HUMAN BRAIN
(Anatomy)

In the sixteenth century, the Italian philosopher and physician, Arcangelo Piccolomini, was the first to make the distinction between white matter and the cortex.

The motor strip (motor cortex) is part of the frontal lobe as is the premotor cortex. The parietal lobe extends from the central sulcus to the occipital gyri. Below the Sylvian fissure is the temporal lobe. The cerebellum, not shown in the figure, sits directly beneath the temporal and occipital lobes. In addition to this simple lobar anatomy of the brain, Rodman (Figure 4) labeled 47 areas of importance such as areas 8 and 24 for contralateral eye and head movements, areas 44 and 45 of the dominant hemisphere as the centers for speech, and areas 17 and 18 of the occipital cortex for vision.

Figure 4

The right cerebral hemisphere, although somewhat larger then the left, is nearly a mirror image of the left hemisphere. The two hemispheres are separated by a thin but sturdy layer of tissue, the cerebral falx which is contiguous with the outermost covering of the brain, the dura mater. The noted Egyptologist R.A. Schwaller de Lubiciz felt that the falx cerebri separated the two brain hemispheres in such a manner that truth was separated from error and in his book The Temple in Man: The Secrets of Ancient Egypt, shows how the cerebral hemispheres are represented by the southernmost portion (Room 20) of the Temple of Luxor (also see G. Daressy's "Notice Explicative des Ruines du Temple de Luxor", 1893). Modern science tells us that the right hemisphere is the creative brain and the left hemisphere the calculating or logical brain. Generally, language centers are located within the left hemisphere and important visual and spatial functions are contained within the right hemisphere. This is a great simplification of what is actually an enormously complex system, but it will suffice for lay purposes. The cerebral hemispheres are separated by the dural falx, and are interconnected by means of the corpus callosum, a thick bundle of white-matter fibers. Deep within the brain are paired gray-matter structures: the thalami, the hippocampi and the amygdali.

Just as an ounce of gold can be hammered into a sheet thin enough to cover a tennis court, the numerous folding and enfolding (gyri and sulci) of the brain allow a large surface area of cortex to be compacted to the allowable space within the cranium (skull cavity). This outer layer or cortex is extremely thin, varying from one to four millimeters in thickness, and is composed of over 30 billion neurons where as the remainder of the brain and cerebellum have over 150 billion neurons. There is a well-organized laminar pattern of cellular and fibrous components (axons and dendrites) with approximately 150,000 neurons beneath each square millimeter of cortex. The layers of the cortex are identified (following Brodmann) as:

Notice that four of the six layers have further subdivisions. Again, the number of layers varies from one to six with one layer only found in the older archicortex of the hippocampus, to the six layers in the larger neocortex. The outermost molecular layer contains sparse cell bodies and is composed mainly of the distal rami long apical dendrites arriving perpendicular to the brain surface from fusiform and pyramidal cells in the deeper cortical layers. Brodmann's 47 areas of the cerebral cortex were based on the cytoarchitecture of each region. Within the cortical layers, axons and dendrites extend vertically and horizontally forming billions upon billions of connections. The importance of this cortical structure is for one to note that this layering pattern, with the resultant multitude of connections, has been clearly demonstrated as a significant feature of the human brain. The important feature of the cortical neurons is the mapping of areas according to function and the dynamic relationship that continues between neurons with excitation and inhibition of the multitude of interneuronal connections.

Within the white matter primarily are the glial or supporting cells of the central nervous system, the astrocytes, which nourish and maintain the neural cells and the oligodendrocytes, which produce the myelin sheaths of the neural axons.

The areas immediately anterior and posterior to the central sulcus are known as the precentral and poscentral gyri and are responsible for sensory and motor functions respectively. The distribution of function is commonly represented by sensory and motor homunculi or "little men" (a phrase coined by Dr. Wilder Penfield) is drawn in a representative fashion, Fig. 5, which shows the motor homunculus along the precentral gyrus or motor strip.

Figure 5

Notice that the relative size of each area directly corresponds to the amount of information being transmitted to or from the brain. For example, the area for the motor control of the thumb is far greater than that for the little finger and indeed, we can perform many complex moves with the first digit than we can with the fifth. There is a homunculus for the premotor cortex also which varies quite a bit from the motor strip's homunculus. This distribution was obtained from various brain stimulation sessions in awake patients. Edelman in his book Bright Air, Brilliant Fire, feels that there may be a large number of homunculi and that the visual system alone may have thirty such representations. Blood-flow studies and functional MRI studies are now shedding more light on the subject of cortical and brain function but we remain in the dark ages when it comes to understanding the human brain.