854 published articles
This muscle is rarely absent (about 2% of individuals), but varies greatly in its size and strength and in its length and direction. It is sometimes found joining flexor digiti minimi brevis. It may insert onto the pisiform bone. The palmaris brevis may be doubled and may simulate a soft-tissue tumor of the hand. Syn.: m. palmaris cutaneus, cavo quadrata manus, Peaucier de la main (Cruveilhier).
These muscles vary in their differentiation and extent and may be fused with other muscles in the region. The two orbicularis oculi muscles may be united by a transverse glabellae muscle. An upward extension of the platysma has been seen to reach the lower eyelid. The palpebral and orbital portions of these muscles may be completely separate. The orbital portion of orbicularis oculi may be absent, and a rudimentary palpebral portion of the same muscle has been reported. A musculus laryrmalis, associated with the medial side of orbicularis oculi, has been described.
These muscles vary considerably in the extent of their development. They may be more or less continuous. The sternal attachment of the sternohyoid is absent more frequently than the clavicular attachment. A strap-like muscle, sternohyoideus azygous, may be found in the midline of the neck from the posterior surface of the manubrium to the hyoid bone.
The occipital belly of this muscle may be absent, and the muscles of the two sides may be fused medially. The frontal belly is rarely is rarely absent and may supply slips to the arch of the frontal bone, maxilla, or nasal bones. Fibers of the frontal part may interdigitate across the midline. Syn.: m. epicranius, cranii cutaneus (Meckel), Schädelmuskel, Oberschädelmuskel, peaucier du crâne (Cruveilhier).
This small muscle is usually absent. It is located between the occipital protuberances and the posterior auricular region, and is usually fused with auricularis posterior It may be found deep or superficial to trapezius. It is also known as the occipitalis transverse muscle. Reported frequencies are remarkable. Occipitalis minor is always "present" in Malays, in 56% of "blacks", 50% of Japanese, 36% of Europeans, and is "nonexistent" in Hottentots and Melanesians.
The external obturator shows little variation.Occasionally, a part of the muscle may be isolated above the point of exit of the obturator nerve. This becomes a potential nerve entrapment site. The muscle may be divided into three parts and this subdivision is considered to be the normal arrangement. Obturator externus may be connected to adductor brevis.
Reported variations are few. LeDouble referred to a case reported by Colombo but he questioned its veracity. Whitnall reported a doubled muscle and Morgagni found a fusion with the muscles trochlea. Clinicians report the complete absence of the muscle and LeDouble cited accessory slips. The superior oblique may be closely accompanied by an offshoot from the levator palpebrae superioris, the comes obliqui superioris (Albinus) or gracillimus orbitis (Henle).
A fibrous band or inscription frequently interrupts the internal oblique and the intercostal muscles. In rare instances a similar inscription has been seen opposite the twelfth or even the eight rib. They apparently represent tendinous intersections between the internal oblique and the internal intercostal muscles. An additional slip occasionally arises from the eight costal cartilage. It has been reported that the internal oblique inserts onto the eleventh rib in 1% of cases, the tenth in 66.5%, the ninth in 31%, and the eighth rib in 1.5% of cases.
Variations in extraocular muscles in man have been reported by various authors cited below. An unusual muscle bundle from the inferior oblique (obliquus accessorius inferior [Rex]) has been found passing from the apex of the orbit to join the inferior rectus. The inferior oblique may be doubled or absent.
The external oblique varies in the number of its attachments to the ribs. Absence of the highest and lowest digitation is common. On the other hand, one or more slips may be doubled, most frequently those from the eight or ninth ribs, or an additional slip may arise from the lumbar fascia below the last rib. In addition to the connection with the pectoralis major, there may be union, frequently by tendinous intersection or more rarely by direct continuity, between the slips of origin of the external oblique and serratus anterior muscles.
