Illustration of the normal human brain, showing the ventricles.

Cutaway illustration of the human lens showing two types of cataracts of the eye, cortical (in the cortex of the lens) and nuclear (in the nucleus of the lens). Nuclear cataracts are the most common type in the aging eye, while cortical cataracts are seen in diabetic patients.

Illustration of the normal human nervous system.

Illustration of the normal human inner ear, showing the semicircular canals and cochlea.

Illustration of normal kidney function, with a detail showing the blood flow through the glomerulus and urine removal.

Illustration of labor, showing the fetus just before birth.

Illustration of the limbic system of the human brain.

Illustration of the neural pathway of pain sensation. Pain sensation stimuli move up the spinal cord to the thalamus and then to the primary somatosensory cortex on the postcentral gyrus, where the signals are processed into sensation.

Illustration of the normal human spinal cord, with the skull, base of brain, spinal column, ribs, and pelvic bones cut away.

Illustration of the visual cortex, the areas of the brain that process visual sensation. Shown are the visual pathways through the brain, from the retinas in the eyes, optic nerves, optic chiasma, to the visual cortex of the occipital lobes of the brain (cut away to expose the internal structures).

Illustration of normal human wrist tendons, showing the extensor (top) and flexor (bottom) tendons. Also shown is the carpal tunnel, formed by the carpal bones and transverse carpal ligament, through which the flexor tendons run. The carpal tunnel is an area often affected by repetetive stress injury (carpal tunnel syndrome).

Illustration showing layers of skin: epidermis (highlighted cutaway view) and dermis, with hair, hair follicles, capillaries, a sebaceous gland, and a sweat gland.

Illustration showing layers of skin, highlighting the epidermis (cutaway section), which includes the corneal layer, the living layer of keratinocytes, and the basal layer. The dermis, subcutaneous tissue, and other structures are also shown, including hair, hair follicles, skin cells, melanocytes, capillaries, and fat.

Illustration of the thymus gland in an elderly man. Part of the immune system, the thymus acts upon T-cells, allowing them to carry out specific immune reactions. It is suspected that the decrease in thymus mass, which begins after puberty, may hinder immune responses in old age.

Illustration of the brain, showing the corpus callosum, top view. The two hemispheres of the brain communicate via strands of neurons that travel between them through the corpus callosum.

Illustration of the heart's conduction system: sinoatrial (SA) node, atrioventricular (AV) node, Purkinje fibers, and bundle of His (atrioventricular bundle). These specialized tissues are composed of cardiac muscle fibers that keep the heart beating via electrical impulses.

Illustration of the normal large intestine (colon) anatomy: mesocolon, cecum, ascending colon, transverse colon, appendix, sigmoid colon, and rectum. Also shown are the colon's blood supply and lymphatic drainage.

Illustration of pain pathways. Stimulation of pain receptors in the skin sends a pain impulse traveling up a sensory nerve to the dorsal root ganglion (spinal ganglion) of the spinal cord; the impulse crosses over to the spinothalamic tract and the reticular formation, then through the thalamus to the sensory cortex of the brain.

Illustration of the peripheral nervous system in a diving figure, showing the major peripheral nerves and spinal cord which control motion.

Illustration of the rectal anatomy, cutaway view. Structures shown are the rectal muscle layers, sphincter muscles, Levator ani muscle, blood vessels, and mucosa. Also included are the anal canal, anal columns, and anorectal venous plexus, the basis of hemorrhoids.