AnatomyThe thoracic vertebrae are located in the middle section of the vertebral column, specifically inferior to the cervical vertebrae and superior to the lumbar vertebrae. These vertebrae span the large majority of the chest cavity area.
The vertebrae are separated by intervertebral discs of fibrocartilage, which are flexible cartilage discs located between the bodies of two adjacent vertebrae that allow movement in the spine and have a shock absorbing or cushioning function as well. An intervertebral disc consists of an inner gelatinous nucleus pulposus surrounded by a ring of fibrocartilage, the annulus fibrosus. In total, the adult human body typically has 23 discs, with the first found between cervical vertebrae 2 and 3, and the last one is found between the lumbar vertebra and the sacrum. In addition to providing shock resistance and cushioning, the discs also help bind adjacent vertebrae together.
There are 12 thoracic vertebrae (denoted as T1-T12) found in adult humans, and they are situated in between the cervical and lumbar vertebrae, with a general sizing larger than the cervical but smaller than the lumbar vertebrae. For each of the 12 thoracic vertebrae, there is a corresponding pair of ribs attached to them. This is unique since no other vertebrae have ribs attached to them. Thoracic vertebrae increase in size as they descend towards the lumbar vertebrae; this is because the lower vertebrae must be able to support more of the body’s weight when a person is standing due to the effects of gravity.
To summarize, the main anatomical components of a thoracic vertebra are:
- Spinous process
- Costal facets: transverse costal facet, superior costal facet, and inferior costal facet
- Transverse process
- Superior and inferior articular facets
- Intervertebral foramen
- Vertebral foramen
Distinguishing features of the thoracic vertebrae include the presence of facets on the sides of the bodies for articulation with the heads of the ribs, and facets on the transverse processes of all, except the 11th and 12th vertebrae, for articulation with the tubercles of the ribs. In addition, the spinous processes are relatively more pointed than other vertebrae, and angle sharply downward.
The body of a thoracic vertebra is somewhat “heart-shaped,” and is larger than the cervical but smaller than the lumbar vertebrae in size. The body also has small, smooth, and somewhat concave costal facets for the attachment of the ribs. Ribs are generally inserted between two vertebrae, such that each vertebra contributes to articulating with half of the articular surface. Each vertebra therefore has a pair of superior articular facets that face posteriorly and a pair of inferior articulating facets that face anteriorly (except for T12). This means that the rib will articulate with the inferior costal facet of the upper vertebrae and the superior costal facet of the lower vertebrae. Transverse processes arise from the arch found behind he superior articular processes and pedicles, and are thick and strong with a clubbed end and a small concave surface for the articulation with the tubercle of a rib. These processes are directed obliquely backward towards the spinous process and lateralward.
It is important to note that the superior and inferior facets are named relative to their position on the vertebral body, and not for which part of the articular they provide for the rib. The costal facets are also slightly different between the vertebrae. Vertebrae T1, T10, T11, and T12 have complete costal facets on the bodies for ribs 1 and 10-12, which articulate on the vertebral bodies instead of between vertebrae. On the other hand, vertebrae T11 and T12 have no transverse costal facets. Similarly, ribs 11 and 12 articulate with the bodies of T11 and T12, but since the ribs do not have tubercles they also cannot attach to the non-existent transverse costal facets of T11 and T12. These ribs also lack a cartilaginous connection to the sternum and higher costal cartilages, and are only embedded in the lumbar muscle, therefore, T11 and T12 are known as the floating ribs.
Superior articular processes are thin plates of bone, which project upward from the junctions of the pedicles and laminae, with facets that are nearly flat and directed backward, lateralward, and upward. Inferior articular processes are mostly fused with the laminae, with the facts directed forward, medialward, and downward. The lamina of the thoracic vertebrae are broad, thick, and imbricated (which means their adjacent sides overlap similar to roof tiles or scales of a fish); lamina function to connect with the pedicles to surround and protect the spinal cord. The spinal cord runs through the vertebral foramen of a vertebra where is it thus protected since it is encased throughout the vertebrae. The nerve roots from the spinal cord exit from intervertebral foramen, where there are two at each intervertebral level, one for each the right and left side.
