Inca leg

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Inca leg in the X-ray
Skull from above (ventral: upper image area; dorsal: lower image area). Inca leg in the lower part of the picture (difficult to see). More accessory bones in the upper part of the picture.

The Inca leg or the Inca bone ( lat. INCAE Os or Os interparietal ; Engl. Inca bone or incarial bone ) is on the skull of the human an accessory (additional) bone (lat. In the lambdoid suture lamdoidea ), the connection between the vertex ( Os parietale ) and occiput ( os occipitale ). This excess bone represents an anatomical variation ( switching bone ) that is not clinically relevant. It can be mentioned as a secondary finding in the radiological assessment of skull images. In many other vertebrates, the interparietal bone occurs regularly.

Because of a persistent sutura mendosa, a single piece of bone remains in the occipital scale. The shape of the Inca leg is mostly triangular, but can vary widely and can also be rectangular, rhombus-shaped or M-shaped. The Inca leg can very rarely be divided into two or three parts or contain even more parts (1 = Os incae totum ; 2 = Os incae bipartitum ; 3 = Os incae tripartitum ), as there are sometimes further one or more longitudinal sutures or an additional transverse suture. It can encompass the entire thickness of the skull bone, or it can only be on the outside or inside of the skull bone.

The Inca leg was made by surgeon P.F. Bellamy was first mentioned in the analysis of the skulls of two Peruvian child mummies ( A brief Account of two Peruvian Mummies in the Museum of the Devon and Cornwall Natural History Society ) and was first described by Johann Jacob von Tschudi in 1851. Le Double described this bone as the interparietal bone in 1903 . The Inca leg was also found in fossil hominini (including Australopithecus and Homo erectus ) and in the early Homo sapiens .

anatomy

The Inca leg has its embryonic origin in the occiput, but is separated from it by an additional bone suture, since an additional ossification center has arisen in the occiput. The inca leg anatomically represents the front part of the occipital scale, which usually grows together early on with the rear part of the occipital scale. In Peruvian mummies the Inca leg was also found separately. This can be explained as a fetal cross suture in the occiput remaining open, which leads to the separation of this part of the rear skull.

The Inca leg is bounded by the occiput, namely by its occipital scales (Latin Squama occipitalis ), as well as by both parietal bones. The suture between the Inca leg and occiput corresponds to the sutura mendosa of the fetus . This transverse suture between the Inca leg and occiput lies at the level of the uppermost Linea nuchea and is then called the Sutura occipitalis transversa - in a sense the right and left Sutura mendosa , which are fused in the midline.

In reptiles and many mammals, the parietal bone ( os interparietale ) occurs regularly.

embryology

In the developmental phase of the occipital scaly ( lat.Squama occipitalis ) of the occiput, a distinction is made between:

  • Upper scale (caused by the desmal ossification of a membrane)
  • Lower scale (caused by chondral ossification of cartilage)

The sutura mendosa runs between the upper and lower scales . It ossifies in the 3rd month of life and is then visible as the Linea nuchae superior (upper line) on the bone.

The upper part of the occipital scale of the occiput develops from a membrane, while the rest of the occiput develops from cartilage. A single ossification center comes into action within the membrane that is to become the upper part of the occipital scale . Usually the upper part of the occipital scale fuses with the originally cartilaginous lower part of the occipital scale. If the fusion does not take place, the Inca leg is formed.

The upper part of the interparietal bone forms the upper scale of the occipital bone . In the developmental phase of the bony skull, the interparietal bone is a paired core of the cervical bone that is based on connective tissue and forms the upper part of the occipital scale. If they do not merge, the Inca leg is created.

The occiput develops from 5 or 6 centers of ossification.

The small fontanel in newborns lies in the same area as the Inca leg, at the point of contact between the parietal bones and the occiput. In some newborns, the fontanelles can be wide open and covered by additional, free-floating bones. These bones are unimportant in themselves, but can appear along with other, more important anomalies.

frequency

The Inca leg, which occurs in 20% of the ancient Peruvian skulls, is a genetic peculiarity that was first discovered among the Incas , as it occurs more frequently among them. It was later found in abundance among Japanese , Koreans, and Tibetans .

