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Skeletal Series Part 3: The Human Skull

22 Apr

In this post I will be discussing the basics of the human skull; its anatomical features, number of elements, terminology, key functions and how to handle a skull.  Alongside the earlier blog on variations in human skeleton and the ethics that should be considered, this should prepare the user for interaction and identification of physical remains.

A skull in situ. From the Gadot archaeological site in Israel.

Individual elements found in the human skull, individual elements discussed below (Pearson Education 2000).

 The human skull is one of the most complex structures in the human skeleton.  It houses the foundations for the sense of smell, sight, taste & hearing, alongside the housing of the brain.  It also provides the framework for the first processes of digestion by mastication of food with the use of the teeth anchored in mandible and maxilla bones (White & Folkens 2005: 75).  White & Folkens (2005) go on to note that it is of value that the key anatomical landmarks of the skull are noted.  These include the Orbits of the eye sockets, the Anterior Nasal Aperture (nose hole), External Auditory Meati (ear canals), the Zygomatic Arches (cheek bones) along with the Foramen Magnum (base of the skull).  It is by these landmarks that we can orientate the skeletal elements if they are disarticulated or have been broken (White & Folkens 2005: 75).

Excavation

Particular care should be taken when excavating the skull, or any human skeletal element.  Careful consideration should be made of its location, burial type, any nearby skeletons, and of course any different stratigraphic (colour/cut/fill) features present should be noted (Mays 1999).  As this is the only chance to lift the skeleton since deposition, careful notes should be made on first impression and any post depositional changes that can be immediately identified.  Careful sieving of the soil matrix around the skull should take place, to help retain any small fragments of bone or lose teeth (whole and partial fragments) (Mays 1999).  Differential preservation, dependent on deposition & burial environment conditions, will mean that it is likely sections of the skull will not survive.  These are often the small, delicate bones located inside the cranial-facial portion of the skull.  The likeliest to survive portions are the mandible and the cranial plate elements because of their tough biological nature.

Handling

When handling the skull it should be noted of the above major landmarks.  For example, you will not damage the skull whilst carefully holding it in both your hands but if you hold it by the orbits you are liable to damage the surrounding bone.  The foramen magnum is usually stable and strong it to withstand creeping fingers as a hold place.  Whilst studying the skull on a desk, a padded surface should be provided for it to rest upon.  Care should be taken when handling the mandible, and temptation should be resisted in testing the mechanical properties of the surrounding bone (Mays 1999).

Anatomical Planes

For use between comparative material, it is useful to use a standardized set of viewing planes.  The human skull is often viewed via the Frankfurt Horizontal (White & Folkens 2005).  The FH is a plane of three osteometric points conceived in 1884 (see above link).  The skull is normally viewed from six standard perspectives.  These include norma verticalis (viewed from above), norma lateralis (viewed from either side), norma occipitalis (viewed from behind), norma basilaris (viewed from underneath) and norma frontalis (viewed from the front). Thus, when considered with osteometric points, measurements can be taken and compared and contrasted (White & Folkens 2005: 86).

Cranial Terminology and Elements

  1. The Skull refers to the entire framework including the lower jaw.
  2. The Mandible is the lower jaw.
  3. The Cranium is the skull without the mandible.
  4. The Calvaria is the cranium without the face portion.
  5. The Calotte is the calvaria without the base of the skull.
  6. The Splanchnocranium is the facial skeleton.
  7. The Neurocranium is the braincase.

The skull in infants is made up of 45 separate elements but as an adult it is normally made up of 28 elements (including the ear ossicles) (White & Folkens 2005: 77).  The Hyoid bone (the ‘voice box’ bone) is generally not included in the count of skull bones.  The identification of the elements can be made hard as idiosyncratic differences, and fusion between plates of the cranium, can lead to differences.  A number of elements in the human skull are paired elements; simply that they are part of two identical bones in the skull.  Alongside this there are also separate elements.  The list is below-

Paired Elements

  1.  Parietal bones- Located form the side and roof of the cranial vault.
  2. Temporal bones- Located laterally and house the Exterior and Interior Auditory Meatus.  They also include the Temporomandibular Joint (TMJ for short), the
  3. Auditory Ossicles– The malleus, incus and stapes (6 bones altogether) are located in both of the ears, very near the temporal bones (Very often never recovered in archaeological samples).
  4. Maxillae bones- Located proximal to the mandile, houses the upper jaw.
  5. Palatine bones- Located inside the mouth and forms the hard palate and part of the nasal cavity.
  6. Inferior Nasal Conchae bones- Located laterally inside the nasal cavity.
  7. Lacrimal bones- Located medially in the orbits.
  8. Nasal bones- Located distally to the frontal bone, helping to form the upper nose.
  9. Zygomatic bones- They are the cheekbones.

