Variation archive

854 published articles

• Encyclopedia entry8 August 2026

  Rhomboideus Major, Rhomboideus Minor

  The rhomboid sheet may extend beyond its usual limits in both a cranial and caudal direction. In some cases, the caudal border may reach the cranial border of latissimus dorsi. The caudal extension of rhomboideus major may be simultaneously associated with a cranial extension of latissimus dorsi. Mori reported the upper limit of the vertebral origin to be as follows: Upper Limit: Cervical 4 right side, 5 left side, 5 Total 10 16.7%. 5 right side, 18 left side, 18 Total 36 60.0%. 6 right side, 7 left side, 7 Total 14 23.3%. Lower Limit: Thoracic 3 right side, 1 left side,1 Total 2 9.3%.

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• Encyclopedia entry8 August 2026

  Rectus Inferioris (Inferior Rectus) Rectus Lateralis (Lateral Rectus) Rectus Medialis (Medial Rectus) Rectus Superioris (Superior Rectus) Obliquuis Inferior (Inferior Oblique) Obliquuis Superiorus (Superior Oblique)

  The four recti may be poorly developed and may be connected to each other by muscular slips. The inferior rectus may provide a large muscular bundle passing lateral to the optic nerve to join the superior rectus. A fasciculus of the lateral rectus may pass to the inferior rectus. Two additional fasciculi sometimes occur: one passing forward to end on the inferior tarsal plate and another inserting on the lateral wall of the orbit.

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• Encyclopedia entry8 August 2026

  Rectus Capitis Lateralis

  This is a short thick muscle arising from the upper, fore part of the transverse process of the atlas and inserted onto the rough under surface of the condylar portion of the occipital bone. The muscle represents the lateral intertransverse muscle of the space between the transverse process of the atlas and the occipital bone. An accessory bundle, rectus capitis lateralis longus, arising from the transverse process of the axis and inserting on the occipital bone has been reported.

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• Encyclopedia entry8 August 2026

  Rectus Capitis Anterior

  This small flat muscle, partly covered by longus capitis, arises from the transverse process of the atlas and is inserted into the basilar process of the occipital bone, between the foramen magnum and the longus capitis muscle.. It represents an anterior intertransverse muscle. It may be absent or reduced to a few fiber bundles, or it may fail to reach the occipital bone, inserting into the occipitoatlantal ligament. A medial part may separate from the primary muscle mass and has been named rectus capitis minimus.

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• Encyclopedia entry8 August 2026

  Rectus Abdominis

  The rectus muscle of lower mammals extends further over the thorax than that of the human subject. In many of the lower primates, it reaches the first rib and the clavicle. It may rise to the fourth rib or even the third rib in humans, and in certain rare cases even higher. On the other hand, the slip attached to the fifth rib may be absent. The muscle usually inserts onto two or three ribs. In one study, six types of insertion were found. In 67%, the insertions were ribs 4 and 5; 4,5 and 6; 5 and 6; or 5, 6 and 7. In 20%, the insertions were ribs 6 and 7; and in 10%, ribs 6, 7 and 8.

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• Encyclopedia entry8 August 2026

  Quadriceps Femoris

  The deep parts of the quadriceps are not subject to many variations. The vasti are sometimes bilaminar. This, in the case of vastus lateralis, is an accentuation of a typical feature. The vasti may be continuous with the rectus femoris, and all three parts may be united into a common fleshy mass. The absence of rectus femoris, as well as the entire muscle mass, has been reported. Macalister reported the variations of the quadriceps as follows; 1. An origin from the anterior superior spine of the ilium was found joining the straight head; 2.

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• Encyclopedia entry8 August 2026

  Quadratus Plantae

  Also known as flexor accessorius this muscle is commonly reduced in size: the lateral head may be missing, as may, the medial head or whole muscle. Heads may be duplicated or divided into superficial or deep parts. The manner of attachment to the tendon of flexor digitorum longus longus varies. The muscle may be inserted in part or wholly into the long flexor of the great toe. Quadratus plantae may recieve (in about 5% of cases) an accessory slip from the fibula or tibia, one of the muscles of the leg or foot, or the medial surface of the calcaneus or related areas (see m.

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• Encyclopedia entry8 August 2026

  Quadratus Lumborum

  This muscle varies according to the degree of development of its vertebral slips. The insertion of the muscle has been found to extend to the eleventh rib. Syn.: m. scalenus lumborum (Meyer, H.), ileolumbalis (Meyer, H.), rectus abdominis posticus (Luschka).

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• Encyclopedia entry8 August 2026

  Quadratus Femoris

  Quadratus femoris may be fused with the inferior gemellus or with adductor magnus, replaced by a large inferior gemellus or oturator internus, or may be absent altogether (1-2%). It may be divided at its femoral insertion into two parts: one posterior, with the normal attachment, and the other anterior, with insertion onto the intertrochanteric crest. Hallett and Schwegel reported it absent and replaced by an unusually thick (large) obturator internus muscle. Syn.: Quarré fémoral.