The Inca leg occurs more frequently in the American population, but rarely in Northeast Asia and Australia. The populations in Tibet , Nepal , Assam, and Sikkim (northeast India) are more likely to have Inca legs than the neighboring populations. In Europe as well as in Central and West Asia, Inca legs are very rare, but more common in Sub-Saharan Africa . Because of this observed frequency distribution, a causal genetic factor for the incabein cannot be excluded.

Depending on the population, the Inca leg can be found in 3 to 36% of the population.

  • London: 8%
  • Burma: men 19%, women 8%
  • Mexico: men 36%, women 28%

The Inca leg represents an anatomical variety, as well as suture bones, the sacralization of a lumbar vertebra or tail vertebra or tooth anomalies. The Inca bone is not a malformation.

Further anatomical anomalies on the skull, which are mostly to be interpreted as setbacks on animal ancestors of humans, are:

Earlier ethnologists listed the Inca leg as a racial characteristic in the racial classification of humans ( racial theories ), as did the Mongolian fold . However, recent studies on the worldwide frequency distribution of the Inca leg refute the fact that the Inca leg is an identification feature of the "Inca race". The interpretation of a mummy find in Norway (2007, St. Nikolas Church in Sarpsborg , Østfold province , report in the Aftenposten ; and a report in The Norway Post ) is also wrong. A 1000 year old mummy was found there and “clearly identified as an Inca Indian because the Inca leg was present, since the Inca bone is only known to members of the Inca”.

causes

Togersen found in his 1951 study that the Inca bone is inherited dominantly and has a penetrance of 50%. On the other hand, artificial skull deformation has also been discussed as a cause (Ossenberg, 1970; Lahr, 1996), since Inca legs are often found in deformed brain skulls. The ancient Peruvian skulls were also often artificially deformed.

literature

Web links

Individual evidence

  1. ^ PF Bellamy: A brief Account of two Peruvian Mummies in the Museum of the Devon and Cornwall Natural History Society. In: The Annals and Magazine of Natural History. Volume 10, No. 63, 1842, pp. 95-100.
  2. ↑ Quoted from: B. Oetteking (1930): The Jesup North Pacific Expedition XI, Craniology of the North Pacific Coast. New York: GE Stechert.
  3. ^ AF Le Double (1903): Traité des variations des os du crâne de l'homme et leur signification au point de vue de l'anthropologie zoologique. Paris: Vigot.
  4. ↑ a b Tsunehiko Hanihara and Hajime Ishida: Os Incae: Variation in frequency in major human population groups. In: Journal of Anatomy. Volume 198, No. 2, 2001, pp. 137-152, doi: 10.1046 / j.1469-7580.2001.19820137.x
  5. ^ Robert Shapiro & Franklin Robinson: Embryogenesis of the human occipital bone In: American Journal of Roentgenology 126, 1976, pp. 1063-1068.
  6. ↑ Meyer's Large Conversation Lexicon: Skull of Man. Description and measurement. 1905-1909, 6th edition
  7. ^ The Norway Post: Archaeological sensation in Oestfold. June 26, 2007
  8. ↑ Torgersen JH (1951): Hereditary factors in the sutural pattern of the skull. Acta Radiologica 36, ​​374-382.
  9. ↑ Ossenberg NS (1970): The influence of artificial cranial deformation on discontinuous morphological traits. American Journal of Physical Anthropology 33, 375-372.
  10. ^ Lahr MM (1996): The Evolution of Modern Human Diversity. A Study of Cranial Variation. Cambridge: Cambridge University Press.
  11. ^ El-Najjar MY, Dawson GL: The effect of artificial cranial deformation on the incidence of wormian bones in the lambdoidal suture. Am J Phys Anthropol 1977; 46: 155-160. Medline