Fibers from nasalis may pass upward on the nasal bones and enter into the formation of procerus. Frequently, the pars alaris and pars transversa are recognized as distinct muscles, depressor alae nasi (myrtiformis [Casserio]), dilatator pinnae [Santorini] and compressor narium [H. Meyer] , respectively. Thin muscular slips on the outer margin of the nostrils are sometimes described as distinct muscles, dilatores naris anterior and posterior (Theile) or levatores alae nasi posterior and anterior (Arnold).
The muscles of mastication are derivatives of a single muscle mass; indications of their common origin can be seen in partial unions of the various muscles. Thus, fibers from the posterior portion of the deeper head of the masseter may join the temporal, fibers from both the temporal and masseter sometimes pass to the anterior border of the fibrocartilage of the mandibular articulation (temporomandibular joint), and the temporal and lateral pterygoid have been observed in some cases to be connected. The temporal muscle may have a more extensive cranial origin than usual.
One or more of the lunbrical muscles has been reported absent; absence of the first lumbrical, however, is very rare. Doubling of the second, third, and fourth has also been reported. Various attachments have been noted: the second, third, or fourth lumbrical may arise from the short instead of the long flexor tendon, and the first may arise from the tendon of tibialis posterior. A frequent observation is the contribution of fleshy or tendinous fibers to the lumbricals from flexor digitorum longus and flexor hallucis longus.
The limbricals show frequent variations (20% of individuals). Wood reported that he found 19 variations in 102 individuals (or 5.3%). They may be reduced to three or even two, and in rare instances all four are absent. Rarely, the number may be increased by the development of accessory slips. In the case of the first lumbrical, an accessory slip may arise from the tendon of flexor pollicis longus, the tendon of fllexor digitorum superficialis, the first metacarpal, opponens pollicis, or the palmar carpal ligament.
The most significant and common variations in prevertebral muscles concern the number of vertebrae to which the tendons of origin and insertion are attached. In addition, there may be fusion with the anterior scalene. In about 4% of cases, a muscle (atlantobasilaris internus [Gruber] or axiobasilaris) extends along the medial border of longus capitis from the anterior tubercle of the atlas to the base of the skull. Another muscle, epistropheobasilaris , is similar but arises from the second cervical vertebra.
When the tendinous slips attached to the lumbar spines are cut, the deep medial surface of the thoracolumbar part of longissimus is exposed, revealing a variable number of slender accessory slips of origin. These come from the transverse processes of the lower thoracic vertebrae, mamillary processes of the lumbar vertebrae, or aponeurosis of the underlying transversospinalis. These may unite to form an independent muscle, transversalis dorsi or thoracis , with a separate insertion onto the upper two or three thoracic vertebrae.
This muscle is composed of two parts that may vary independently of each other. Krause and Wood have classified these two as follows: A ventral fasciculus. This slip arises from the cervical vertebrae and runs downward and inserts on the ventral surface of subscapularis or serratus posterior superior. Other variations have been described : Macalister has found the following variations for the levator scapulae: Syn.: m. levator anguli scapulae, patientiae, Schulterheber, angulaire.
Absence of the levator palpebrae superioris has been noted. When the muscle is present, a slip (tensor trochleae [Budge]) may arise from its belly or proximal part and may pass to the sclera, trochlea of the superior oblique, or neighboring tissues. Another slip (transversus orbitis [Bochdalek]) may be found between the medial and lateral walls off the orbita and connects with the levator in the midline.The reported frequency of these muscles (15 of 20 cases, 75%) is probably much too high as the rarity of these muscles has also been reported (Isomura, about 3%, 5 in 85 individuals).
The muscle is divided into three parts: iliococcygeus, pubococcygeus and puborectalis. Because the muscle shows great individual variation in structure, the muscle is described differently by different authors and these descriptions may differ substantially from a dissected specimen.
Variations in the extent of this muscle and its attachments are considerable. The muscle may arise only from the ribs rather from the vertebral column, ribs, scapula, and the crest of the ilium as is most common. The costal origin may be reduced to rib 12. Mori has reported the vertebral, costal, and scapular origins for latissimus dorsi from 50 Japanese cadavers, 100 halves, as follows: Vertebral Origin Number of Sides Upper Limit Right Left Total % T.