Individual Thoracic Vertebrae
- First thoracic vertebrae (T1): Has on either side of the body, an entire articular fact for the head of the first rib, and a demi-facet for the upper half of the head of the second rib. The spinous process is thick, long, and almost horizontal. The transverse processes are long, with the upper vertebral notches deeper than any of those found on the other thoracic vertebrae. The thoracic spinal nerve 1 passes through underneath T1.
- Second thoracic vertebrae (T2): The T2 vertebra is larger than T1 vertebra. The thoracic spinal nerve 2 passes through underneath T2.
- Third thoracic vertebrae (T3): The thoracic spinal nerve 3 passes through underneath T3.
- Fourth thoracic vertebrae (T4): The T4 and T5 vertebrae are at the same levels as the sternal angle. The thoracic spinal nerve 4 passes through underneath T4.
- Fifth thoracic vertebrae (T5): The T5 and T4 vertebrae are at the same levels as the sternal angle. The trachea branches into the two main bronchi at the level of T5. The thoracic spinal nerve 5 passes through underneath T5.
- Sixth thoracic vertebrae (T6): The thoracic spinal nerve 6 passes through underneath T6.
- Seventh thoracic vertebrae (T7): The thoracic spinal nerve 7 passes through underneath T7.
- Eighth thoracic vertebrae (T8): The T8 and T9 vertebrae are found at the same level as the xiphoid process. The thoracic spinal nerve 8 passes through underneath T8.
- Ninth thoracic vertebrae (T9): The T9 vertebra may have no demi-facts below it, but some people do have two demi-facets on either side, which subsequently means T10 will not have facets below it. The xiphoid process of the sternum is at the same level as T9 in the axial plane. The thoracic spinal nerve 9 passes through underneath T9.
- Tenth thoracic vertebrae (T10): T10 has an entire articular facet (not demi-facet) on either side, which is located on the lateral pedicle surface. However, it will not have any type of facet below it, since the ribs following only have a single facet on their heads. The thoracic spinal nerve 10 passes through underneath T10.
- Eleventh thoracic vertebrae (T11): At this level, the T11 begins to resemble the size and structure of a lumbar vertebrae. The articular facets for the heads of the ribs are large and primarily on the pedicles, which are thicker and stronger in T11 and T12, than in T1-T10. The spinous process in T11 is short and nearly horizontal. The transverse processes are very short, tuberculated at its extremities, and devoid of articular facets. The thoracic spinal nerve 11 passes through underneath T11.
- Twelfth thoracic vertebrae (T12): T12 is similar to T11, but can be distinguished by its inferior articular surfaces, which are convex and directed lateralward, similar to those of lumbar vertebrae. This vertebrae will most closely resemble a lumbar vertebra also by its transverse process that is subdivided into 3 elevations—the superior, inferior, and lateral tubercles. The superior and inferior tubercles correspond to mammillary and accessory processes of the lumbar vertebrae. T12 is accordingly a logical anatomical progression into lumbar vertebrae structure and function. The thoracic spinal nerve 12 passes through underneath T12.
Muscles Affecting Thoracic Vertebrae Function
- Spinalis muscle: This is a long muscle that is part of the erector spinae bundle of muscles and tendons, which helps the spine with movement and also helps in maintaining posture. The ligaments of the spinalis muscle attach to multiple thoracic vertebrae, and while it is narrow near the first thoracic vertebra and the lumbar vertebrae, the muscle is wide in the middle.
- Longissimus muscle: This is a long muscle lateral from the semispinalis, which travels upwards the spine from the middle of the lumbar spine and is found to run along both sides of the spinalis muscle.
When the intervertebral discs are under heavy stress, such as when lifting a heavy object, the disc will bulge laterally and excessive stress will cause the annulus to crack and the nucleus to ooze out from the cracked space in the annulus fibrosus—this is called a herniated disc. Given the lack of a functional disc between two vertebrae increased pressure can cause a lot of spinal cord pain, and create a might higher risk of bone fracturing and damaging of the adjacent vertebral bones since they will rub on each other.
Fun fact: People are on average roughly 1% shorter in height when they go to bed at night than when they first wakeup in the morning. This is since the weight of the body compresses the intervertebral discs during the day as they move about through their usual business, and the compression on these discs squeezes the water out of them. During slumber or times when one is laying down in a supine position, the weight is no longer focused on the spine, which allows the discs to reabsorb water and swell, thus restoring a person’s height.