The spinal cord extends from the medulla or brain
stem down to the second lumbar vertebra. It is surrounded and protected
by the meninges (same as covering the brain) and the vertebrae of
vertebral column. It is a hollow, oval shaped cylinder, tapering
slightly as it descends.
A transverse section of the spinal cord (Figure
23.4) has an H-shaped region of grey matter. The grey matter contains
nerve cells and non-myelinated fibers.
A small central canal containing cerebrospinal
fluid runs through the center of the chord. Outside the grey matter
is white matter comprised of numerous medullated nerve fibers running
up (ascending) and down (descending) the cord or passing out to
the spinal nerves. These fibers usually cross over from one part
of the spinal cord to the other along their pathways to and from
the brain. Thus, the right side of brain receives impulses
that originate on the left side of the body and vice versa.
Functions : (1) The spinal chord serves
as a pathway for the condition of impulses between the receptors
and effector organs and the brain. (2) It acts as a reflex center
(spinal reflexes) for many local simple reflexes.
(i) Cranial nerves : There are 12 pairs
of cranial nerves in mammals (10 pairs in fishes and amphibians)
and most of them lead away from the medulla oblongata (Figure 23.5).
Of these 12 pairs, some are purely sensory, some are purely motor
and the others are mixed. The table shows the name, number, nature,
function and distribution of the cranial nerves.
Click here for enlargement
Figure 23.4 Transverse section of a Spinal Cord
Cranial nerves of man
| No. |
Name |
Nature |
Function |
Distribution |
| 1) |
Olfactory |
Sensory |
Smell |
From nose |
| 2) |
Optic |
Sensory |
Vision |
From eye |
| 3) |
Oculomotor |
Motor |
Eye movement |
To muscles of eye ball |
| 4) |
Trochlear |
Motor |
Eye movement |
To muscles of eye ball |
| 5) |
Trigeminal |
Mixed |
Sensitive and jaw movement |
From and to face, teeth,
lips, tongue,jaws |
| 6) |
Abducens |
Motor |
Eye movement |
To muscles of eye ball |
| 7) |
Facial |
Mixed |
Taste sensation, jaw
movement |
From taste buds, to sali
vary glands and face |
| 8) |
Auditory |
Sensory |
Hearing and balance |
From ear |
| 9) |
Glossopharyngeal |
Mixed |
Muscle movement and sensations
|
From and to pharynx,
from taste buds Salivary glands |
| 10) |
Vagus |
Mixed |
Sensory to chest and
abodomen |
From and to visceral
organs |
| 11) |
Spinal |
Motor |
Movement of shoulder
muscles |
To shoulder muscles |
| 12) |
Hypoglossal |
Motor |
Movement of tongue |
To tongue |
