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Skeletal Series Part 4: The Human Spine

30 Apr

Axial skeletal elements (Click to enlarge).

As we started with the skull in this series of posts, we shall continue with the axial skeleton (see right), and discuss the vertebral column (or spine) in the human skeleton.  The human spine consists of vertebrae that help support the back muscles, protect the spinal column, and provides the rod function for the axial skeleton (Mays 1999).  We’ll firstly discuss the moveable vertebrae, then move onto the hyoid, sacrum and coccyx elements.

On a personal note, I have difficulty with identifying vertebrae and reconfiguring them in the right order.  This is something I’m hoping to improve soon!  Meantime, here is a short guide…

As the basic excavation methods have already been discussed in the previous post, we will miss them out here.  Once again, care should be taken with handling, and it is unlikely all the elements would survive.  The hyoid bone is particularly liable to be missing or broken as it is a fairly small and fragile bone (Larsen 1997).

Vertebrae Anatomy, Terminology and Elements

The human spine consists of 33 vertebrae in total; 24 are considered to be part of the upper spine, whilst the other 11 are found in the Sacrum & Coccyx (discussed below).  The 24 vertebrae can be split into three seperate groups based on morphology, spinal curvature and position; those of the Cervical, Thoracic & Lumbar vertebrae (White & Folkens 2005).  The vertebrae are often referred to by their first distinction, and by the number in that area.  So the second cervical vertebrae is referred to as the C2 etc.

The main bone landmark features of a typical vertebra are presented in the diagram below.  Please note that there are variations depending on the class of vertebrae under discussion.

Typical bone landmarks found on vertebrae and anatomical information (Eckalbar et al 2012. Scoliosis and Segmentation Defects of The Vertebrae. Developmental Biology. 1 (3): 402.)

The vertebrae in these groups are typically found as below (proximal to distal):

Cervical: 7 Vertebrae present.  The normal cervical vertebrae include interlocking vertebral bodies, with saddle shaped superior and inferior surfaces; alongside this the canal (see below) is triangular and of a similar size to the vertebral body (Mays 1999).  The spinous process are shorter then in thoracic and not as massive as lumbar vertebrae processes (White & Folkens 2005: 163).

Thoracic: 12 Vertebrae present.  The elements in this section of spine again increase in body size (vertebral body).  Each thoracic element articulates with a pair of ribs in the human skeleton.  It has been noted that ‘upper thoracic bodies are roughly triangular in a superior outline whilst the lower thoracic vertebral bodies are more circular’ (White & Folkens 2005: 170).  The vertebrae canal (or arch which the spinal column runs through posteriorly) are smaller relative to the vertebral body, and importantly, more circular then in cervical vertebrae (Waldron 2009).

Lumbar: 5 Vertebrae present.   Finally the vertebral bodies of the lumbar vertebrae once again increase in size from superior to inferior (as all vertebrae do).  They are the largest of all the unfused vertebrae, and should be easily identifiable by their size and features (larger spinous process, vertebral bodies, & smaller transverse process and lumbar arches) (White & Folkens 2005: 178).

It must be remembered that due to variation, an added or missing vertebra, is a possibility.  Between the individual vertebrae bones are intervetebral discs present.  In life, they help to provide movement between bones and act as ligaments.

Cervical, Thoracic & Lumbar Vertebrae & Anatomical Features.

As discussed above, there are three vertebrae types, as seen in the diagram above and below.  Important changes in the morphology and shape of these elements can be noted, thus each part can be ascertained to the differing thirds of the spine (White & Folkens 2005).  As it can be seen from the diagram below, the thoracic & lumbar vertebrae have a much more developed Vertebral Body (cementum in other animals), alongside a much more elongated Spinous Process.

Differences In Body Shape for Cervical (C4), Thoracic (T6) & Lumbar (L2) Vertebrae.

Distinctive Elements

The Atlas and Axis Vertebrae (diagram below), the first and second cervical vertebrae, are the most distinctive in terms of size and morphology.  The Atlas bone (C1) lies between the cranium (connecting with the condyles of the occipital bone) and the Axis  vertebrae (C2).  The atlas lacks a ‘vertebral body, a spinous process and has no articular disks either superior or inferior to it’ (White & Folkens 2005: 163).  The Axis bone also lacks a typical vertebral body.  It’s most distinctive feature is the Odontoid peg, as seen in the diagram below.  This allows the head to rotate, moving the atlas about the odontoid peg of the axis bone (White & Folkens 2005: 169).