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• Encyclopedia entry8 August 2026

  Pyramidalis

  The first anatomist to refer to the pyramidalis was Nocolai Massa (1538) who assumed that this muscle assisted in the erection of the penis while Vesalius (1543) thought that it had the function of expressing urine from the penis, "pene urieversum efformans musculum". Riolan (1649) noted that the muscle is occasionally absent, as did Crooke (1650), and Rolfinck (1656). Rolfinck also noted that occasionally only one of the two muscles was present, almost invariably on the right side. Diemerbroek (1672) repeated these observations Ruysch (1704) reported that on one side the muscle was doubled.

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• Encyclopedia entry8 August 2026

  Pronator Teres

  Macalister has found the following variations for this muscle: The most interesting variation of this muscle is the extension of its origin, in a proximal direction, to a supracondylar process or to the ligament (of Struthers) that connects it to the medial epicondyle. In these cases, the course of the brachial artery is usually changed: accompanied by the median nerve, it passes beneath the process , deep to the accessory portion of the muscle, and reaches the antecubital space.

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• Encyclopedia entry8 August 2026

  Pronator Quadratus

  In very rare instances, even when the radius is normal, the absence of a pronator quadratus has been noted. It is occasionaly laminated into two or more layers consisting of fasciculi running in different directions. It may be continued down on the carpus or metacarpus, in some cases as a radiocarpal fasciculus, in other cases as an ulnocarpal fasciculus. Aberrant fibers arising from the distal edge of the muscle may insert into a thenar muscle mass and act as an accessory adductor of the thumb.

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• Encyclopedia entry8 August 2026

  Popliteus

  Popliteus represents the pronator element of the flexor-pronator mass and corresponds to the upper part of the head of pronator teres of the arm. A rare variation is the presence of a second head, or accessory slip (popliteus minor [Calori,Wood] or popliteus biceps [Gruber]), which may arise from the upper and posterior part of the lateral condyle of the femur on the medial side of plantaris, from the lateral head of gastrocnemius or, in some cases, from the posterior ligament of the knee joint.

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• Encyclopedia entry8 August 2026

  Piriformis

  The most frequent variation is the division of the muscle into two parts. This is usually associated with a high division of the sciatic nerve, with the peroneal portion of the sciatic emerging between the two parts. The muscle is rarely divided by the tibial division of the sciatic, or by the entire sciatic nerve. The cleavage generally affects only the muscle belly, but may involve the entire muscle. Subdivision into several fascicles has also been reported. The muscle has been reported absent as well.

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• Encyclopedia entry8 August 2026

  Peroneus Quartus (Otto)

  Gruber found the peroneus quartus muscle to be present in about 13% of his bodies. The muscle arises from the posterior intermuscular septum and inserts on the fibular aspect of the calcaneus. Hence it has also been called peroneocalcaneus (See also the discussion of m. flexor hallucis longus). This muscle may also exist as a peroneocuboideus when it inserts onto the cuboid bone. Peroneus quartus muscles are located in the pre-achilles fat pad of the lower calf and may appear radiologically as a soft-tissue mass.

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• Encyclopedia entry8 August 2026

  Peroneus Brevis and Longus

  In rare cases, fasciculi have been seen arising from the lateral femoral condyle, and it has been suggested that the fibular collateral ligament represents a primitive femoral attachment of the muscle. Peroneus longus, in the plantar foot, may serve as the origin of flexor digiti quinti or the plantar interosseus muscles. The tendon of peroneus brevis may deviate to insert onto the dorsum of the fifth metatarsal or into the fourth dorsal interosseus muscle. It may also attach to the flexor digiti quinti. Its tendon may bifurcate, with one part also joining peroneus tertius.

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• Encyclopedia entry8 August 2026

  Pectoralis Major and Pectoralis Minor

  From the surface of pectoralis major, some superficial fibers arise from the fascia of the clavicular head and are inserted into the fascia of the deltoid muscle. M. Sternalis (in 3-5% of individuals) is a fleshy band of longitudinal fibers, often bilateral, of varying length and width, located close to and, generally, parallel to the sternum.

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• Encyclopedia entry8 August 2026

  Pectineus

  The pectineus may be divided into two parts, each of which receives its nerve supply from different sources: the medial part from the obturator and the lateral part from the femoral nerve. This division occurs normally in many mammals including some primates. Pectineus may be supplied entirely by an accessory obturator nerve. A common variation of this muscle is the union with adductor longus. In joining adductor longus it crosses over the deep (profundus) femoral artery and may compress it.

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• Encyclopedia entry8 August 2026

  Panniculus Carnosus

  The embryologic origin of these muscles probably accounts for the special degree of variability not only in extent and kinds of attachments but in the variety of separate aberrant slips that are occasionally present. In lower mammals, a great sheet of muscle arises from the early pectoral muscle mass or group and spreads beyond this group. This sheet of muscle, the panniculus carnosus, regresses in higher primates and humans, and its functional significance is diminished since the arm is so free that almost any portion of the body surface may be reached by the hand.

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• Encyclopedia entry8 August 2026

  Palmaris Longus

  This is one of the most variable muscles in the body. It is absent in 11.2% of bodies (all sources) but has been reported absent in only about 5 % of Japanese and 2.2 % of chinese. Its absence was, reportedly, recorded as early as 1559 by Colombus (De Re Anatomica Libri). In a splendid recent review however, on the absence of palmaris longus, Brinkman and Hage (2015) reported that Vesalius (De Fabrica) actually described this variation 16 years before Columbus.

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