the most external of the fasciae which cover the spermatic cord.

Relations. By its external surface, with the superficial fascia, superficial epigastric and circumflexa ilii vessels, and some cutaneous nerves. By its internal surface, with the Internal oblique, the lower part of the eight inferior ribs and Intercostal muscles, the cremaster, the spermatic cord in the male, and round ligament in the female. Its posterior border is occasionally overlapped by the Latissimus dorsi; sometimes an interval exists between the two muscles, in which is seen a portion of the Internal oblique.

Dissection. The External oblique should now Ira detached by dividing it across, just in front of its attachment to the ribs, as far as its posterior border, and by separating it below from the crest of the ilium as far as the spine; the muscle should then be carefully separated from the Internal oblique, which lies beneath, and turned towards the opposite side.

The Internal Oblique Muscle(Obliqus ascendens), thinner and smaller thun the preceding, beneath which it lies, is of an irregularly quadrilateral form, and situated at the anterior lateral and posterior parts of the abdomen. It arises, by fleshy fibres, from the outer half of Poupart's ligament, being attached to the groove on its upper surface; from the anterior two-thirds of the middle lip of the crest of the ilium, and from the lumbar fascia. From this origin, the fibres diverge in different directions. Those from Poupart's ligament, few in number and paler in colour than the rest, arch downwards and inwards across the spermatic cord, to be inserted, conjointly with those of the Transversalis, into the crest of the os pubis and pectineal line, to the extent of half an inch, forming the conjoined tendon of the Internal oblique and Transversalis; those from the anterior superior iliac spine are horizontal in their direction; whilst those which arise from the front part of the crest of the ilium pass obliquely upwards and inwards, and terminate in an aponeurosis, which is continued forwards to the linea alba; the most posterior fibres ascend almost vertically upwards, to be inserted into the lower borders of the cartilages of the four lower ribs, being continuous with the internal intercostal muscles.

The conjoined tendon of the Internal oblique and Transversalis is inserted into the crest of the os pubis and pectineal line immediately behind the external abdominal ring, serving to protect what would otherwise be a weak point in the abdomen. Sometimes this tendon is insufficient to resist the pressure from within, and is carried forward in front of the protrusion through the extern;;; ring, forming one of the coverings of direct inguinal hernia.

The aponeurosis of the Internal oblique is continued forward to the middle line of the abdomen, where it joins with the aponeurosis of the opposite muscle at the linea alba, and extends from the margin of the thorax to the pubes. At the outer margin of the sheath of the Rectus muscle, for the upper three-fourths of its extent, this aponeurosis divides into two lamellae, which pass, one in front and the other behind it, enclosing it in a kind of sheath, and reuniting on its inner border at the linea alba: the anterior layer is blended with the aponeurosis of the External oblique muscle; the posterior layer with that of the Transversalis. Along the lower fourth, the aponeurosis passes altogether in front of the Rectus without any separation.

Relations, By its external surface, with the External oblique, Latissimus dorsi, spermatic cord, and external ring. By its internal surface, with the Transversalis muscle, fascia transversalis, internal ring, and spermatic cord. Its lower border forms the upper boundary of the spermatic canal.

Dissection. The Internal oblique should now be detached in order to expose the Transversalis muscle beneath. This may be effected by dividing the muscle, above, at its attachment to the ribs; below, at its connexion with Poupart's ligament and the crest of the ilium; and behind, by a vertical incision extending from the last rib to the crest of the ilium. The muscle should previously be made tense by drawing upon it with the fingers of the left hand, and if its division is carefully effected, the cellular interval between it and the Transversalis, as well as the direction of the fibres of the latter muscle, will afford a clear guide to their separation; along the crest of the ilium the circumflex ilii vessels are interposed between them, and form an important aid in separating them. The muscle should then be thrown forwards towards the linea alba.

