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From the simple to the complex
Sub-Topics
Anatomy by Level of Organization

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The brain’s complex anatomy, with its nested structures whose boundaries blur into one another, can be somewhat bewildering at first. This complexity is the result of the long process by which the brain develops as humans grow from embryos into adults. And this process in turn has resulted from the evolution of all animal species.
THE BRAIN
THE NERVOUS SYSTEM

Unlike the brain’s four external lobes, the limbic lobes, also called the cingulate gyri, are visible only when a sagittal section of the brain is made.

Cross-section of meninges

VENTRICLES AND MENINGES

The brain is the best protected organ in the body. The first layer of protection is the skull, which acts as armour shielding the brain from blows. Next come the meninges, three membranes that surround the brain to keep it from being damaged by contact with the inside of the skull. It is these membranes that become infected when someone gets meningitis, and it is because the meninges are in direct contact with the brain that meningitis is so dangerous.


For even more protection, the brain (and the spinal cord) are bathed in cerebro-spinal fluid. This fluid circulates through a series of communicating cavities called ventricles. Cerebro-spinal fluid also circulates between the pia mater and the arachnoid mater of the meninges. In addition to cushioning blows, this fluid reduces the pressure at the base of the brain by causing the nerve tissue to “float”. Cerebro-spinal fluid is secreted by the choroid plexus in the upper ventricles and absorbed by the venous system at the base of the brain. As this fluid flows downward, it carries away toxic wastes and moves hormones between widely separated regions of the brain.

The Ventricular System



       



The parts of the central nervous system that contain grey matter (composed of neuron cell bodies) are often called nuclei or ganglia. Certain groups of axons found in the brain’s white matter are called pathways or bundles.
THE NERVOUS SYSTEM
THE BRAIN

In addition to making the major distinction between the central and peripheral nervous systems, scientists also frequently subdivide the body’s nerves into two other main categories:

THE SOMATIC NERVOUS SYSTEM

These nerves participate in the organism’s relationship with its external environment. They send information to the brain from the body’s various sensory detectors. These nerves also enable us to respond to these stimuli by moving through our environment.

THE AUTONOMIC NERVOUS SYSTEM

These nerves are more involved in regulating vital internal functions. They help to maintain internal equilibrium by coordinating such activities as digestion, respiration, blood circulation, excretion, and the secretion of hormones. The autonomic nervous system in turn is divided into two categories.


In the central nervous system, the “grey matter” is composed of the neurons’ cell bodies and their dense network of dendrites. The grey matter includes the centre of the spinal cord and the thin outer layer of the cerebral hemispheres, commonly known as the cortex. The white matter consists of the myelin sheathing that covers the axons of these same neurons to enable them to conduct nerve impulses more rapidly. These myelinated axons are grouped into bundles (the equivalent of nerves) that make connections with other groups of neurons.
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