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Minor variations in the ossicles, foramina and ridges of the cranium have aroused the curiosity of anatomists for many decades. These non-metric variants help us to study the genetic relationships among ancient populations. Since these traits show considerable frequency differences in different populations, they can be used as anthropological characters in epidemiological studies. These variants indirectly reflect the part of underlying genotype of a given population thus implying their usefulness in biological comparisons of related groups. They can be used for the assessment of the existence of the parental structures within a community or as taxonomic indicators. For anthropological studies, the traits should be genetically determined, vary in frequency between different populations and should not show age, sex, and side dependency. The present study was conducted on hundred dry adult human skulls from Northern India. They were sexed and studied for the presence of hyperostotic traits (double hypoglossal canal, jugular foramen bridging, and paracondylar process). Sexual and side dimorphism was observed. None of the traits had shown statistically significant side and sexual dimorphism. Since the dimorphism is exhibited by none of them, it can be postulated that these traits are predominantly under genetic control and can be effectively used for population studies.
Anthropological and paleoanthropological studies of epigenetic cranial traits or non-metrical cranial traits have increased in the last decade. Assessment of discontinuous traits is popular in anthropological studies due to their apparent ease of scoring and quick analysis. Such traits have been used for the investigation of population history and the measurement of biological distances between different populations [
The non-metric traits of the skull include hypostatic variants, hyperostotic variants, supernumerary ossicles and variants related to the foramina of nerves and vessels. This work is concerned with hyperostotic variants-double hypoglossal canal, jugular foramen bridging and paracondylar process-their side and sexual dimorphism and their use in anthropological studies. These traits did not show any sexual or side dimorphism (
The study was conducted in the Department of Anatomy, Lady Hardinge Medical College, Delhi on 100 dry adult human skulls (57 males, 33 females, and 10 uncertain) with no deformities or fractures. They were sexed and studied for the presence or absence of hyperostotic traits: double hypoglossal canal (
1, 2, 3 = Trough shaped profile suggestive of male 5, 6, 7 = Angulation of the inner aspect suggestive of female 1 = Definite male 4 = Uncertain 7 = Definite female
Seventy percent accuracy of sex identification using this method was achieved.
Each trait was noted for its presence or absence in the skulls and chi-square testing assessed side and sex dimorphism. After sexing, the skulls were studied for hyperostotic variants: double hypoglossal canal, jugular foramen bridging, and paracondylar process. The variants were scored and if they occurred only on one side the dimorphism was also noted. The traits' occurrences were coupled with the sex information to show side and sexual dimorphisms, which were assessed by chi-square testing.
Double hypoglossal canal occurred in 3.5% of the right and 7.0% if the left sides of the male skulls; it was bilaterally present in 5.3%. In the female skulls, the incidence was 6.1% on the right, 6.1% on the left, and 3% bilaterally. In the skulls of uncertain sex, its occurrence was 10% on the right side and 10% bilaterally (
The incidence of double hypoglossal canal was 10.5% in males and 9.1% in females (
Though these traits showed sexual and side dimorphism it was never statistically significant (
Three hyperostotic variants-double hypoglossal canal, jugular foramen bridging and paracondylar process-were assessed for side and sexual dimorphism.
