The vertebrae are irregularly shaped bones which stack together to form the spinal column.  The vertebrae are connected together by ligaments and muscles which control the degree of flexibility of the spine.  The vertebrae are cushioned from each other by cartilage disks which act as shock absorbers to protect the vertebrae in the spine.  The vertebrae may be separate (cervical, thoracic, and lumbar vertebrae), semi-articulated (as in some coccygeal vertebrae), or fused (as in the sacrum and coccyx).  The typical vertebra has a body of solid bony material, which supports the weight of the spine, and an arch, which forms the vertebral foramen.  It is the adjoining vertebral foramina which creates a canal down through the spinal column which houses and protects the spinal cord.  The thoracic vertebrae feature facets to which the ribs attach, called costal facets (because of their relation to the ribs).

The vertebral body is the weight-bearing portion of the vertebra.  Forming the anterior (forward) part of the vertebra, the body of each vertebra has two articular surfaces.  These surfaces are the faces of the vertebrae which articulate, or join with, the other vertebrae.  The top surface, which bears the weight of the spinal column above it, is called the superior articular surface.  The bottom face of the vertebral body is called the inferior articular surface, and rests on the vertebra below it.  Between the articular surfaces of separate vertebrae are the intervertebral disks, made of cartilage, which cushion the articular surfaces of the vertebrae from shock trauma.

Each vertebra features a projection of bone from the arch of the vertebra toward the posterior (rear).  This projection, located at the apex of the arch and bordered on the arch by the laminae, is known as the spinous process.  It is the series of these processes which is visible as the bumps beneath the skin on the back.  The spinous process serves as an anchor point for ligaments which help control the flexibility of the spine.  The vertebrae feature different types of spinous processes. Typical cervical vertebrae have bifid (forked) spinous processes, with the exception of the atlas (first cervical vertebra) which has no spinous process.  Thoracic vertebrae generally have a single tubercle (that is, they are not forked), and point downward as well as back.  The lumbar vertebrae feature spinous processes which are rectangular or hatchet-shaped and have a section which points back and one which points down.  The spinous processes of the sacrum and coccyx are generally fused together, to form a ridge called, in the sacrum, the sacral crest.  Often, the coccygeal vertebrae (fused or not) show no evidence of any ridge, crest, or spinous process.

Most vertebrae exhibit pronounced lateral protrusions (or processes), one on each side of the vertebra.  These transverse processes serve as the attachment sites for ligaments (intertransverse ligaments) and muscles, which control the bending and twisting of the vertebral column.  The base of each transverse process in most vertebrae is just off of the main body of the vertebra, located instead at the pedicle.  The pedicle is part of the ring-like structure of a vertebra, which also includes the body and lamina of a vertebra, forming the vertebral foramen which protects the spinal cord.

Cartilage disks are located between the vertebrae, and serve to cushion the spinal column from shock.  Each disk features an inner, pulpy center, called the nucleus pulposus, and a fibrous outer ring, called the annulus fibrosus, which is visible in a lateral view of the spine.  These intervertebral disks are easily torn or dislocated when the vertebra column is subjected to
inordinate stresses, such as those encountered in lifting a heavy load improperly, or twisting the back sharply, as occurs in many sporting injuries. Such a "slipped" disk is only one of many causes of back pain.  Others include arthritis, spinal meningitis, and inflammation of a tendon or muscle.  Such back problems can cause extreme pain, which may be increased by changes in the
weather or poor diet, making lifting, walking, and sitting an excruciating ordeal.  Since so many muscles place stress on the spine when they operate, simple activities such as going to the bathroom, coughing, laughing, and even breathing may be intolerable with such a condition.  More recently, however, medical advances have enabled a number of these ailments to be alleviated and chiropracty, arthroscopic surgery, and other forms of physical therapy make much suffering unnecessary.

