The muscles of the lower extremity are subdivided into groups corresponding wit,b the different regions of the limb.

I. Muscles of the iliac regionIII. Muscles of the leg
II. Muscles of the thighIV. Muscles of the foot


Psoas majorPsoas minorIliacus

The fascia iliaca covers the Psoas and Iliacus. It is thin above, but becomes gradually thicker as it approaches the inguinal ligament.

The portion covering the Psoas is thickened above to form the medial arcuate ligament (medial lumbocostal arch), which stretches from the transverse process of the first to the body of the first or second lumbar vertebra. Medially, the fascia covering the Psoas is attached by a, series of arched processes to the intervertebral discs, and prominent margins of the bodies of the vertebrae, and to the upper part of the sacrum. Laterally, above the crest of the ilium, it blends with the fascia covering the front of the Quadratus lumborum: below the crest, with the fascia covering the Iliacus.

The portion covering the Iliacus is connected, laterally, to the whole length of the inner lip of the iliac crest; and medially, to the brim of the true pelvis, where it blends with the periosteum. It is attached to the iliopubic (iliopectineal) eminence and there receives a slip from the tendon of insertion of the Psoas minor, when that muscle exists. The external iliac vessels lie in front of the fascia but the branches of the lumbar plexus of nerves are behind it: it is separated from the peritoneum by the extraperitoneal tissue.

Lateral to the femoral vessels, the iliac fascia is intimately connected to the posterior margin of the inguinal ligament, and is continuous with the transversalis fascia. It passes behind the femoral vessels, and beyond the inguinal ligament becomes the iliopectineal fascia. This fascia divides the space between the inguinal ligament and the hip-bone into a medial and a lateral part; the medial part transmits the femoral vessels, the lateral the Psoas major, the Iliacus and the femoral nerve. Medial to the vessels, the iliopectineal fascia is attached to the pectineal line of the pubis and is continuous with the pectineal fascia. In the thigh the iliopectineal fascia, covers the Iliacus and the Psoas major, and forms the posterior wall of the femoral sheath.

The Psoas major (fig. 643) is a long fusiform muscle placed on the side of the lumbar region of the vertebral column and the brim of the true pelvis. It arises (1) from the anterior surfaces and lower borders of the transverse processes of all the lumbar vertebrae : (2) by five slips or digitations, each of which takes,origin from the bodies of two vertebra and their intervertebral disc; the highest slip arises from the lower margin of the body of the twelfth thoracic vertebra, the upper margin of the body of the first lumbar vertebra and the interposed disc, the lowest slip from the adjacent margins of the bodies of the fourth and fifth lumbar vertebrae and the interposed disc; (3) from a series of tendinous arches extending across the constricted parts of the bodies of the lumbar vertebral between the preceding slips; the lumbar arteries and veins, and filaments from the sympathetic trunk, pass beneath these arches. The muscle proceeds downwards across the brim of the true pelvis, passes behind the inguinal ligament and in front of the capsule of the hip-joint, and ends in a tendon. The latter receives nearly the whole of the fibers of the Iliacus and is inserted into the lesser trochanter of the femur. A large bursa, which occasionally communicates with the cavity of the hip-joint, separates the tendon from the pubis and, the capsule of the joint.

Relations.-The uppermost part of the Psoas major lies behind the diaphragm and occupies the lowest part of the posterior mediastinum. It may be in contact with the posterior, lower limit of the pleural sac. In the abdomen the Psoas major is in relation by its anterolateral surface with the medial arcuate ligament (medial lumbocostal arch), the fascia covering the muscle, the extraperitoneal tissue and peritoneum, the kidney, Psoas minor, renal vessels, ureter, testicular (or ovarian) vessels, and genitofemoral nerve. In front the right Psoas is overlapped by the inferior vena cava and crossed by the terminal portion of the ileum, and the left is crossed by the colon. Its posterior surface is in relation with the transverse processes of the lumbar vertebrae, and the quadratus lumborum. The lumbar plexus is situated in the posterior part of the substance of the muscle. Medially, the muscle is in relation with the bodies of the lumbar vertebrae, the lumbar arteries, the sympathetic trunk, the aortic lymph-glands and, along the brim of the pelvis, with the external iliac artery; medially the right muscle is related to the inferior vena cava, and the left muscle to the aorta.

In the thigh it is in relation, in front, with the fascia lata and the femoral artery; behind with the capsule of the hip-joint from which it is separated by a bursa; by its medial border, with the Pectineus and medial circumflex femoral artery, and also with the femoral vein, which may overlap it, slightly; by its lateral border with the femoral nerve and the Iliacus.

The femoral nerve descends at first through; the fibers of Psoas major, and then lies between it and the Iliacus.

