The human body's nervous system comprises a complex network of nerves and cells responsible for transmitting messages between the brain, spinal cord, and other areas of the body. The nervous system has two primary components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

The Central Nervous System (CNS) consists of the brain and the spinal cord. The brain, the body's most specialized organ, has three primary parts: the forebrain (including the cerebrum, olfactory lobes, thalami, and hypothalami), the midbrain (including the optic lobes, pineal body, and pituitary gland), and the hindbrain (including the cerebellum, pons varolii, and medulla oblongata). The CNS controls virtually every bodily process.

The spinal cord, a tube-shaped structure made up of soft tissue, extends from the medulla oblongata to the tail region. The spinal cord has three layers (meninges) that protect and support it: the dura mater (with two layers, periosteal and meningeal, and an epidural space), the arachnoid mater, and the pia mater (with a subarachnoid space that contains cerebrospinal fluid). The spinal cord coordinates simple reflexes like the knee-jerk and automatic reflexes like sweating and acts as a bridge between the brain and the spinal nerves.

The Peripheral Nervous System (PNS) consists of all nerves outside of the brain and spinal cord, including two types of peripheral nerves: the cranial nerves related to the brain and the spinal nerves connected to the spinal cord. The PNS has two components: the Somatic Nervous System, which regulates voluntary activities and external stimuli responses, and the Autonomic Nervous System, which regulates involuntary activities like heartbeat and breathing and consists of the sympathetic nervous system (stimulating the body parts during emergencies) and parasympathetic nervous system (relaxing the body parts after an emergency).

The heart and blood vessels work together to circulate blood through the body. Blood vessels act as tubes and carry blood away from and to the heart. There are three primary types of blood vessels: arteries, veins, and capillaries. Arteries' muscular walls with a small canal and elasticity makes them potent enough to withstand high blood pressure from the heart, with exception of the pulmonary artery that carries oxygenated blood from the heart. Veins have a larger canal and less elasticity, with valves that manage blood flow through reduced pressure differential and carry deoxygenated blood, except for the pulmonary vein.

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3. The Lymphatic Vessels

The lymphatic system is a complex network of organs, vessels, and tissues that work harmoniously to facilitate the movement of a clear, almost colorless fluid known as lymph.

Essentially, the lymphatic system comprises multiple components, including:

- Lymph: This fluid, also known as lymphatic fluid, is mostly made up of cellular and tissue fluids, as well as other essential substances such as minerals, proteins, cancer cells, and damaged cells. It also contains lymphocytes, which are a type of immune cell.

- Lymph nodes: These bean-shaped structures work to purify and filter lymph as it flows through them. They remove cancerous and defective cells from the fluid and harbor, generate, and store lymphocytes to combat bacteria. Lymph node groups are known as chains and can exist as individual units.

- Lymphatic vessels: A network of capillaries and a vast array of tubes, lymphatic vessels move lymph away from the tissues.

- Collecting ducts: These ducts transport lymph to the subclavian vein, where it is returned to the bloodstream.

Other constituents of the lymphatic system include the spleen, thymus, tonsils and adenoid, bone marrow, peyer's patches, and the appendix.

4. The Anatomical Positions

When standing upright and facing forward, with arms resting at sides and palms facing forward, legs parallel and feet flat on the floor, the body is said to be in anatomical position.

The four imaginary flat surfaces or planes that pass through the body in anatomical position are:

- The median plane: Often referred to as the mid-sagittal plane, the median plane is a sagittal plane, which divides the body vertically through the midline marked by the belly button. This division creates an equal left and right side of the body.

- The sagittal plane: A longitudinal plane, the sagittal plane divides the body into left and right parts.

- The coronal plane: A vertical plane, the coronal plane divides the body into a front section and a back section.

- The transverse plane: The transverse plane divides the body into upper and lower parts.