‘Norma Lateralis’ view of the human skull (Pearson Education 2000).

Single Elements

  1. Frontal bone- Located anterior, it is the brow of the skull.
  2. Occipital bone- Located to the rear of the skull, houses the Foramen Magnum.
  3. Vomer bone- Located in the splanchnocranium, and divides the nasal cavity.
  4. Ethmoid bone- A light and spongy bone located between the orbits.
  5. Sphenoid bone- Located inside the front of the splanchnocranium, a very complex bone.
  6. Mandible bone- The lower jaw.

‘Norma Frontalis’ view of the human skull, note the large orbits (Pearson Education 2000).

‘Norma Basilaris’ view of the human skull, note the foramen magnum where the spinal chord enters the skulls to connect with the brain (Pearson Education 2000).

‘Intracranial Superior’ view of the human skull, again note the foramen magnum where the spinal chord enters the skull to join the brain and the thickness of the outer and inner cortical bones of the skull (Pearson Education 2000).

General Discussion

The human skull is a complex part of the body.  It is key in identification of sex by the size of the Mastoid Process, Supraorbital Torus, tooth size, and the squareness of the mandible amongst others; it can also be used in describing age at death by tooth wear, Cranial Suture closure and general porosity of the bone (Roberts & Manchester 2010, White & Folkens 2005, Jurmain et al 2011).  A later post will detail exactly how in further detail.

It has also changed as our species, Homo Sapiens, evolved from earlier hominids.  The morphology of the human skull has certainly become more gracile, and as an indicator and outcome of the agricultural revolution, it seems our mandibular size and muscle robusticity has slowly become less pronounced (Larsen 1999: 230, Jurmain et al 2011).  As Larsen remarks (1999: 226), it is the influence of environment and mechanical behaviour that helps determine the morphology of the skull, alongside considered genetic factors.  It is important we keep this in mind as we look at archaeological material.  Studying population trends in both temporal, cultural and geographic contexts can have important results and can also highlight long term trends.

One such trend is the discussion that a change to a more ‘globular cranial change in the Holocene represents a compensatory response to decrease in functional demands as foods become softer’ (Larsen 1999: 268).  This is underscored in archaeological populations worldwide that consumed abrasive foods with populations that consumed non abrasive foods.  By being affected by food production processes & the nature of the food itself, the morphology of the cranial facial biomechanics has changed to adjust to differing food sources.  This change has influenced cranio-facial size and morphology, occlusal abnormalities, tooth size, dental trauma, and gross wear from masticatory and non-masticatory functions (Larsen 1999: 269, Waldron 2009).

Case Study: A Mesolithic-Neolithic population trend in Ancient Japan

One example of the importance of cranial studies, and of the skull in general in archaeology, is the discussion of population change during the end of the Jomon period of Japan.  Lasting roughly from 14,000 BC to 300 BC, the Jomon culture has evidence for the earliest use of pottery in the world, and made extensive use of the large variety of environments in the Japanese archipelago (Mithen 2003).  This culture has been classed as largely hunter-gather-forager in lifestyle, until roughly the Yayoi period around 300 BC; when the adoption to agriculture was fully implemented with intensive rice agriculture, weaving and the introduction of metallurgy (Mays 1998: 90).

The evidence suggests that the Yayoi were settlers from mainland Asia, with the evidence from craniometric studies and dental studies of both Jomon and Yayoi populations, alongside a comparative study with the modern day aboriginal Ainu people who inhabit the island of Hakkaido, north of mainland Japan.  The Ainu population themselves maintain that they are the descendents of the Jomon people, and with the skeletal data of skull morphology in the modern population compared to the Jomon archaeological data set, the evidence seems to match (Mays 1998: 92).  Population pressures during the end of the Jomon period and movement of the Jomon culture is therefore suggested as a geographic movement.  The skeletal data from the modern day Ainu population, concentrated in Hokkaido, provide evidence of a Jomon movement north due to pressure, as mainland Japanese modern population cranial measurements shows a mix of origin (Mays 1998: 90).

The importance of this work highlights the movement of the adaptation of agriculture in a relatively late time frame, in comparison to mainland Asia and Europe.  The palaeoenvironmental evidence suggests the richness and diversity of the Japanese archipelago, with heavy densities of the Jomon population in 3500 BC located in central and eastern Japan (Kaner & Ishikawa 2007: 2).

