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

Back muscles are organized into three layers, superficial, intermediate and intrinsic, running from just under the skull down the spine to just above the hips. They provide the main structural support for the trunk, let you bend, twist and move your head, arms and legs, keep you upright and help you breathe. Knowing the layers helps explain back pain.

An aponeurosis is a flat sheet of dense fibrous connective tissue that connects muscles to bones or cartilage. Unlike rounded tendons, these thin sheaths act like springs to absorb energy and provide stability for posture and movement. Found throughout the body, they protect underlying nerves and vessels, support muscle fibers and help transfer mechanical loads during activity.

Apocrine glands are specialized sweat glands located mainly in the armpits and genital regions that produce thick, oily sweat. Unlike eccrine glands, which regulate temperature, they activate during puberty and respond to emotional stress or excitement, releasing secretions into hair follicles. Variations of these glands also produce earwax and protective substances for the eyes.

Ankle ligaments are strong, fibrous bands of connective tissue that connect the foot bones to the lower leg bones. They stabilize the joint, prevent twisting or collapsing and absorb shock during walking and running. They can overstretch or tear, causing a sprain. Strengthening exercises, flexibility training and supportive footwear help keep them healthy.

Amniotic fluid is a water-like substance that surrounds and protects a fetus during pregnancy. It cushions the fetus, regulates temperature and prevents umbilical cord compression. It is made mainly of water and fetal urine and contains hormones and antibodies. Normally clear or pale yellow, changes in its color or volume can signal complications, so levels are monitored in prenatal care.

The spine, or backbone, is the central support structure of the musculoskeletal system, extending from the base of the skull to the tailbone. It consists of 33 vertebrae that form a protective canal for the spinal cord while allowing flexibility and maintaining posture. Intervertebral disks absorb shock, facet joints allow movement, and muscles and ligaments provide stability.

The vulva is the external part of the female genitals, separate from the vagina, the internal canal. Vulvar anatomy includes the mons pubis, labia majora and minora, clitoris, and urethral and vaginal openings. The vulva protects internal reproductive organs and plays a central role in sexual pleasure, reproduction, menstruation and urination. Gentle hygiene and avoiding irritants keep it healthy.

The vestibular system is the sensory network in the inner ear and brain that maintains balance and coordinates eye movements with head motion. Three semicircular canals sense rotation, and two otolith organs sense straight-line movement and gravity. The brain combines these signals with input from the eyes, muscles and joints. When it malfunctions, dizziness, vertigo and imbalance can result.

The appendix is a thin pouch of tissue, approximately three to four inches long, attached to the cecum, the first part of the large intestine. It stores beneficial gut bacteria that can repopulate the intestines after illness and supports the immune system with B and T cells. Appendicitis, its most common disorder, is a medical emergency.

The vagus nerve is the 10th cranial nerve and the primary component of the parasympathetic nervous system, responsible for “rest and digest” activities. It originates in the brainstem and extends through the neck, chest, and abdomen, reaching as far as the large intestine. The vagus nerve controls involuntary functions such as heart rate, respiratory pace, digestion, and immune responses.

The vagina is a stretchy, muscular canal, about 3 to 4 inches (7 to 10 cm) deep when unaroused, connecting the vulva to the cervix and uterus. It carries menstrual flow out of the body, takes part in intercourse and serves as the birth canal. Its health depends on estrogen, its mucosal lining and healthy bacteria.

The uvea is the middle layer of the eye's wall, between the sclera and the retina, also called the vascular tunic because parts are rich in blood vessels. It has three parts: the iris, which controls light and gives eye color, the ciliary body, which focuses the lens, and the choroid, which supplies the retina. Uveitis can cause vision loss.

Tumor suppressor genes are genes that make proteins to restrain cell growth and prevent tumors. They protect the body by repairing DNA errors, making sure damaged cells die and keeping cell multiplication in check. Mutations in these genes, whether inherited or acquired, are a major driver of many cancers.

The trochlear nerve, or cranial nerve IV, is a motor nerve that signals the superior oblique muscle of each eye, letting you look down and move the eye toward or away from the nose. Its long, thin path makes it vulnerable to injury, especially head trauma. Damage causes double vision, misaligned eyes and a compensating head tilt.

The spleen is a fist-sized organ inside your left rib cage, above the stomach. It filters your blood, removing old or damaged cells and waste, stores healthy blood, and makes white blood cells and antibodies that fight infection. It also helps maintain the body’s fluid levels. You can live without a spleen, but with a higher risk of serious infections.

The urethra is a thin-walled tube that carries urine from the bladder out of the body through the urethral meatus. In females it is about 1.5 inches (3 to 4 centimeters) long and carries only urine. In males it is about 7 to 8 inches (20 centimeters) long, passes through the prostate and penis and carries urine and semen.

The stages of labor describe delivering a baby and the placenta. Stage 1 starts with regular contractions and ends when the cervix is fully effaced and open to 10 centimeters. Stage 2 is pushing and ends with the baby's birth. Stage 3, the shortest, ends with delivery of the placenta. Every labor is different.

The somatic nervous system is part of the peripheral nervous system that carries sensory information to the brain and delivers the brain’s commands to your skeletal muscles. It uses cranial and spinal nerves to relay touch, taste, smell and sound, and it makes every voluntary movement possible. Damage to these nerves, called peripheral neuropathy, can cause numbness, tingling and weakness.

The urachus is a fibrous tube that runs between the bladder and belly button in a developing fetus, helping drain fetal urine through the umbilical cord. In most people it closes before birth and becomes the median umbilical ligament, but about 33% of adults retain some urachal tissue. Unclosed remnants such as cysts can cause symptoms or infection.

The substantia nigra is a small, dark band of tissue in the midbrain that is part of the basal ganglia and helps control movement. Its pars reticulata uses GABA to block unwanted signals to muscles, while its pars compacta makes dopamine, which influences movement, muscle tone, motivation and risk and reward. Its dark color comes from melanin.

The scrotum is a thick-walled sac below the penis that contains the testicles, epididymis, and spermatic cords. It has two functions: protecting the reproductive organs from external injury and regulating their temperature. By moving the testicles closer to or further from the body, the scrotum maintains the environment for sperm development, which requires a few degrees below body temperature.

The small intestine is a coiled tube about 22 feet long between the stomach and the large intestine. It uses digestive juices and enzymes to break food down, then absorbs nutrients and water into the bloodstream. It is only about 2 centimeters wide, and more than 90% of the nutrients and water your body takes in are absorbed here.

The umbilical cord is a flexible, coiled, tubelike structure that serves as the lifeline between mother and fetus during pregnancy. It typically contains three blood vessels: one vein carrying oxygen-rich blood to the fetus and two arteries returning oxygen-poor blood and waste to the placenta, all protected by Wharton's jelly. It supplies the fetus until birth.

The scapula is a large, triangular bone on the upper back that forms the posterior part of the shoulder girdle. It connects the humerus (upper arm bone) to the clavicle (collarbone), providing a stable, mobile platform for arm movement. It is connected to 17 muscles, allowing the shoulder to move in six directions, more than any other joint.