The coccyx (or "tailbone") is composed of from three to five rudimentary vertebrae.  Often, the first of these coccygeal vertebrae is separate, while the remainder are fused together.  The articulation between the coccygeal vertebrae and the sacrum allow some flexibility in the coccyx, which is particularly beneficial in taking the stresses of sitting and falling.  The coccyx is extremely susceptible to shock fracture, as might be induced from a fall.  Furthermore, since a number of nerve pathways pass near this area, damage to the coccyx threatens damage to the nerves of the lower body.  The juncture of the first coccygeal vertebra with the sacrum occurs at the lower facet of the sacrum.

The sacrum is the portion of the vertebral column between the lumbar vertebrae and the structures of the coccyx.  It is composed of five vertebrae which are fused together to form a single bone structure.  The sacrum features a median
crest (running down the back, or posterior of the sacrum) which is made of the fused spinous processes of its component vertebrae.  Beneath this crest is the sacral canal, a tunnel which runs lengthwise from the top of the sacrum to a hiatus (opening) near the bottom.  Four pairs of holes (sacral foramina) pierce the sacrum, flanking the medial (center) line, where the intermediate sacral crests are formed by the fused articular processes of the component vertebrae.  To the outside of the intermediate sacral crests are the lateral crests, formed by the fused transverse processes of the component vertebrae.
In the sacrum, therefore, unlike the upper vertebrae in the spine, the intertransverse ligaments have been replaced by fusion of these processes together.  The crests are not represented on the front (pelvic) surface of the sacrum, though the sacral foramina are evident.

The lumbar vertebrae are the five vertebrae which are below the thoracic vertebrae and above the fused vertebrae of the sacrum.  The lumbar vertebrae feature no facets on the body or transverse processes (as the thoracic vertebrae have) and the bodies of the lumbar vertebrae are much larger than those of the cervical or thoracic vertebrae.  The vertebral foramen is usually triangular, while the spinous process points backward and is rectangular or hatchet-shaped.  The transverse processes of the lumbar vertebrae (which also represent their rib elements) lack the foramina which characterize the cervical vertebrae.  The large body of each lumbar vertebra bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord.

The thoracic vertebrae are the middle twelve in the vertebral column.  Most of the thoracic vertebrae feature costal (relating to the ribs) facets on the body and transverse processes of the vertebra, no foramina in the transverse processes (as the cervical vertebra have), a spinous process which points back and down, and a round vertebral foramen.  The body of each thoracic vertebra in the spinal column bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord.

The cervical vertebrae are the first (upper) seven in the vertebral column.  The first cervical vertebra is the atlas, so called because it directly bears the weight of the skull.  The second cervical vertebra is called the axis, because it admits the rotation of the skull by allowing the atlas to pivot upon it.  The other five cervical vertebrae have no names, but are called by their number (i.e., third cervical vertebra).  Each of the cervical vertebra features a body (anterior, or frontal, portion) and an arch (posterior, or rear, portion).  The body of each vertebra in the column bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord.  Every cervical vertebra has a foramen (opening) in each of its transverse processes (lateral protrusions).  The arch of the vertebra features a small knob or prominence, called an anterior tubercle.  The anterior tubercles on the sixth cervical vertebra are particularly large and are known as the carotid tubercles.

The axis is the second of the seven cervical vertebrae, and is called such because it allows axial (rotational) movement of the skull.  The axis lies directly beneath the atlas vertebra, their junction occurring at lateral articular surfaces and an unique juncture between a concave facet (on the atlas) and an upward-protruding dens (on the axis).  This articulation is regulated by the alar ligament, which attaches to both atlas and axis.

The atlas is the first of the seven cervical vertebrae, and is called such because it bears the direct weight of the skull, just as the mythical Greek hero Atlas bore the world on his shoulders.  The atlas vertebra meets with the occipital condyles which flank the foramen magnum in the basilar part of the occipital bone of the skull.  This junction forms the atlanto-occipital joint, and is responsible for the primary articulation between the spine and the skull.  It is the only vertebra in the spine which has no vertebral body.  The atlas vertebra, in turn rests upon the axis vertebra, which is the second of the cervical vertebra in the spine, with the articulation between these two vertebra occurring at lateral articular surfaces and an unique juncture between a concave facet (on the atlas) and an upward-protruding structure on the axis called a dens.

