Variation archive

The articles published on the portal, most recent first.

854 published articles

Encyclopedia entry8 August 2026
Supracostalis (Wood)
Slips are occasionally found that extend from the ventral end of the first rib to the fourth. Other slips may terminate on the second or third rib. M. supracostalis has also been found attached to the deep fascia of the neck or to the scalene muscles. Supracostalis anterior is found on the anterior surface of the thoracic wall. It usually passes from the anterior end of the first rib downward to the third or fourth rib. Mori reported 126 supracostalis anterior muscles in 1200 cadavers (2400 sides). The form of the muscle is simple but the origin and insertion are variable.
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Encyclopedia entry8 August 2026
Supinator
The division of the muscle into two lamellae may be more pronounced than is normally the case. An accessory fasciculus from the lateral epicondyle has been observed, as well as fibers inserted into the tendon of insertion of the biceps, the bursa under the tendon, and the tuberosity of the radius. Spinner (1968) has pointed out that the most proximal part of the superficial head of the supinator muscle may be tendinous and form a fibrous arch and is implicated in the paralysis of the posterior interosseous nerve.
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Encyclopedia entry8 August 2026
Subscapularis
The most important variation is an additional muscle, subscapularis minor or s. secundus, which arises from the upper part of the axillary border of the scapula and is inserted into the capsule of the joint, onto the crest of the lesser tubercle of the humerus, or somewhat below the lesser tubercle. It was found by Gruber in 10 of 200 limbs. A rarer variation is a slip arising from the subscapular tendon and passing to the axillary fascia and skin at the base of the axilla.
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Encyclopedia entry8 August 2026
Suboccipitales (Obliquus Capitis Inferior, Obliquus Capitis Superior, Rectus Capitis Posterior Major, Rectus Capitis Posterior Minor
Division or doubling of the rectus muscles has been reported as frequent, although doubling of the oblique muscles is rare. Divison of the muscles into three parts is extremely rare. In Mori's 112 cadavers, doubling occurred on both sides in 2.6%; on the right side in ~1%; on the left side in 1.7%. According to Mori, rectus capitis posterior major conforms to the textbook description, blaterally in 76.5% of his 112 cadavers.
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Encyclopedia entry8 August 2026
Subclavius
This little muscle is regarded as a derivative of the deep lamella of the pectoral sheet. Its insertion may spread from the clavicle to the coracoclavicular ligament , coracoid process, transverse ligament of the scapula, or the superior of the scapula and the humerus. It has been described as being replaced by a ligament and by pectoralis minimus. It may also be doubled. The subclavius may arise from the second rib and insert on the coracoclavicular ligament.
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Encyclopedia entry8 August 2026
Stylopharyngeus
This muscle is frequently divided into two or three slips. Supernumerary muscles of the pharynx are common. They frequently originate from the petrous portion of the temporal bone in front of the carotid canal, the vaginal process of the temporal spine (petropharyngeus), the basilar process (occipitopharyngeus), and, very rarely, the mastoid process (mastoidopharyngeus). Occasionally a small median strip of muscle arises form the pharyngeal tubercle of the occipital bone and inserts on the raphe or posterior wall of the pharynx (azygopharyngis).
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Encyclopedia entry8 August 2026
Stylohyoideus
A close relationship exists between the stylohyoid and the posterior belly of the digastric; one or the other occasionally fails to separate from a common muscle mass. These two muscles develop from the deep aspect of the premuscle mass located at the base of the hyoid arch. Stylohyoideus arises from the lateral and dorsal part of the styloid process (processus styloideus ossis temporalis) by a round tendon, and attaches to the ventral surface of the body of the hyoid bone near its junction with the greater cornu.
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Encyclopedia entry8 August 2026
Styloglossus
The styloglossus may be doubled or it may be absent on one or both sides. In the latter two cases, it may be replaced by myloglossus (Wood) s. mandibuloglossus, which arises from the inner surface of the angle of the mandible or from the stylomandibular ligament and is inserted into the sides of the tongue. In some cases the whole muscle arises form one of these locations, or from the surface of the medial pterygoid. This muscle is usually in the form of small bundles of fibers. The stylohyoid ligament may pass in front of or behind the digastric muscle. The styloauricularis (Hyrtl) s.
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Encyclopedia entry8 August 2026
Sternocleidomastoideus
Comparative anatomical studies have concluded the the sternocleidomastoid muscle is composed of five parts arranged in two layers. The amount of fusion of the two heads of this muscle varies considerably. They are frequently separated into cleidomastoid and sternomastoid parts; this has been regarded as normal by some authors.
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Encyclopedia entry8 August 2026
Stapedius
Middle ear variations have a reported incidence of about 5.6% (28/500 temporal bones). This muscle may be doubled or absent., or its tendon may fail to develop. Ectopic muscle rests were found in 19/500 bones near the facial nerve and are believed to be derived from mesenchyme originating from the hyostapedial ligament. Syn.: m. Pyramido-stapéal, Steigbügelmuskel.
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Encyclopedia entry8 August 2026