Stable village sites with pits dwellings, storage areas and burial facilities have been excavated and studied, yet there is only a hint of cultivating nuts and plants.  Ongoing date conflicts with AMS results from human and animal bone have suggested the impact of the Yayoi culture to be pushed back to 1000 BC or 900 BC.  However the results could be contaminated with the ‘marine radiocarbon reservoir effect’, a natural distortion of dates and thus a possible need to recalibrate existing dates (Kaner & Ishikawa 2007: 4).  The outcome of the timing of adoption of agriculture in the Late Jomon/Yayoi period is still hotly debated. Yet the archaeological and osteoarchaeological evidence presents a hunter gather society managing to thrive without agriculture in diverse environments until later cultures and migrations of people came into contact with the Jomon culture (Mays 1998).

Further Information

Bibliography

Jurmain, R. Kilgore, L. & Trevathan, W.  2011. Essentials of Physical Anthropology International Edition. London: Wadworth.

Kaner, S. and Ishikawa, T. 2007. ‘Reassessing the concept of ‘Neolithic’ in the Jomon of Western Japan’. Documenta Preahistorica. 2007. 1-7.

Larsen, C. 1997. Bioarchaeology: Interpreting Behaviour From The Human Skeleton. Cambridge: Cambridge University Press.

Mays, S. 1999. The Archaeology of Human Bones. Glasgow: Bell & Bain Ltd.

Mithen, S. 2003. After The Ice: A Global Human History, 20,000-5000 BC.London: Weidenfeld & Nicolson.

Roberts, C. & Manchester, K. 2010. The Archaeology of Disease Third Edition. Stroud: The History Press.

Waldron, T. 2009. Palaeopathology: Cambridge Manuals in Archaeology. Cambridge: Cambridge University Press.

White, T. & Folkens, P. 2005. The Human Bone Manual. London: Elsevier Academic Press.

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Skeletal Series Part 1: Bone Variation & Biomechanics

10 Apr

In the following series of blog posts I aim to cover each of the main skeletal elements.  Each post will have a single focus on a bony element, from the skull down to the bones of the foot.  Firstly though we must deal with the variation that human osteologists and bioarchaeologists will see in individual skeletons, and in a population series.  It is both useful and informative to learn the differences and the effects caused by the 4 main variation factors in the morphology of human bones.  As it is by ascertaining the degree of influences that the variation factors can cause that we can begin to understand the individual, and the skeletal series of a population, in a more informative and considered way.  The second part of this entry will focus on the basics of biomechanics, and the influences certain lifeways can have on bone.

The basic biology of bones was previously discussed in this post here, and of teeth here.  Bone in its natural state must be recognised as a changing living organism throughout life that responds to stress, both nutritional and mechanical, and remodels accordingly.  It is also must be remembered that bone is a composite material, and is able to heal itself.

Variation 1 : Ontogeny

Ontogeny is simply growth and development of an organism, in this case Homo sapiens.  The archaeological record of skeleton remains include unborn individuals right through to individuals in their 70th year and beyond.  Typically there are 7 classification groups of human age groups.  They run from Fetus (before birth), Infant (0-3 years), Child (3-12 years), Adolescent (12-20 years), Young Adult (20-35 years), Middle Adult (35-50 years), and Old Adult (50+ years) (White & Folkens 2005: 364).  Differences in bone structure, and in the growth of different bone elements often manifest themselves in changes in size and shape.

Adult with Two Juvenile Remains, From Southern Sahara

Basic Growth Profile for Homo Sapiens, Notice the Large Cranium and the Way the Body Catches Up.

Variation 2: Sexual Dimorphism

Humans are sexually dimorphic, that is there are differences between the female and male body size.  Although not as distinct between our cousins such as the gorillas, female human skeletal remains are relatively smaller in both bones and teeth size (Jurmain et a 2010).  Such skeletal variation is also manifest in the requirement of reproductive functions in the female skeleton, thus we are also often able to tell sex from skeletal remains (White & Folkens 2005: 32).

Generalised Male:Female Sexual Dimorphism

Variation 3: Idiosyncractic Differences

The idiosyncratic  (or individual) differences found in skeletons are simply natural variations, in the understanding that every body is different, and rarely are people exactly the same (identical twins excluded).  Idiosyncratic differences in bone affect the size and shape of the bone, and the topography of the bone surface.  Again, such variation is very common in human skeletal remains (White & Folkens 2005: 32).

Disarticulated human bone from the site of Armana, ancient Egypt.

Variation 4: Geographic or Population-Based

As White & Folkens point out ‘different human groups can differ in many skeletal and dental characteristics’ (2005: 32).  Thus this geographic variation can be employed to assess population affinities between skeletal series.  This trait can be quite useful in determining commingling of certain populations in prehistoric skeletal series as certain environmental and genetic traits can be passed on.