Anterior to the occipital condyle is a hypoglossal canal medial to and below the lower border of the jugular foramen at the junction of the basilar and the lateral parts of the occipital bone. The canal is directed laterally and forwards from the posterior cranial fossa. It may be divided partly or wholly by a spicule of bone and transmits the hypoglossal nerve and meningeal branch of the ascending pharyngeal artery, an emissary vessel connecting the intracranial basilar plexus with the extracranial internal jugular vein [
The bridging termed ponticuli foraminis jugularis is established by the contact of the intrajugular process of the temporal bone situated posterior to the triangular depression with the bony process of the occipital bone projecting either from just above the hypoglossal canal (type I, anterior type) or from a site posterior to the hypoglossal canal (type II, posterior type) [
A bony exostosis of the cranium base projects from the jugular process (lateral to the occipital condyle) towards the transverse process of the atlas. It may vary from a small tubercle to a large elongated process articulating with the transverse process of the first cervical vertebra. The paracondylar process can rarely be fused with the transverse process of the atlas, functionally limiting neck movement. This may present clinical symptoms due to altered posture and restricted motion. A tubercle in this position has been shown to be a constant feature of 21-82 mm embryos. Once established as a morphological character of the individual, a paracondylar process behaves like a hyperostotic trait (
These traits are characterized by excess ossification over the anomalous conditions, i.e., ossification of cartilage, ligament, duramater or bone. They might be reminiscent of the bony spurs and osteophytes that form in the skeletons of older people and are related to degenerative changes in such as arthritis. This suggests that they achieve expression at different times during postnatal development [
The hyperostotic traits can be regarded as non-metric anthropological markers to measure genetic distinctiveness or divergence between populations. These variants are either clearly present or absent, easily defined and standardized. Hence their scoring and analysis is quick and easy. For population studies, the trait should be genetically determined and vary in frequency between populations; it should not show age, sex or side dependence. Variants' usefulness is markedly reduced if environmental factors significantly affect their expression. In humans, it is difficult to demonstrate the degree of genetic control. However, since the human variants are morphologically analogous to those in mice, the genetic background of non-metric traits in humans can be strongly supported based on the study of mice [
There is no concencus regarding these traits' environmental and genetic predisposition. They have been reported not to show sexual or side dimorphism [
While several differences, both sexual and side, do occur in human crania, there is little consistency in the occurrence of these dimorphisms. The incidence of a variant may be observed more in females in one sample and more in males of another sample. Such lack of consistency suggests that the variants have genetic, epigenetic and also environmental origins and are well removed from the primary site of gene action [
It can thus be concluded that hyperostotic variants are predominantly under genetic control and can be used effectively in population studies despite being influenced by environmental factors and exhibiting dimorphism. However, the dimorphism is not statistically significant (
The hypoglossal canal passing as two roots into the occipital bone and emerging as one trunk may be of clinical importance. The nerve roots might get trapped in the occipital bone on one or both sides during ossification, resulting in slight alterations of tongue movement. While not sufficient to casue any major problems, these differences are sufficient to affect speech. There are no reported clinical studies of the entrapment of the hypoglossal nerve in the occipital bone. It is worth considering this variation in elderly patients with minor difficulties in movements of the tongue or speech.
Jugular foramen bridging can press upon the cranial nerves causing nerve palsies (Vernet syndrome) characterized by loss of taste at the posterior third of the tongue (cranial nerve ninth), vocal cord paralysis and dysphasia (cranial nerve X), weakness of sternocleidomastoid and trapezius (cranial nerve XI) [
A large paracondylar process may cause vascular insufficiency symptoms due to compression of the vertebral arteries. The third atlanto-occipital joint, if present, may disrupt the biomechanics between the occiput and the atlas resulting in mechanical pain and increased rigidity. The condition may also be associated with an osseous torticolis [
Hypoglossal canal divided into two on the right side.
Jugular foramen bridged completely on the left side.
Paracondylar process projecting towards the atlas vertebra on the left side.
Side dimorphism of hyperostotic variants. M, male; F, female; U, unilateral; B, bilateral; RT, right; LT, left; DHC, double hypoglossal canal; JFB, jugular foramen bridging; PP, paracondylar process.
Sexual dimorphism of hyperostotic variants. DHC, double hypoglossal canal; JFB, jugular foramen bridging; PP, paracondylar process.
Incidence of hyperostotic variants
Values are presented as number (%).
Sexual dimorphism of the hyperostotic variants
Values are presented as number (%).
Statistical significance of side dimorphism in males and females
Values are presented as number (%). Incidence recorded on 2n basis *
Statistical significance of sexual dimorphism
Values are presented as number (%).