Figure 643
Left thigh muscles anterior view - Figure 643
Nerve-supply.—-The Psoas major is supplied by branches from the second and third lumbar nerves.

Actions.-The Psoas major acts conjointly with the Iliacus.

The Psoas minor (fig. 643) is placed in front of the Psoas major within the abdomen. It arises from the sides of the bodies of the twelfth thoracic and first lumbar vertebrm and from the disc between them. It ends in a long, flat tendon which is inserted into the pectineal line and iliopubic eminence; and, by its lateral border, into the iliac fascia. This muscle is absent in about 40 per cent. of subjects.

The Psoas major and Iliacus are sometimes described as a single muscle named the Iliopsoas.

Nerve-supply.-The Psoas minor is supplied by a branch from the first lumbar nerve.

Action.–The Psoas minor is a tensor of the iliac fascia and a weak flexor of the trunk.

The Iliacus (fig. 643) is a flat, triangular muscle which fills the iliac fossa. It arise: from the upper two-thirds of the iliac fossa, from the inner lip of the iliac crest, from the anterior sacro-iliac and iliolumbar ligaments, and from the upper surface of the lateral mass of the sacrum (fig. 279); in front, it reaches as far as the anterior superior and anterior inferior iliac spines, and receives a few fibers from the upper part of the capsular ligament of the hip-joint. Most of its fibers converge to the inserted into the lateral side of the tendon of the Psoas major, but some of them are attached to the shaft of the femur for 2.3 cm, below and in front of the lesser trochanter.

Relations.-Within the abdomen, the Iliacus is in relation, by its anterior surface, with the iliac fascia, which separates the muscle from the extraperitoneaI tissue and peritoneum and with the lateral femoral cutaneous nerve : on the right side, with the cecum; on the left side, with the iliac part of the descending colon; by its posterior surface, with the iliac fossa; by its medial border, with the Psoas major and femoral nerve.

In the thigh, it is in relation, by its anterior surface with the fascia lata, Rectus femoris, Sartorius and arteria profunda femoris; by its posterior surface, with the capsule of the hip-joint, a bursa common to it and the Psoas major being interposed.

Nerve-supply.-The Iliacus is supplied by branches of the femoral nerve (L. 2 and 3).

Actions.-The Psoas major, acting from above, flexes the thigh upon the pelvis, being assisted by the lliacus; acting from below, with the femur fixed it bends the lumbar portion of the vertebral column forwards and to its own side, and then, in conjunction with the Iliacus, tilts the pelvis forwards. When the Psoas major apd Iliacus of both sides act from below, they serve to maintain the erect posture by supporting the vertebral column and pelvis upon the femora, or in continued action bend the trunk and pelvis forwards, as in raising the trunk from the recumbent posture.

Applied Anatomy.- There is no definite septum between the portions of fascia covering the Psoas and Iliacus respectively, and the fascia is only connected to the subjacent inuscles by a quantity of loose connective tissue. When an abscess forms beneath this fascia, as it is very- apt to do, the pus is contained in an osseofibrous cavity which is closed on all sides within the abdomen, and is open only at its lower part, where the fascia is prolonged over the muscles into the thigh. When the disease is in the thoracic vertebrae, the pus tracks down the posterior mediastinum in front of the bodies of the vertebrae, and, passing behind the medial arcuate ligament, enters the sheath of the Psoas, down which it travels as far as the pelvic brim; it then passes deep to the iliac portion of the fascia, and fills up the iliac fossa. In consequence of the attachment of the fascia to the arcuate line (linea terminalis), it rarely finds its way into the true pelvis, but passes by a narrow opening under the inguinal ligament into the thigh, lateral to the femoral vessels. It thus follows that a psoas abscess may be described as consisting of four parts, (1) a somewhat narrow channel at its upper part, in the psoas sheath; (2) a dilated sac in the iliac fossa; (3) a constricted neck behind the inguinal ligament; and (4) a dilated sac in the upper part of the thigh. When the lumbar vertebra are the seat of the disease, the matter finds its way directly into the substance of the Psoas. The muscular fibers are destroyed, and the nerves contained in the abscess are isolated and exposed in its interior; the iliac vessels, which lie in front of the fascia. remain intact; and the peritoneum seldom becomes implicated. All psoas abscesses do not, however, pursue this course; the pus may leave the sheath of the muscle above the crest of the ilium, and tracking backwards may point in the loin (lumbar abscess); or it may point above the inguinal ligament in the inguinal region; or it may follow the course of the branches of the internal iliac vessels into the true pelvis, and passing through the greater sciatic foramen, discharge itself on the back of the thigh.


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