Features Of The Atlas (C1) & (C2) Axis Vertebrae

Transitional vertebrae include the sixth cervical vertebrae, which has a larger cementum (body) and a spinous process resembling a thoracic vertebrae.  The twelfth thoracic vertebrae resembles the eleventh vertebrae but the inferior articular facets assume the lumbar pattern rather then the typical thoracic pattern (White & Folkens 2005: 170).

The Hyoid bone (diagram below) is the located in the neck, immediately above the Adams apple on the anterior surface of the neck.  it is the only bone in the human body that does not articulate with any other bone whatsoever.  It consists of the body, the lesser horns and greater horns, as can be discerned in the diagram below (White & Folkens 2005: 155).  Unfortunately, it is often broken (fractured) during strangulation, and can be used as key indicator in murder cases.

Details of The Hyoid Bone

The Sacrum (diagram below) consists of five fused vertebrae in a wedge like shape at the bottom of the vertebral column.  These fuse in adolescents, and can consists of between four to six segments, although five is the normal average.  To both lateral sides are the pelvic bones (Os Coxa), whilst inferiorly lies the coccyx.

The Coccyx (diagram below) articulates distally to the Sacrum, and consists of 3 to 5 fused elements (variation is common).  It is the vestigial tail, and highly variable in shape.  In later life, the Coccyx may fuse to the Sacrum.  As with the above bone, the coccyx decreases in size inferiorly (White & Folkens 2005: 245).

Sacrum Terminology

Discussion of Osteoarthritis

As biped hominids, homo sapiens are at the mercy of numerous back problems.  The wear and tear, stresses and strains, that the vertebrae have to take are often manifested through various disease & maladies (Jurmain et al 2010).  Specifically, there is an increased susceptibility  in spinal joint disease.  Osteoarthritis is one such example, (OA) presenting in vertebrae that often occurs as a direct response to spinal stress (Roberts & Manchester 2010: 139).  The spine is recognised as ‘exhibiting a backward curve in the chest or thoracic region and a forward curve in the lumbar and cervical regions’; which lead to the C5, T8 & L4 vertebrae as being the most affected by joint disease (Roberts & Manchester 2010: 139).  These points are the areas of maximum and minimum stress, and this is seen as the variation in the frequency of OA in the spinal column.  As Waldron (2009: 27-30) states, ‘osteoarthritis is primarily a disease of the articular cartilage which breaks down as the disease progresses’.  The incipient factor is the enzymatic breakdown of the cartilage matrix, and affects the bone in the following ways.

The five main steps are outlined below:

1.  ‘Formation of new bone (after mixed signals from enzymes) around the margins of the joint; often called maringal ostephytes.

2.  Formation of new bone on the joint surface due to the vascularisation of the subchondral bone.

3. Pitting on the joint surface manifested as a series of holes on the joint surface.

4.  Changes in the normal contour of the joint, often widening and flattening of he contour.

5.  The production of eburnation, a highly polished area on the joint surface, usually sharply demarcated from the non-eburnated surface.  This area is sometimes grooved towards the motion and direction of the joint; presumably due to debris or crystals between the two articulating surfaces’ (Waldron 2009: 27-29).

Eburnation On The ‘Peg’ of the Axis Vertebrae (Shiny & Smooth Wearing)

Factors that are known to be important as precipitants to OA include age, race, sex, genetics, obesity, trauma and most importantly, movement itself.  Age is particularly important, as at the older standard range of human health there is scarcely anyone left with normal joints (Roberts & Manchester 2010, Waldron 2009).  An example of OA occurring in populations will now be discussed.  Lovell (1994) discuss a site in Pakistan, dating from 4000-5000 years ago (Bronze Age), where a pattern of OA was seen in the population.  The disease was noted as mechanical in nature (as normal), but focused on the cervical vertebrae.  This may be a reflection of activity in which the people carried heavy loads upon their heads (Roberts & Manchester 2010: 141).

The excavation of medieval rural site of Wharram Percy, in North Yorkshire, uncovered a large series population (May 1999).  Examination of features on the spinal column indicate OA was prevalent in the population at around 55 per cent for males and 39 per cent for females.  Because the site was rural in nature, and had indication of being used as an agricultural centre, it was identified that this population had developed OA through their lifestyle choices (Roberts & Manchester 2010: 143).  Useful as these sites, and features are, it should be reminded that ‘spinal joint disease was not the ideal part of the skeleton to observe as a marker of activity-related stress’ (Roberts & Manchester 2010: 143).

Bibliography

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.

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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