Splenius
The absence of this muscle has been reported at least twice (Testut and LeDouble). Its origins may be displaced proximally by one or two vertebrae, and frequently the origins do not correspond on both sides. The spinal origin may extend to the cranial end of ligamentum nuchae. The muscle is sometimes separated into two parts, the superior part overlapping the inferior at its origin. Slips joining longissimus capitis, iliocostalis, or levator scapulae are sometimes found. Splenius cervicis may send a slip to the levator scapulae (Wood).
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Encyclopedia entry8 August 2026
Sphincter Ani Externus
This muscle shows considerable individual variations in structure Peikert, Platzek, Besséde and May studied the relationship between the bulbospongiosus muscle and the external anal sphincter. They reported that the perineum contains several fascial layers composed of elastic and collagen fiber including bundles of smooth muscle fibers. The bulbospongiosus was found subdivided into a ventral and dorsal portion, which developed into four variants.
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Encyclopedia entry8 August 2026
Serratus Posterior Inferior and Serratus Posterior Superior
There is considerable variation in the number of digitations of these muscles. The number of slips of the serratus superior may be reduced to five or six slips. Mori (1964) describes the vertebral origin of serratus posterior superior, in his cases, as follows: Cervical Thoracic Right side Left side Total % IV - VII 2 2 4 4 V - VII 5 7 12 12 VI I 18 15 33 33 V I 20 21 41 41 VI I 2 1 3 3 IV II 1 2 2 2 V II 2 2 4 4 Upper Limit Cervical Right side Left side Total % IV 24 18 42 42 V 27 30 57 57 VI 2 1 3 3 Lower Limit Cervical/ Thoracic Right side Left side Total % VII 7 9 16 16 I 40 37 77 77 II 3…
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Encyclopedia entry8 August 2026
Serratus Anterior
This muscle may receive a slip from the tenth rib. Likewise, the highest digitation often has no attachment to the first rib, or the lower slips may be absent and the muscle may not be found below the seventh rib. In one study it was found that the number of digitations varied between seven and 12. The last digitation stopped at the seventh rib in 1%, eighth in 40%, ninth in 38%, tenth in 10%, and the eleventh rib in 0.5% of cases. The first digitation attached to the first rib in 75%, or to the fascia of the first rib in 21% of cases.
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Encyclopedia entry8 August 2026
Semitendinosus
Complete separation of semitendinosus from biceps femoris has been noted and this is of interest because the two muscles are independent in certain groups of mammals. Semitendinosus may be partially fused with semimembranosus and cases have been reported of muscular slips joining biceps femoris and semitendinosus. Accessory slips have been observed arising from the coccyx, sacrotuberous ligament, or ischial tuberosity and joining the belly of semitendinosus. In 39% of 62 limbs, the semitendinosus took origin from the sacrotuberous ligament.
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Encyclopedia entry8 August 2026
Semispinalis Capitis (Arnold)
The slips of attachment vary considerably in number. The muscle may receive accessory slips from longissimus or spinalis (dorsi s. thoracis). A rare variation is the presence of a second lamella under the medial portion of the muscle, extending from one or more of the upper thoracic transverse processes to the occipital bone.
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Encyclopedia entry8 August 2026
Semimembranosus
Reduction and complete absence of the semimembranosus have been recorded. It may be doubled or split along its length. The scond belly may have the form of a small fleshy slip with proximal and distal tendons, or it may be inserted in various ways. In one case, it was attached by one slip to the main tendon, with a larger slip to the oblique popliteal ligament and a delicate reflected slip inserted onto the medial supracondylar ridge. Sometimes the tendon joins that of the adductor magnus.
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Encyclopedia entry8 August 2026
Scalenus Anterior, Scalenus Medius Scalenus Posterior
The scalenes vary in the number of their costal and vertebral attachments, as well as in the interrelationships caused by the fusion of numerous fasciculi. The problem of definition has been compounded by the variety of descriptions of various muscles and their relation to the scalene mass by various authors. Frequently (about 30% of cases) a poorly defined muscle mass is present. The anterior scalene is sometimes located behind the subclavian artery or it may be split into two parts for the arery to pass through between them.
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Encyclopedia entry8 August 2026
Sartorius
In rare cases, this muscle may be partially divided distally or may even be completely divided longitudinally into two parts, the accessory portion being inserted into the fascia lata, the femur above or near the medial condyle, the patellar ligament the tendon of semitendinosus, or the tendon of its twin. Separate or accessory heads of origin occasionally arise from the notch below the anterior superior spine, the iliopectineal line, the pubic bone close to the symphysis, or the inguinal ligament.
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Encyclopedia entry8 August 2026
Risorius (Santorini/Theile)
This muscle is frequently absent or may be represented by some scattered muscular bands. Its intimate association with depressor anguli oris suggests derivation from that muscle. The muscle varies in the extent of its distribution and has been found arising from the fascia over the sterno- mastoid, muscle, or the mastoid process, the outer ear or zygomatic process. It has been found doubled and tripled.
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