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Illustrated Encyclopedia of Human Anatomic Variation: Opus I: Muscular System: Alphabetical Listing of Muscles: P

Psoas Minor

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD

Peer Review Status: Internally Peer Reviewed


This muscle is not constant in humans. In a report of 182 subjects it was present on both sides in 70 subjects, on the right side in 12, on the left side in 8, and absent on both sides in 92. Various sources report that the muscle is present in about 56% of bodies. In one study of 2627 cadavers, bilateral agenesis occured in 54.5%. The muscle was absent in 49.9% of 722 body-halves in Asians, 57% of 4507 body-halves in Whites, and 66.6% of 674 body-halves in Blacks.

Mori found the muscle absent in 53.4% of 958 body-halves in Japanese and he cites Loth's (1931) summary of other groups of people as follows:

The data provided by Loth was prepared at a time when the majority of the people that were dissected, in each grouping, were (in general) geographically isolated from each other, so that there was little mixing of the gene pool. Inspite of this, the numbers are remarkably similar.

When present, the muscle varies considerably in its site of origin. It may be connected only with the first lumbar vertebra, or with the second lumbar and intervertebral disc (above it); it has been observed to arise from two heads, separate from each other, or to be divided throughout. At its insertion it may end on the iliac fascia, inguinal ligament, neck of the femur, or lesser trochanter with psoas major The tendon of insertion may bifurcate, with one terminating as usual and the second attaching to the synchondrosis between the fifth lumbar vertebra and the sacrum.

According to Macalister the following variations in psoas minor has been described:

  1. This muscle may be doubled, the second part lying deep to the first. This has been described by Winslow (1766), Kelch, Cruvelhier and Macalister. Theile saw a subdivision or splitting of its tendon with one part going to the fifth lumbar and the first sacral vertebrae, and the second going to the iliopectineal line. This was also seen by Macalister;
  2. Blandin saw this muscle replaced by a tendon;
  3. Macalister has seen its insertion lost in the pelvic fascia, or directly into the iliac fascia, through which, it was attached to the crural arch;
  4. This muscle is very often absent, being one of the five that are, of all muscles, the most frequently deficient (pyramidalis, psoas parvus, peroneus tertius, palmaris longus, and plantaris). When absent, Macalister has seen a expansion from the inner edge of psoas magnus (major) taking its place;
  5. John Bell (1793) reports the muscle is more frequently absent in males than females;
  6. Moser saw it present on the left and absent on the right. Macalister indicates that he can find no rule for its presence or absence, and does not believe that sex has anything to do with it. Macalister says that the muscle is especilaly developed in leaping animals such as the kangaroo, jerboa, macrocelides.

Syn.: Psoas parvus.

Image 131

Psoas Minor, 1.
from Wagstaffe and Reid, 1875


References

(1861) D'anomalie musculaire de la cuisse. Bulletins et Mem. de la Société Anatomique de Paris XXVI(2):42.

Anson, B.J.,Ed. (1966) Morris' Human Anatomy,12th ed., The Blakiston Division, McGraw-Hill Book Company. New York.

Henle, J. (1871) Handbuch der Muskellehre des Menschen, in Handbuch der systematischen Anatomie des Menschen. Verlag von Friedrich Vieweg und Sohn, Braunschweig.

LeDouble, A.F. (1897) Traité des Variations du Système Musculaire de l'Homme et Leur Signification au Point de Vue de l'Anthropologie Zoologique. Libraire C. Reinwald, Schleicher Freres, Paris.

Macalister, A. (1875) Observations on muscular anomalies in the human anatomy. Third series with a catalogue on the principal muscular variations hitherto published. Trans. Roy. Irish Acad. Sci. 25:1-130.

Mori, M. (1964) Statistics on the Musculature of the Japanese. Okajimas' Fol. Anat. Jap. 40:195-300.

Seib, G.A. (1934) Incidence of the m. psoas minor in man. Am. J. Phys. Anthropol. 19:229-246.

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