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. The textbook description of this muscle is that it arises from the spine of the axis, and its fibers pass upward and laterally, to insert onto the lateral half of the inferior nuchal line. In 6.2%, only the right side has this form. In 5.3% only the left side conforms to the textbook description. Rectus capitis posterior major receives accessory slips from rectus capitis minor on both the right and left sides in 3 cadavers, (2.6%); on the right side in 2 cadavers, (1.7%); on the left side in 9 cadavers (8%), and the muscle on the left side received a slip from obliquus capitis inferior in 1 cadaver (0.9%).
In rectus capitis posterior minor, which arises from the posterior tubercle of the atlas, and its diverging fibers pass upward and laterally, to insert below the medial third of the inferior nuchal line, several variations occur. In Mori's 112 cadavers, only 48.2% conformed to the textbook description bilaterally: on the right side only 14.2%; and on the left side only 8.9%. The muscle was absent bilaterally in 1.7%, on the right side in 0.8% (1 cadaver), and on the left side in 2.4%.
The doubling of rectus capitis posterior minor occurred bilaterally in 15.1%, on the right side in 0.9%, and on the left side in 1%. Mori reported that when the muscle is doubled, the lateral fasiculus of the divided muscle is generally smaller. The lateral fasiculus passes laterally and crainially and joins the medial margin of rectus capitis posterior major or it may insert directly onto the inferior nucal line. The rectus capitis posterior minor may receive accessory slips from the axis or from the ligamentum (septum) nuchae in 6.6% (112 cadavers).
Accessory slips may be found as a continuation of spinalis cervicis. They may extend from the spines of one or two cervical vertebrae and ligamentum nuchae to the occipital bone. A small muscle has been seen passing from the spine of the axis to the mastoid process along the lateral border of the inferior oblique and lateral rectus, and inserting onto the mastoid process.
Atlantomastoid is a small muscle, often present, arising from the transverse process of the atlas, between the superior oblique and lateral rectus, and inserting into the posterior part of the mastoid process. According to Mori, the muscle is best seen when splenius capitis is removed and the medial part of longissimus capitis is examined.The muscle is separated fron obliquus capitis inferior by the occipital artery; occasionaly the occipital artery separates atlantomastoid from longissimus capitis. In 54 cadavers, Mori found this muscle on 19 sides (10 right, 9 left) or in 17.5 % of his subjects. Mori suggested that atlantomastoideus, obliquus capitis superior, and longissimus capitis arise from the same anlage based on their innervation by the dorsal division of C1. Atlantomastoideus may also have a connection with splenius capitis, a union more difficult to explain embryologically. The muscle has been seen in the gorilla (Sömmerring), chimpanzee (Gratiolet et Alix), and seminopithecus (Kohbrugge).
Syn.:
m. Rectus capitis posterior major s. rectus capitis posterior superficialis s. inferior,
m. Obliquus capitis inferioris s. obliquus capitis major,
m. Rectus capitis posterior minor s. rectus capitis posterior profundus,
m. Obliquus capitis superior s. obliquus capitis minimus.
Atlantomastoideus
Modified from Mori, 1964.
References
Anson, B.J., Ed. (1966) Morris' Human Anatomy, 12th ed. The Blakiston Division, McGraw-Hill Book Company. New York.
Gruber, W. (1876) Über den Musculus atlantico-mastoideus. Arch. Anat. Physiol. Wissen. Med. 1876:733-738.
Henle, J. (1871) Handbuch der Muskellehre des menschen, in Handbuch der systematischen Anatomie des Menschen. Verlag von Friedrich Vieweg und Sohn. Braunschweig.
Macalister, A. (1875) Observations on the muscular anomalies in the human anatomy. Third series with a catalogue of the principal muscular variations hitherto published. Trans. Roy. Irish Acad. Sci. 25:1-130.
Miyata, H. (1951) Some variations of the M. rectus occipitalis minor. Kanazawa Daigaku Igakubu Kaibogaku Kyoshitsu Gyosekishu 39:15-17. In Japanese.
Mori, M. (1964) Statistics on the musculature of the Japanese. Okajimas Folia Anatomica Japonica 40:195-300.
Schaefer, E.A., Symington, J. and T.H. Bryce, Eds. (1923) Quain's Elements of Anatomy, 11th ed. Longmans, Green and Co. London.