The trunk forms the major part of the body and consists of the chest and abdomen, but does not include the arms. It contains and protects the delicate organs vital for body maintainance and survival. The bony part of the trunk consists of the backbone, the rib cage and the pelvis.
The backbone is composed of a column of 33 individual vertebrae, which are separated by pads of cartilage known as the intervertebral discs. Because all the vertebrae are interconnected by ligaments, tendons and muscles, the backbone acts as one long flexible bone. To provide shock-absorbing elasticity the backbone, in side-view, resembles an elongated letter S. In the region of the chest the curve is backwards (kyphosis), in the loin region it curves forwards (lordosis). The backbone contains the spinal cord; it runs down the spinal canal which consists of a series of holes inside the vertebrae. Nerves pass out of the spinal canal through gaps between the vertebrae to innervate the organs, muscles andof the trunk and the muscles of the limbs.
The rib cage is connected to the backbone. Part of the rib cage forms the chest cavity, the site of the heart, lungs and majorvessels. These are protected by the ribs. The lower ribs protect the kidneys, liver and spleen.
The pelvis is a bony structure situated at the base of the abdomen, containing the bladder and, in a woman, the reproductive organs. Although basically similar in form, a woman’s pelvis is usually broader than a man’s, and has a larger space in the middle. This is to accommodate the baby’s head during birth.
The trunk can be divided into two compartments: the chest and the abdomen, which are separated by the diaphragm, a muscular dome. Apart from shielding the lungs, the heart and the abdominal organs, the rib cage is very important for respiratory movements. Most people think that during inspiration the lungs actively expand, but this is not the case. When drawing breath, the diaphragm moves downwards and the rib cage moves outwards, which creates more space in the chest cavity. The lungs are surrounded by two layers of pleura tissue: the pleura parietalis attached to the ribcage, and the pleura viscelaris attached to the lungs. A thin film of fluid is contained in the tiny space between these layers which prevents friction. The chest cavity contains a vacuum, so when its volume changes the lungs have to follow, thereby sucking in air. In expiration (breathing out), the reverse process is followed.
The wearing of corsets or other tightly fitting clothing greatly impairs respiratory movements because the abdomen is unable to expand. This is the reason why women in Victorian times frequently fainted: their tight-laced corsets prevented normal breathing andflow. The diaphragm is penetrated by the oesophagus, major arteries and veins and nerves.
The abdomen contains most of the alimentary tract, the excretory system amd the female reproductive system.
The abdomen is not a totally bony structure like the chest, because it needs to expand – for example, after eating, during inspiration and in pregnancy. The lower ribs protect the liver, spleen and kidneys. The rest of the abdominal cavity is formed by muscles which both protect the inner structures, and enable them to expand.
The abdominal cavity is lined by a membrane known as the peritoneum. It envelops the alimentary organs, and in the female also the ovaries. Like the pleura, the peritoneum consists of two layers: the peritoneum parietale lining the abdominal wall, and the peritoneum viscerale which covers the stomach and the intestines. It exudes a small amount of fluid which lubricates the constantly moving intestines thus avoiding friction. It also has an essential role in protecting the intestines against micro-organisms. In this it is assisted by the omentum majus, the ‘big net’, an apron-like tissue which hangs from the outside of the stomach, containing fat, blood vessels and lymphocytes. If an infection occurs, the omentum majus moves towards the site of the infection and starts working. An intraperitoneal infection such as a burst appendix causes the peritoneum to become extremely painful. Upon touching the belly the abdominal muscles contract rigidly, in order to protect the infected area.
As the peritoneum is capable of absorbing large quantities of fluid, and transporting it to the bloodstream, it can be used in dialysis. Normally the kidneys, lying behind the peritoneum, filter the blood of waste products and excrete them in the urine. When the kidneys do not function properly the blood can be filtered artificially. This can be done by connecting blood vessels to the dialysis apperatus, and also by injecting a few litres of fluid into the peritoneal cavity. Waste products of the blood are then excreted by the peritoneum and after several hours the dialysis fluid can be removed again.
Weakness in the abdominal wall may cause a pouch of the peritoneal lining to protrude through the muscles under the skin, forming a bulge called a hernia. Such weaknesses may be inborn or occur for instance after pregnancy. The commonest site for this condition is in the groin, if a section of intestine protrudes through the peritoneum, and it is called an inguinial hernia.