Osteology Notes (Part 2)

The human skeleton has 206 bones in total, divided into three major parts: the axial skeleton (trunk bones), the skull, and the appendicular skeleton (limb bones). The trunk bones total 51, and are further divided into vertebrae, ribs, and sternum. There are 26 vertebrae in total: 7 cervical, 12 thoracic, 5 lumbar, 1 sacrum, and 1 coccyx, which together form the vertebral column; there are 12 pairs of ribs totaling 24 bones, of which pairs 1-7 are true ribs, pairs 8-10 are false ribs, and pairs 11-12 are floating ribs; the sternum is a single bone, divided into three parts: the manubrium, the body, and the xiphoid process. There are 29 skull bones, divided into the cranial bones (8) and the facial bones (15), plus 6 auditory ossicles. The appendicular skeleton totals 126 bones, divided into the upper limb bones (64) and the lower limb bones (62).

Among these human bones, the humerus, ulna, radius, femur, fibula, tibia, metacarpals (5 in total), phalanges of the hand (14 in total), metatarsals (5 in total), and phalanges of the foot (14 in total) in the appendicular skeleton all belong to typical long bones, with a shaft and two ends, and a medullary cavity in the middle. Whether the clavicle in the appendicular skeleton belongs to long bones is somewhat controversial, because although the clavicle also has a shaft and two ends, there is no medullary cavity in the middle; the ribs, the parietal and frontal bones of the skull, and the scapula in the appendicular skeleton belong to flat bones; the carpal bones of the hand (8 in total) and the tarsal bones of the foot (7 in total) belong to short bones; the patella is a sesamoid bone; the rest are basically irregular bones.

There are 26 vertebrae in an adult: 7 cervical (C1-C7), 12 thoracic (T1-T12), 5 lumbar (L1-L5), plus one sacrum and one coccyx each. The adult sacrum is formed by the fusion of the 5 sacral vertebrae of childhood during development; the adult coccyx is formed by the fusion of the 3-4 coccygeal vertebrae of childhood.

These vertebrae together form our spinal column, which plays an important supporting role in our body. Because each segment of the vertebrae bears different body weight, the vertebral bodies gradually increase in size from top to bottom; this structural feature is determined by their function.

A vertebra is divided into two parts: the vertebral body and the vertebral arch (the atlas of the cervical spine has no vertebral arch). The vertebral body is the main weight-bearing part of the vertebra, and there is a vertebral foramen in the center of the body. The vertebral foramina connect up and down to form the vertebral canal, which contains the spinal cord, the meninges, cerebrospinal fluid, spinal nerve roots, blood vessels, and a small amount of connective tissue.

There is also fat in the vertebral canal; healthy intraspinal fat is defined as "small amount, evenly distributed, no obvious compression or space-occupying effect." Moderate fat has a certain protective effect on the tissues within the vertebral canal, but excessive deposition of intraspinal fat can cause spinal stenosis or nerve compression, leading to a series of conditions such as lower back and leg pain and foot numbness, which is also one of the common diseases of our time.

The inside of the vertebral body is filled with spongy bone; the compact bone on the surface is relatively thin, and the upper and lower surfaces are rough, connecting with adjacent vertebrae through intervertebral discs. The vertebral arch is an arched bony plate; its narrowed part closely connected to the vertebral body is called the pedicle of the vertebral arch, and the upper and lower margins of the pedicle are called the superior vertebral notch and inferior vertebral notch respectively. The superior and inferior notches of adjacent vertebrae together form the intervertebral foramen, through which the spinal nerves and blood vessels pass.

The vertebral arch gives off 7 processes (not every vertebral segment has a vertebral arch): 1. One spinous process, extending from the midline of the posterior vertebral arch backward or posteroinferiorly, the tip of which can be palpated on the body surface. 2. One pair of transverse processes, totaling 2, distributed symmetrically on both sides of the vertebral arch. The spinous processes and transverse processes are both attachment sites for muscles and ligaments. 3. Two pairs of articular processes, totaling 4, projecting upward and downward respectively at the junction of the pedicle and the lamina; the upward-projecting ones are the superior articular processes, and the downward-projecting ones are the inferior articular processes; adjacent articular processes form the articular processes joints.

The spinal column has four physiological curvatures: the cervical lordosis, thoracic kyphosis, lumbar lordosis, and sacral kyphosis. Among them, the cervical lordosis and lumbar lordosis curve forward, while the thoracic kyphosis and sacral kyphosis curve backward. Long-term poor sitting posture, ankylosing spondylitis, trauma, degenerative changes, and congenital abnormalities can cause straightening of the physiological curvatures of the spine, bringing all kinds of inconvenience and suffering to patients.

The functions of the four physiological curvatures are different. The cervical lordosis is the uppermost curvature of the spine; this curvature allows the head to balance and have a certain degree of flexibility. The thoracic kyphosis is the curvature in the middle of the spine; this curvature gives the thoracic cage enough space to accommodate important organs such as the heart and lungs. The lumbar lordosis is the curvature in the lower part of the spine; this curvature helps support body weight and balance, and absorb pressure from the upper body. The sacral kyphosis is the lowermost curvature of the spine; this curvature allows the sacrum to connect stably with the pelvis and provide support for the ischium.

Variations can occur in vertebrae during embryonic development. For example, if the posterior ends of the two sides of the vertebral arch fail to fuse completely, spina bifida is formed, which is common in the lumbosacral region. Milder cases are spina bifida occulta, in which patients often have lower back pain; in more severe cases, the meninges or even the spinal cord and cauda equina herniate through it. The number of vertebrae can also vary: for example, if the first sacral vertebra does not fuse with the other sacral vertebrae but forms a sixth lumbar vertebra, it is called lumbarization of the sacral vertebra. Conversely, if a lumbar vertebra fuses with the sacrum to become part of the sacrum, it is called sacralization of the lumbar vertebra.