The Transversalis muscle, so called from the direction of its fibres, is the most internal flat muscle of the abdomen, being placed immediately beneath the Internal Oblique. It arises by fleshy fibres from the outer third of Poupart's ligament, from the inner lip of the crest of the ilium, its anterior two-thirds, from the inner surface of the cartilages of the six lower ribs, interdigitating with the Diaphragm, and by a broad aponeurosis from the spinous and transverse processes of the lumbar vertebrae. The lower fibres curve downwards, and are inserted together with those of the Internal oblique, into the crest of the os pubis and pec-tineal line, forming what was before mentioned as the conjoined tendon of these muscles. Throughout the rest of its extent the fibres pass horizontally inwards, and near the outer margin of the Rectus, terminate in an aponeurosis, which is inserted into the linea alba; its upper three-fourths passing behind the Rectus muscle, blending with the posterior lamella of the Internal oblique; its lower fourth passing in front of the Rectus.

Relations. By its external surface, with the Internal oblique, the inner surfaces of the lower ribs, and Internal intercostal muscles. By its internal surface, it is lined by the fascia transversalis, which separates it from the peritoneum. Its lower border forms the upper boundary of the spermatic canal.

Lumbar Fascia. The vertebral aponeurosis of the Transversalis divides into three layers, an anterior, very thin, which is attached to the front part of the apices of the transverse processes of the lumbar vertebrae, and, above, to the lower margin of the last rib, forming the ligamentum arcuatum externum; a middle layer, much stronger, which is attached to the apices of the transverse processes; and a posterior layer, attached to the apices of the spinous processes. Between the anterior and middle layers is situated the Quadratus lumborum, between the middle and posterior, the Erector spinae. The posterior lamella of this aponeurosis receives the attachment of the Internal oblique; it is also blended with the aponeurosis of the Serratus posticus inferior and with that of the Latissimus dorsi, forming the Lumbar fascia; the two anterior layers are connected solely with the Transversalis.

Dissection. To expose the Rectus muscle, its sheath should be opened by a vertical incision extending from the margin of the thorax to the pubes, the two portions should then be reflected from the surface of the muscle, which is easily effected, excepting at the linese tansversae, where so close ail adhesion exists, that the greatest care is requisite in separating them. The outer edge of the muscle should now be raised, when the posterior layer of the sheath will be seen. By dividing the muscle in the centre, and turning its lower part downwards, the point where the posterior wall of the sheath terminates in a thin curved margin will be seen.

The Rectus Abdominis is a long, flat muscle, which extends along the whole length of the anterior wall of the abdomen, being separated from its fellow of the opposite side by the linea alba. It is much broader above than below, and arises by two tendons, the external or larger being attached to the crest of the os pubis; the internal, smaller portion, interlacing with its fellow of the opposite side, and being connected with the ligaments covering the symphysis pubis. The fibres ascend vertically upwards, and the muscle becoming broader and thinner at its upper part, is inserted by three portions of unequal size into the cartilages of the fifth, sixth, and seventh ribs. Some fibres arc also occasionally connected with the costo-xiphoid ligaments, and side of the ensiform cartilage.

The Rectus muscle is traversed by a series of tendinous intersections, which vary from two to five in number, and have received the name lineze transversae. One of these is usually situated opposite the umbilicus, and two above that point; of these, one corresponds to the ensiform cartilage, and the other, to the interval between the ensiform cartilage und the umbilicus; there is occasionally one below the umbilicus. These intersections pass transversely or obliquely across the muscle in a zigzag course; they rarely extend completely through its substance, sometimes pass only half way across it, and are intimately adherent to the sheath in which the muscle is enclosed.

The Rectus is enclosed in a sheath formed by the aponeuroses of the Oblique and Transversalis muscles, which are arranged in the following manner. When the aponeurosis of the Internal oblique arrives at the margin of the Rectus, it divides into two lamellas, 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 Transversalis; and these, joining again at its inner border, are inserted into the linea alba. This arrangement of the fasciae exists along the upper three-fourths of this muscle; at the commencement of the lower fourth, the posterior wall of the sheath terminates in a thin curved margin, the concavity of which looks downwards towards the pubes; the aponeuroses of all three muscles passing in front of the Rectus without any separation. The Rectus muscle in the situation where its sheath is deficient, is separated from the peritoneum by the transversalis fascia.

