Chapter III - Chest & Pelvis
Rib cage And Pelvic Girdle
The upper part the trunk between the neck and the abdomen is referred to as the thorax. It contains the chief circulatory and respiratory systems and is separated from the abdomen by the diaphragm. It houses 12 thoracic vertebrae, l2 pairs of the ribs, sternum, and the muscles and the fascia attached to these.
The ribs are the long curved bones, which form the rib cage. Ribs surround the chest of land vertebrates, and protect the lungs, heart, and other internal organs of the thoracic cavity.
Types of Ribs
The human skeleton has 24 ribs, 12 on each side. (A small proportion have one pair more or fewer.) They are attached behind the vertebral column. The first seven pairs are connected to the sternum in front and are known as true ribs (I-VII). The eighth, ninth, and tenth are attached in front to the cartilaginous portion of the next rib above and are known as false ribs (VIII-X). The lower two, that is the eleventh and twelfth, are not attached in front and are called floating ribs (costae fluitantes, XI-XII). The spaces between the ribs are known as intercostal spaces; they contain the intercostal muscles, nerves, and arteries. The rib cage allows for breathing due to its elasticity. In some humans, the rib remnant of the 7th neck vertebra on one or both sides is replaced by a free extra rib called a cervical rib, which can cause trouble for the nerves going to the arm.
The third through ninth ribs are "typical ribs" since they share the same structure. They each have a head that has two facets separated by a crest. One head articulates with the rib's corresponding vertebra and one head articulates with the vertebra superior (above) to it.
The atypical ribs are the 1st, 2nd, and 10th to 12th.
The first rib has a shaft that is wide and nearly horizontal, and has the sharpest curve of the seven true ribs. Its head has a single facet to articulate with the first thoracic vertebra (T1). It also has two grooves for the subclavian vessels, which are separated by the scalene tubercle.
The second rib is thinner, less curved, and longer than the first rib. It has two facets to articulate with T2 and T1, and a tubercle for muscles to attach to.
The 10th to 12th ribs have only one facet on their head, and the 11th and 12th ribs are short with no necks or tubercles.
The basin-shaped structures of right and left hip bones joined by the pubic symphysis is called pelvic girdle.The appendicular structures of the lower limbs attach to this pelvic girdle. The pelvic girdle also supports the structures of the abdominopelvic cavity. Male and female pelvic structures are different. The female pelvis is shallower but wider in all directions. This is to support the enlarged uterus and to provide the passage for the fetus during the parturition. Both male and female pelvis comprises three bones, viz., ileum, ischium, and pubis. As the child grows up, these bones fuse together (forming an innominate bone), and it becomes difficult to set their boundaries. These three bones form a cup-like socket called acetabulum.
The pelvis is symmetrical and each side is actually made up of three separate bones — the upper half (the broad "wings") is the ilium; the middle (the top half of the lower "loops") is the pubis, and the bottom (the lower half of the "loops") is the ischium. These three bones fuse together with age and are collectively known as the hip bone, ossa coxae or the innominate bone. The pelvis is joined to the sacrum bone by ligaments (the sacroiliac joint), and the hip bones nest in specially shaped sockets (the acetabulum) on each side. The upper edge of the ilium is known as the iliac crest. The place at the front of the pelvis where the two sides join together is called the symphysis pubis. This is normally a very inflexible joint, but it softens and becomes more flexible during late pregnancy, allowing it to expand during labour for the baby's head to pass through. A female pelvis is also wider and shallower than a male pelvis.
The pelvis protects the digestive and reproductive organs in the lower part of the body, and many large nerves and blood vessels pass through it to supply the legs. It is also an important load-bearing part of the skeletal system.
Chapter III - Chest & Pelvis