Doctor-Reviewed Health Guides: Conditions, Procedures & Health News

The sternum, or breastbone, is a flat, T-shaped bone at the front and center of the chest that works with the ribcage to protect the heart and lungs and connects to the collarbones and ribs. It measures about 6 inches long. Sternum pain has many causes, including costochondritis and fractures; most are not serious, but persistent pain should be evaluated.

The saphenous vein is a superficial blood vessel in the leg that carries oxygen-poor blood toward the heart. There are two saphenous veins in each leg: the great saphenous vein, the longest vein in the body, and the small saphenous vein. They drain blood from the feet and lower legs into deeper veins.

The ulna is the longer of the two forearm bones, running from the elbow to the wrist on the pinkie side. It helps move the arm, wrist and hand and anchors more than a dozen muscles plus tendons and ligaments. Its notched upper end forms the point of the elbow, and its narrow lower end forms part of the wrist.

The splenic flexure, or left colic flexure, is the bend at the top left of the large intestine where the transverse colon turns down to become the descending colon, near the spleen and pancreas. Waste passes through it toward the rectum. It is clinically important as a site for trapped gas, volvulus, obstruction and colon cancer.

The skeletal system is a framework of bones, joints, and connective tissues that provide structural support, allow movement, and shield internal organs. Bone marrow produces red and white blood cells, and bones store minerals such as calcium. Comprising 206 to 213 bones and more than 600 muscles in adults, this system is the body’s foundation.

The optic nerve, or cranial nerve II, carries visual information from the retina at the back of the eye to the brain. Made of millions of nerve fibers, it forms as an extension of the brain and is part of the central nervous system. It enables sight, controls automatic pupil adjustments and relays light signals that help maintain circadian rhythm.

The neck muscles are a group of about 30 voluntary skeletal muscles extending from the base of the skull and jaw to the shoulder blades and collarbone. Organized into front, side and back regions, they stabilize and move the head and support the upper spine. They also help with breathing, chewing and swallowing.

The occipital lobe is the rearmost part of the brain and decodes visual signals received from the retinas through the optic nerves. It handles spatial and color processing, distance perception and object recognition. Although it makes up only 10% to 18% of brain volume, damage to it can cause significant visual impairment, including cortical blindness.

The nose is a pyramid-shaped structure of bone, cartilage and fatty tissue that begins the breathing process, with two nasal cavities separated by a septum. It filters air through cilia and nose hairs, warms and moistens air with the turbinates and detects scents through specialized nerve cells. Hydration, hygiene and avoiding irritants like smoke keep it healthy.

The nail matrix is the specialized tissue that produces the cells forming fingernails and toenails. Its germinal matrix drives about 90% of nail growth, while the sterile matrix contributes the remaining 10% and helps keep the nail attached to the skin beneath. It produces 196 layers of cells for each nail, making it the foundation of new nail growth.

The mitral valve, also called the left atrioventricular valve, sits between the heart's left atrium and left ventricle. It acts as a one-way door, opening so oxygen-rich blood can flow into the left ventricle and closing to stop blood from leaking backward. Unlike the heart's other three valves, it has only two thin but strong leaflets.

The myelin sheath is a protective layer of fat and protein that wraps around nerve cell axons and lets electrical signals travel quickly and efficiently. Much like the plastic coating on a wire, it insulates the neuron so each impulse keeps its strength over long distances. Many nerve cells rely on myelin to control movement, sensation and organ function.

The metacarpals are the five bones in the palm of each hand that connect the wrist to the fingers and thumb. They give the palm its arched shape and support, and they anchor the tendons, ligaments, and muscles that move the hand. You can feel them by pressing on the back of your hand, where one sits behind each knuckle.

The Mullerian duct, or paramesonephric duct, is a paired fetal structure that develops into the female internal reproductive organs. Without anti-Mullerian hormone, the ducts fuse to form the fallopian tubes, uterus, cervix and upper vagina, roughly between the third and twelfth weeks of gestation. When the hormone is present, as in male fetuses, the ducts regress.

The lymphatic system is part of your immune and circulatory systems, made of vessels and organs like the spleen, thymus, and lymph nodes. Its job is to collect lymph from your body's tissues, filter out germs and waste, and return it to your bloodstream. By absorbing fats from the intestines and producing infection-fighting white blood cells, it protects your body.

The mesentery is a continuous fold of tissue in the abdomen that connects the intestines to the back wall of the belly. It keeps the intestines from collapsing or twisting, carries blood vessels, lymph vessels and nerves, and helps regulate inflammation. Stretched out, the adult mesentery is about six feet long and is made mostly of fat and connective tissue.

The membranous urethra, also called the intermediate urethra, is the shortest, middle section of the male urethra, connecting the prostatic urethra to the penile urethra. It sits just below the prostate and passes through the pelvic floor. Surrounding muscle helps hold in urine, tightening to retain it and relaxing when you are ready to pass it.

The mediastinum is the middle compartment of the thoracic cavity, situated between the two lungs. It serves as a secure “housing” for the heart, thymus gland, esophagus, trachea, and major blood vessels. By providing structural support and cushioning, the mediastinum protects these vital organs while facilitating the passage of air, food, and blood between the neck, chest, and abdomen.

The maxillary artery is a major vessel deep in each side of the face, starting near the parotid gland in front of the ear. It delivers oxygenated blood to the jaw, teeth, lips, chewing muscles, and middle ear structures. It is about 40 to 47 millimeters long and is a key anatomical landmark in medical treatments.

The linea alba is a vertical band of connective tissue, primarily composed of collagen and elastin, that extends from the sternum to the pubic bone. It separates the left and right rectus abdominis muscles while serving as a central anchor for the obliques and transverse abdominis. It stabilizes the torso and may darken or stretch during pregnancy.

The iris is the flat, circular, colored part of the eye located behind the cornea and in front of the lens. It regulates how much light enters the eye by automatically adjusting pupil size, using specialized muscles to widen the pupil in low light and narrow it in bright light. This supports clear vision and protects internal eye structures.

The macula is a round, yellow spot at the center of the retina at the back of your eye. It processes central vision, letting you see objects directly in front of you in sharp detail. It supports reading, driving, and recognizing colors or faces. By converting light into electrical signals for the brain, it lets you see fine detail.

The inguinal ligament is a pair of fibrous bands in the groin where the abdomen meets the inner thigh. Its main purpose is to anchor the abdominal oblique muscles to the pelvis, providing stability for the lower abdomen and hips. It also serves as a protective bridge for nerves and blood vessels passing from the torso into the legs.

The gut microbiome is the community of trillions of bacteria, viruses, fungi and parasites living in the intestines, mostly the large intestine. It works like a virtual organ, helping digest complex carbohydrates, making vitamins such as B12 and K, and supporting the gut, which houses about 80% of immune cells. Diet and lifestyle shape it most.