Illustration showing the pathways of cerebrospinal fluid (CSF) circulation around the brain.

Illustration of the peripheral nervous system (left) and central nervous system (right).

Illustration showing an algorithmic organization of the central nervous system.

Illustration showing the anatomy of the motor end plate, the point at which a motor neuron makes contact with a striated muscle fiber.

Illustration showing the autonomic nerves of the heart.

Illustration of the autonomic nervous system, showing the parasympathetic nervous system.

Illustration of the autonomic nervous system, showing the sympathetic nervous system.

Illustration of the thoracic cage, showing the mechanism involved in lung inhalation.

Medical illustration of the major arteries of the neck and associated structures, left lateral view. Labeled are the vertebral and basilar arteries and the internal, external, and common carotid arteries.

Medical illustration of the major arteries of the neck and associated structures, left lateral view. Shown are the vertebral and basilar arteries and the internal, external, and common carotid arteries.

Medical illustration of the eyelid, sagittal section, with a detailed view of the exocrine glands. Structures shown include the glands of Manz, glands of Moll, and glands of Zeis.

Medical illustration of the posterior longitudinal ligament of the spinal column, posterior dorsolateral view (A) and cutaway view showing paths taken (arrows) in disk prolapse (B). Structures labeled include the vertebral (spinal) foramen, intervertebral foramen, nucleus pulposus, and annulus fibrosus.

Medical illustration of the posterior longitudinal ligament of the spinal column, posterior dorsolateral view (left) and cutaway view showing paths taken (arrows) in disk prolapse (right). Structures shown include the vertebral (spinal) foramen, intervertebral foramen, nucleus pulposus, and annulus fibrosus.

Medical illustration of contraction of intrafusal muscle fibers. Labeled are gamma (fusimotor) fiber, group Ia afferent fiber, and nuclear bag intrafusal fiber.

Medical illustration of the contraction of intrafusal muscle fibers. Shown are gamma (fusimotor) fiber, group Ia afferent fiber, and nuclear bag intrafusal fiber.

Medical illustration showing the mechanism of gamma motor neuron function in skeletal muscle, showing (A) resting extrafusal and intrafusal fibers, (B) extrafusal contraction, and (C) intrafusal response. Structures shown include the alpha efferent, gamma efferent, and group Ia afferent.

Medical illustration of a muscle spindle and its associated structures. Shown are the spindle in the first dorsal interosseus muscle of the hand (left) and muscle spindle location and anatomy (right), with labels for the connective tissue capsule, intrafusal and extrafusal muscle fibers, and sensory neurons.

Medical illustration of a muscle spindle and its associated structures. Shown are the spindle in the first dorsal interosseus muscle of the hand (left) and muscle spindle location and anatomy (right), with structures including the connective tissue capsule, intrafusal and extrafusal muscle fibers, and sensory neurons.

Medical illustration of the parts of the central nervous system (CNS) that consist of gray matter. Shown are the brain, frontal section (top), and the spinal cord, horizontal section (bottom), with labels for cortex, nucleus, and column.

Medical illustration of the parts of the central nervous system (CNS) that consist of gray matter. Shown are the brain, frontal section (top), and the spinal cord, horizontal section (bottom).