The main divisions of the central nervous system are the:

A. Horizontal fissure: The horizontal fissure, also called the minor fissure, separates the superior (upper) and middle lobes of the right lung. It runs horizontally from the hilum toward the anterior and lateral surfaces of the lung, creating a clear anatomical boundary between these lobes. This fissure is unique to the right lung, as the left lung only has an oblique fissure.

B. Oblique fissure: The oblique fissure, also called the major fissure, separates the inferior (lower) lobe from the superior and middle lobes in the right lung, and the superior from the inferior lobe in the left lung. It runs diagonally from the hilum to the posterior and inferior lung surfaces and does not separate the superior and middle lobes.

C. Bronchopulmonary segments: Bronchopulmonary segments are subdivisions within each lung lobe, each supplied by its own segmental bronchus and artery. They do not define the borders between the lobes themselves but rather the smaller functional units within lobes.

D. Hilum: The hilum is the entry and exit site for the bronchi, pulmonary arteries, veins, and lymphatic vessels. It is a central structure on the medial lung surface, not a fissure that separates lobes. It serves as a passageway rather than a boundary between lobes.

A. Astrocyte: This is the most abundant neuroglial cell in the central nervous system, characterized by its star-shaped morphology and perivascular feet. These cells are essential for maintaining the blood-brain barrier, regulating the chemical environment of the interstitial fluid, and providing structural support to neurons. Their primary role is homeostatic and metabolic.

B. Schwann Cells: Schwann cells are glial cells of the peripheral nervous system that form the myelin sheath around axons. This myelination increases the speed of electrical impulse conduction along the nerve fiber. Schwann cells also aid in the regeneration of damaged peripheral nerves by guiding axonal growth.

C. Microglial cell: This cell functions as the resident macrophage and primary immune defense within the central nervous system. These small, mobile cells constantly scavenge for plaque, damaged neurons, and infectious agents to maintain neural health through phagocytosis.

D. Ependymal cells: Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. These ciliated epithelial-like cells are responsible for the production, circulation, and monitoring of cerebrospinal fluid (CSF), creating a permeable barrier between the CSF and the nervous tissue. Their specialization is related to fluid dynamics and ventricular lining.

E. Oligodendrocyte: This is a specialized glial cell that extends multiple cytoplasmic processes to wrap around axons in the central nervous system. These wraps form the myelin sheath, a lipid-rich insulating layer that significantly increases the velocity of action potential conduction through saltatory conduction.