Thorax - transverse section

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Introduction: Transverse section through the thorax (from the Visible Human Project). Note the location of internal organs such as the lungs and the heart, including the differing sizes of the right and left ventricles. Anteriorly, the pectoralis major muscle overlies the rib cage. Laterally attachments of the serratus anterior muscle to the 3rd - 8th ribs, can be seen. Posteriorly, the latissimus dorsi, trapezius and erector spinae muscles may be seen.

Annotations

azygous_vein: The azygous vein runs within the thorax, to the right of the vertebral column. It empties into the superior vena cava which leads to the right atrium of the heart. The hemiazygous vein runs along the left side, and ultimately drains into the azygous vein. This system of drainage beomes very important should the inferior vena cava (draining blood from the lower extremities), or the hepatic portal vein (draining blood from the GI tract) become blocked. In such a case the azygous system provides a collateral route for blood to drain into the heart via the superior vena cava.

biceps: The biceps brachialis, favorite muscle of Arnold Swartznegger, flexes the elbow (bends the arm).

deltoid: The insertion of the deltiod muscle to the humerus of the arm can be seen in this section. The deltoid muscle forms the rounded musclular area over the shoulders. It helps abduct the shoulder joint, as occurs when you raise your arms to the side of your body.

descending_aorta: Blood exiting the left side of the heart enters the aorta - the largest artery in the body. The carotid arteries to the head, and subclavian arteries to the arms branch off the aortic arch. The aorta (shown here) then descends through the abdomen until it splits into the iliac arteries which supply blood to the legs.

erector_spinae: The erector spinae muscles run the length of the vertebral column. They allow the bending and rotation of the spine. They are also the most common cause of muscle spasms and lower back pain.

esophagus: The esophagus is a long musclular tube which connects your mouth to your stomach. It contracts after you swallow to push the food into the stomach. Note its location behind the heart. Acid reflux, a condition where acid from the stomach enters the esophagus, irritates its lining, producing heartburn.

fashia: Note the subcutaneous fatty tissue beneath the skin. This layer of fat and connective tissue helps insulate the body. It is also metabolized for energy during periods of starvation.

humerus: The humerus is the long bone of the arm.

intercostal_muscle: The intercostal muscles lie between the ribs. They help elevate the ribs during inspiration. This action allows the lungs to expand within the thoracic cavity.

latissimus_dorsi: The latissimus dorsi muscle is one of the primary muscles of the back. It extends in a large traingle from the 6th thoracic vertebra (a point roughly between the scapulae) to the sacrum, and laterally to its attachment on the humerus of the arm. The muscle adducts the shoulders, done by military recruits standing at attention, and also extends the shoulder joint, an action used to raise the arm.

left_atrium: The left atrium receives newly oxygenated blood. The right and left pulmonary veins return blood from the lungs. These veins are the only ones that carry oxygenated blood. Ateries always carry blood Away from the heart while veins return blood to the heart.

The left atrium then pumps the oxygenated blood into the left ventricle where it will be pumped to the rest of the body. The mitral value regulates the flow of blood between the two chambers.

left_ventricle: The left ventricle of the heart pumps oxygenated blood returning from the lungs to the rest of the body. Blood enters via the left atrium and exits through the aortic value to the aorta, the largest artery in the body. You can feel the left ventricle in action by taking your pulse, or by placing your hand over your ribs, just beneath the breast. The site where you sense the heart beating is called the point of maximal impulse (PMI).

Note the greater thickness of the left ventricle as compared to the right ventricle. The right ventricle pumps returning venous blood to the lungs, a job that requires less work, and thus less muscle.

lungs: The right and left lungs can be seen in this section. Note their location with respect to the heart. The deep red color of the lungs comes from the great amount of blood in the pulmonary vasculature. In the lungs red blood cells exchange carbon dioxide for oxygen.

pectoralis_major: The pectoralis major muscle lies over the anterior wall of the chest.

pericardium: The heart is surrounded by a thick fibrous membrane - the pericardium. A small amount of fluid exists in the space between the beating heart and this membrane. Should the amount of fluid increase due to infections (pericarditis) or rupture of the heart muscle as can happen with serious damage to the heart muscle, a life-threatening condition called cardiac tamponade can occur. Due to the limited space, the increased fluid prevents the heart from filling with blood and pumping it to the body.

ribs: The ribs form much of the protective cage surrounding the heart and lungs. This particular section cuts through many ribs. Note that they do not lie perectly in the transverse plane.

The 12 ribs articulate posteriorly with the 12 thoracic vertebrae of the vertebral column. Anteriorly, the 7 true ribs articulate with the sternum, or breast bone, through the costal cartilages. The cartilagenous connections permit the ribs to swivel slightly. This action helps the lungs expand more fully during inspriation.

Following the 7 true ribs are 5 false ribs. The first three false ribs articulate with the 7th true rib. They do not directly attach to the sternum. The last two false ribs ribs - floating ribs lack anterior connections to the sternum.

right_atrium: Venous blood from the body enters the right atrium. Blood from the head, neck and arms collects in the superiour vena cava while blood from the lower regions of the body collects in the inferior vena cava.

Blood subsequently enters the right ventricle where it will be pumped to the lungs for oxygenation. The tricuspid valve regulates the flow of blood between the two chambers.

right_ventricle: The right ventricle pumps venous/deoxygenated blood to the lungs. Note the difference in size between the right ventricle, and its larger counterpart, the left ventricle. The increased size is due to the greates amount of pressure needed to pump blood to the entire body, versus solely to the lungs

scapula: The scapular bone forms your shoulders. Many muscles are attached to this bone to help you raise and lower your arms, swing them to the side, and shrug your shoulders.

serratus_anterior: The serratus anterior muscles can be felt in the following manner. With one hand, push against a wall (protraction of the scapula (shoulder) bone). Place the other hand along the side of the chest, just below the arm pit. The flexed muscle can be felt along the 3rd - 8th ribs.

spinal_cord: The greyish white oval within the vertebra on this section represent the spinal cord. Note how small the cord is with respect to its bony encasement. The spinal cord is part of the central nervous system. Peripheral nerves in the skin, muscles and organs communicate with nerves in the spinal cord to send messages to the brain.

sternum: The sternum is commonly callekd 'breast bone.' The seven true ribs of the rib cage articulate with the sternum. The 8th, 9th, and 10th false ribs join only with the seventh true rib. The last two false ribs 11 & 12 are floating ribs. They do not articulate anteriorly with any elements of the rib cage.

trapezius: The trapezius muscle forms the sloping region between the neck and the shoulders. It elevates the shoulders (a shrug), raises the arms over the head, depresses the shoulders, and retracts the shoulder blades, as occurs when stretching the arms beind the body.

triceps: The triceps muscle is the primary extensor of the elbow. It straightens the arm. The muscle has two heads, one of which inserts on the humerus just below the shoulder joint, and the other of which inserts on the scapula. The two heads can commonly be felt by placing a hand around the back of the arm while holding the elbow in a locked position.

vertebra: The vertebral column forms the backbone. This bony structure protects the spinal cord. The column runs from the neck to the pelvis (hips). It includes the 7 cervical vertebrae of the neck, 12 thoracic vertebrae which articulate posteriorly with the ribs, 5 lumbar vertebrae of the lower back, the sacrum, which consists of 5 fused bones, and is responsible for transmitting the weight of the body to the hips, and lastly the coccyx - 2-4 fused bones which form the vestigal remains of a tail.

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