The Pyramidalis is a small muscle, triangular in form, situated at the lower part of the abdomen, one on each side of the linea alba. It arises by tendinous fibres from the front of the os pubis and anterior pubic ligament; the fleshy portion of the muscle passes upwards, diminishing in size as it ascends, and terminates by a pointed extremity, which is inserted into the linea alba, midway between the umbilicus and the os pubis. It rests against the lower part of the front of the Rectus, and is contained in the same sheath with that muscle. This muscle is sometimes found wanting on one or both sides; the lower end of the Uectus then becomes proportionally increased in size. Occasionally it has been found double on one side, or the muscles of the two sides are of unequal size. Sometimes its length exceeds that stated above.

The Quadratus Lumborum is situated in the lumbar region of the spine, it is irregularly quadrilateral in shape, broader below than above, and consists of two portions. One portion arises by aponeurotic fibres from the ilio-lumbar ligament, and the adjacent portion of the crest of the ilium for about two inches, and is inserted into the lower border of the last rib, about half its length, and by four small tendons, into the apices of the transverse processes of the third, fourth, and fifth lumbar vertebras. The other portion of the muscle, situated anterior to the preceding, arises from the upper borders of the transverse processes of the third, fourth, and fifth lumbar vertebrae, and is inserted into the lower margin of the last rib. The Quadratus lumborum is contained in a sheath formed by the anterior and middle lamellse of the vertebral aponeurosis of the Transversalis.

Nerves. All the abdominal muscles are supplied by the lower intercostal, ilio-hypo-gastric, and ilio-inguinal nerves, excepting the Quadratus lumborum, which receives filaments from the anterior primary branches of the lumbar nerves.

In the description of the abdominal muscles, mention has frequently been made of the linea alba, lineae semilunares, lineas transversas; when the dissection of these muscles is completed, these structures should be examined.

The Linea Alba is a tendinous raphe or cord seen along the middle line of the abdomen, extending from the ensiform cartilage to the symphysis pubis. It is placed between the inner borders of the Itecti muscles, and formed by the blending of the anterior aponeuroses of the Oblique and Transversalis muscles. It is narrow below, corresponding to the narrow interval existing between the Recti, but broader above, as these muscles diverge from one another in their ascent, becoming of considerable breadth after great distension of the abdomen from pregnancy or ascites. It presents numerous apertures for the passage of vessels and nerves; the largest of these is the umbilicus, which in the fcetus transmits the umbilical vessels, but in the adult is obliterated, the cicatrix being stronger than the neighbouring parts; hence the occurrence of umbilical hernia in the adult above the umbilicus, whilst in the foetus it occurs at the umbilicus. The linea alba is in relation, in front, with the integument to which it is adherent, especially at the umbilicus; behind, it is separated from the peritoneum by the transversalis fascia; and below, by the urachus, and the bladder, when that organ is distended.

The Linea Semilunares are two curved tendinous lines, placed one on each side of, and a little external to the linea alba. Each extends from the cartilage of the eighth rib to the pubes, and corresponds with the outer border of the Rectus muscle. They are formed by the aponeurosis of the Internal oblique at its point of division to enclose the Rectus.

The Linea: Transversae are three or four narrow transverse lines which intersect the Rectus muscle as already mentioned, they connect the linear semilunares with the linea alba.

Actions. The abdominal muscles perform a three-fold action.

When the pelvis and thorax are fixed, they can compress the abdominal viscera, by constricting the cavity of the abdomen, in which action they are materially assisted by the descent of the diaphragm. By these means, the foetus is expelled from the uterus, the faeces from the rectum, the urine from the bladder, and the ingesta from the stomach in vomiting.

If the spine be fixed, these muscles compress the lower part of the thorax, materially assisting in the process of expiration. If the spine be not fixed, the thorax is bent directly forward, if the muscles of both sides act, or to either side if they act alternately, rotation of the trunk at the same time taking place to the opposite side.

If the thorax be fixed, these muscles act upon the pelvis, as in climbing, when the pelvis is drawn directly upwards, or to one or the other side. The Recti muscles may draw the pelvis forwards, and flex it upon the vertebral column. The Pyramidales are tensors of the linea alba.

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