There is 30,000 million neurons and five to ten times that number of glacial cells constitute
the brain of grey and white tissue. Each of these connect with others - as many as 60,000 times.
Each neuron looks like a spider attached to a filament. The spider is the cell body,
the filament the axon, the legs the dendrites. The legs pick up a single from adjacent neurons,
pass it to the body ; the signal in turn passed along by the axiol or the conducting fiber
( a few mm to a meter long) at speeds up to 225 miles per hour. Skins, liver tissue, blood
cells can be replaced after damage or loss, but not one of these nerve cells are lost.
However, in old age when enough cells are destroyed, the senses begin to fail; hearing may fade,
and there may be more difficulty in remembering names, dates telephone numbers, and so on, but
the really important jobs are carried out to the end.
You know that you have two eyes, two ears, two lungs, two kidneys,
but in a sense your brain is also a 'paired' organ with distinct right and
left hemispheres shaped like a walnut with baffling intertwining of nerve cells
axons mentioned above. The left half of the brain controls your ability to talk,
to write and to do mathematics, and much of other activities on the right side of the body.
With right-handed people, the left hemisphere is dominant; with left-handed people
the reverse is true. The cerebral hemisphere receives all the sensory impressions of
the body and controls voluntary movements. It is also concerned with consciousness,
thought, intellect and memory. A specific area being predominantly involved with each
of these functions (Fig. 1). The so-called 'silent area' of the cerebral hemisphere is
thought to be the seat of the higher human faculties; damage to it has been found to
produce moral degeneration.
The cerebellum, below and behind the cerebrum, also divided lengthwise into halves,
coordinates muscular movements and maintains posture and equilibrium.
In the brain-stem are the centers controlling the heart and respiration; 12 pairs of
cranial nerves coming from the base of the brain provide sensory paths for smell,
sight, hearing and equilibrium; sensation in face, head and tongue ; motor paths to
the muscles of the eyes, face, tongue, larynx, pharynx and certain neck muscles.