Above the umbilicus the linea alba is from two to four lines broad in the greater part of its extent; and below the umbilicus it gradually tapers down to the pubis, at the same time in creasing in thickness.* 5. Pyramidalis (pubio-sub-umbilical). At the inferior extremity of the recti, and separa ting their origin, are two small muscles of a pyramidal form ; their bases are inferior, and attached to the symphysis and body of the pubis, and uniting ligaments, and their apices superior and inserted into the linea alba by small tendons, from two to three inches above the symphysis pubis. Each muscle is enve loped in a distinct sheath, and lies a little more prominently than the origin of the rectos of the same side. These muscles are not unfrequently absent. Sometimes, on the contrary, there have been two on one side and one on the other, or even two on each side.-f The muscles which enter into the composi tion of the posterior wall of the abdomen are chiefly those which occupy the lumbar region of the back, filling up that empty space which in the skeleton is observed on each side of the spinal column between the crista ilii and the last rib. In dissecting from behind forwards in this region, having removed the skin and lax cellular tissue already described, we come upon the strong fibrous expansion, the fascia lumbo rum. This has extensive osseous attachments, and thus firmly binds down the subjacent mus cles. When it is removed, the lumbar portions of the sacrolumbalis and longissimus dorsi, and a little of the spinalis dorsi, are brought into view, the two former of which are described by some as a single muscle—the sacrospinalis. The external of these muscles is the sacrolum balis, and its outer margin may be said to con stitute the limit of the posterior wall of the abdomen in that direction. In this situation the posterior and middle layers of the tendon of the transversalis separate from each other to ensheath these muscles, the posterior layer forming the fascia lumborum. We must refer to the article BACK for a particular description of these muscles.
When the lumbar mass of muscles (as the three preceding have been called) has been re moved, the next part brought into view is the anterior layer of their fibrous sheath formed by the middle lamina of the transversalis tendon, which is inserted into the apices of the trans verse processes. This lamina is thin and semi transparent, so that the fibres of the muscle which lies immediately before it, are seen through it. This muscle is the Quadratus lumborum ilio-lurnbi costal). The term quadratus is applied to this muscle, more from its quadrilateral form than from any nearer resemblance to a square, in asmuch as all its sides are unequal. The most fixed attachment of this muscle is its inferior, where it is inserted by tendinous fibres into the iliolumbar ligament and into the inner lip of the crista ilii for about an inch to the outer side of the insertion of that ligament. From these points the fibres proceed vertically upwards, the ex ternal ones going to be inserted into the inferior margin of the last rib for nearly its entire length, and the internal fibres, those in parti cular which are attached to the ligament, ter minating by four aponeurotic tongue-like bun dles, which are inserted into the anterior surface of the transverse processes of the four superior lumbar vertebrae near their bases. The several bundles which end in these tongue-like pro cesses vary in length; those which are external being the longest, as going to higher vertebrae. This muscle is covered on its anterior or abdo minal surface by the anterior lamina of the tendon of the transversalis muscle, by which it is separated from the diaphragm as well as from the psoas magnus.* The last dorsal
nerve and the first two branches of the lumbar plexus, pass between the quadratus and the aponeurotic lamina which covers it.
Psoas magnus, (4noc, lumbus) (prelombo, trochanterien, lumbaris.) The greatest por tion of this muscle belongs to the abdominal region ; it lies along the side of, not only the lumbar but also of a small portion of the dorsal region of the spine, lodged in the angle between the transverse processes and bodies. It passes as high up as the twelfth dorsal vertebra, to the body of which as well as to those of the four suc ceeding lumbar and to their interven ing fibro-cartilages, the muscle is attached : it likewise is attached to the bases of the corres ponding transverse processes, so that the inter vals between the portions that are attached to the bodies, and those to the transverse processes, correspond to the intervertebral foramina or points of exit of the lumbar nerves, the an terior branches of which plunge at once into the substance of the psoas muscle to form the lumbar plexus. The several bundles which thus take their origin from the vertebrae form a thick rounded muscle, which passes nearly vertically downwards, inclining a little out wards, over the brim of the true pelvis, so as often to appear to encroach upon the circum ference of the upper outlet of that cavity. A little way above Poupart's ,ligament the mus cular fibres are inserted around a strong thick tendon. This tendon, which had commenced high up by distinct portions in the interior of the muscle, passes under Poupart's ligament over the horizontal ramus of the pubis. It descends over the capsular ligament of the hip joint (from which as well as from the ramus of the pubis it is separated by a bursa) over the head and along the inner side of the neck of the femur, and is inserted into the posterior part of the trochanter minor at its base, being separated by a small bursa from the surface of that process. As the tendon is passing over the ramus of the pubis, it receives by its outer margin a series of fibres from the iliacus in ternus muscle. At its superior portion the psoas muscle is covered by a thin fibrous ex pansion, which is attached on the one hand to the apices of the transverse processes, and on the other to the bodies of the upper lumbar vertebrae ; this expansion, the arcus interior of Senac and also called ligamentum arcuatum, separates the psoas from the dia phragm. Below this the psoas muscle is covered with a lax, and in some degree fatty cellular tissue, which separates the muscle from the kidney externally, and from the peritoneum and ureter within, excepting where the psoas parvus covers it, and on the right side where the vena cava lies upon it. Along its internal margin are the lumbar portion of the sympathetic, the crura of the diaphragm, more especially on the left side, and on this side too the aorta ap proaches a little its internal margin. The common and external iliac arteries and veins lie along the internal margin of the pelvic portion of the muscle, which is covered by the fascia iliaca. The several branches of the lumbar plexus issue from this muscle at its external margin, and the genito-crural nerve descends in front of it inferiorly. We refer to the article on the muscles of the thigh for a further account of this muscle, its relations in the upper part of the thigh, and its actions.