Biomechanic Basics

So these are the four main variations we should be aware of when we are looking and studying individual skeletons or a series of a population.  By considering these four main variations we can study the individual’s life pathway alongside other lines of investigation.  What we must also take into account next are the basics of biomechanics.  Biomechanics is the application of engineering principles to biological materials, whilst remembering that bone can remodel and change according to pressures put upon the bone.  As Larsen states that ‘the density of bone tissue differs within the skeleton and within individual bones in response to the varying mechanical demands’ (1997: 197).  It must be remembered that the response of human bone to ‘increased loading is in the distribution of bone (geometric) rather than density or any other intrinsic material property of bone’ (Larsen 1997: 197).

Importantly it is noted that Human bone is anisotropic, meaning its mechanical properties vary according to the direction of the load.  Importantly, Wolff’s Law highlights how bone replaces itself in the direction of functional demand.  A classic example of the remodelling capabilities of bone is that of the tennis player who has thicker cortical bone in their dominant arm.  This manifests itself in thicker cortical bone alongside hypertrophy of the muscle attachment sites.  One study carried out found that ‘males have a 35% increase in the cortical bone in the distal humerus of the playing arm vs the non-playing arm’, helping to exemplify Wolff’s Law (Larsen 1997: 196).  That study was an example of bilateral asymmetry humeral loading.  Alongside, it is the action of the main forces acting on human bone that help to change the bone, these  include a) compression, b) tension, c) shear, d) torsion & E) compression + tension+ bending.

Wolff’s law states that healthy load bearing bone (LBB) responds to strain by ‘placing or displacing themselves (at a mechanical level) in the direction of the functional pressure, & increase or decrease their mass to reflect the amount of functional pressure’, often muscular strain and/or weight bearing pressures (Mays 1999: 3).  As a part of this Frost (2004: 3) argues that the ‘mechanostat’, a tissue level negative feedback system, involves ‘two thresholds that make a bone’s strains determine its strength by switching on and off the biologic mechanisms that increase or decrease its strength’.  However, Skerry (2006: 123) has argued that there are many ‘mechanostats’ operating on the LBB and that different elements throughout the skeleton require different strain magnitudes for maintenance. Furthermore Skerry (2006: 126) also notes that differences are apparent between the sexes, and that genetic constitution, concomitant disease, exercise & activity patterns must be considered.

A recent article has also highlighted how the femoral neck width of obese people changes to accommodate the added weight.  In this case the width of the femoral neck has increased to dissipate weight throughout the bony area by increasing surface area and strength through redistribution of bone.  This is an example of active bone remodelling adapting to changes that the person has gone through in life.

An archaeological example of the above will now be taken from Larsen 1997.  ‘In the Pickwick Basin of northwestern Alabama, analysis carried out on both femora and humeri cross-sectional geometry has helped to reveal a number of differences between earlier Archaic Period hunter-gatherers and later Mississippian Period agriculturalists‘ (1997: 213).  From the femora measurements it seems that the both female and male agriculturalists had a greater bone strength, whilst analysis of male humeri shows little difference between the two series.  This has helped to show that activity levels increased for males but only in the lower limbs, as evidenced by the cross-section geometry.  However, for females of both time periods both humeri and femora strengths increased.  The article cited in Larsen (1997), Bridge 1991b, findings indicate that changes are from a greater range of activity undertaken by females than males.  With palaeopathological signs of osteoarthritis, it is concluded that the shift to food production,in particular maize production, may have had a relatively greater impact physically on women in this setting.

The next post will focus on ethics in human osteology, and from there we will consider each of the anatomical skeletal elements in context of their relative limb.

Bibliography

Frost, H. M. 2004. (A 2003). Update on Bone Physiology and Wolff’s law for Clinicians. Angle Orthodontist.  February 2004. 74 (1): 1-15.

Jurmain, R. Kilgore, L. & Trevathan, W.  2011. Essentials of Physical Anthropology International Edition. London: Wadworth.

Larsen, C. 1997. Bioarchaeology: Interpreting Behaviour From The Human Skeleton. Cambridge: Cambridge University Press.

Mays, S. 1999. The Archaeology of Human Bones.Glasgow: Bell & Bain Ltd.

Skerry, T. M. 2006. One Mechanostat or Many? Modifications of the Site-Specific Response of Bone to Mechanical Loading by Nature and Nurture. Journal of Musculoskeletal & Neuronal Interaction. 6 : 122-127. (Open Access).

White, T. & Folkens, P. 2005. The Human Bone Manual. London: Elsevier Academic Press.