Spinal Cord

The vertebral body is the weight-bearing portion of the vertebra.  Forming the anterior (forward) part of the vertebra, the body of each vertebra has two articular surfaces.  These surfaces are the faces of the vertebrae which articulate, or join with, the other vertebrae.  The top surface, which bears the weight of the spinal column above it, is called the superior articular surface.  The bottom face of the vertebral body is called the inferior articular surface, and rests on the vertebra below it.  Between the articular surfaces of separate vertebrae are the intervertebral disks, made of cartilage, which cushion the articular surfaces of the vertebrae from shock trauma.

Each vertebra features a projection of bone from the arch of the vertebra toward the posterior (rear).  This projection, located at the apex of the arch and bordered on the arch by the laminae, is known as the spinous process.  It is the series of these processes which is visible as the bumps beneath the skin on the back.  The spinous process serves as an anchor point for ligaments which help control the flexibility of the spine.  The vertebrae feature different types of spinous processes.  Typical cervical vertebrae have bifid (forked) spinous processes, with the exception of the atlas (first cervical vertebra) which has no spinous process.  Thoracic vertebrae generally have a single tubercle (that is, they are not forked), and point downward as well as back.  The lumbar vertebrae feature spinous processes which are rectangular or hatchet-shaped and have a section which points back and one which points down.  The spinous processes of the sacrum and coccyx are generally fused together, to form a ridge called, in the sacrum, the sacral crest.  Often, the coccygeal vertebrae (fused or not) show no evidence of any ridge, crest, or spinous process.

The brain and spinal cord are covered by three protecting membranes, or meninges.  These are the dura mater, the pia mater, and the arachnoid.  The innermost of these membranes is the thin, compact pia mater which carries blood vessels which supply the central nervous system.  The intermediate layer is the arachnoid, so called because it resembles a spider's web.  The outer meningeal membrane is the dura mater, a tough, durable layer of connective tissue which protects the layers beneath from the bones surrounding them.  The space between the pia mater and arachnoid is called the subarachnoid space and the space between the arachnoid and the dura mater is called the subdural space.  Inflammation of these membranes is a condition called meningitis.

The spinal nerves are those which originate in the spinal cord and carry impulses to muscles (and other structures) and from sensory organs.  They feature both efferent and afferent nerves.  Efferent nerves send impulses out to the muscles and other structures and connect with the spinal cord in the anterior roots.  Afferent nerves carry impulses from sensory organs to the spinal cord and connect in posterior roots.  The spinal nerves include, in alphabetical order, the celiac, the femoral, the genitofemoral, the gluteal, the hemmorhoidal, the hepatic, the hypogastric, the iliohypogastric, the ilioinguinal, the intercostal, the intercostobrachial, the interosseus, the median, the mesenteric, the musculocutaneous, the obturator, the pelvic, the peroneal, the phrenic, the pudendal, the radial, the renal, the saphenous, the scapular, the thoracic, the tibial, and the ulnar nerves.

The spinal nerves pass through the meninges of the spinal cord and into the main canal at anterior and posterior junctures along the spinal cord.  The anterior (front) roots or these nerves are the efferent nerves, which convert impulses from the spinal cord to the muscles and other organs.  The posterior nerve roots are for afferent nerves, which carry nerve impulses back to the spinal cord from the sense organs.

When nerves bundle together outside of the spinal cord, the bundle is called a ganglion.  A number of these ganglia are located along the exterior spinal cord, connected together in the nerve pathway called the nerve trunk.  The sympathetic nerve trunk runs along the vertebral column and features five main ganglia.  These include the celiac, the cervical, the splanchnic, the superior mesenteric, and the inferior mesenteric ganglia.  The celiac ganglion is responsible for supplying nerves to the adrenal glands, the duodenum, the kidneys, the pancreas, and the stomach.  The cervical ganglion supplies nerves to the heart, the face, the neck, and the tympanum.  The splanchnic ganglion supplies nerves to the viscera, and the superior and inferior mesenteric ganglia supply nerves to the intestines and to the bladder and gonads, respectively.  The parasympathetic ganglia are located in the brain and medulla oblongata and include the ciliary, optic, and sphenopalatine ganglia.
 
 

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