Gross Structure Of The Nervous System





DIVISIONS OF THE NERVOUS SYSTEM.--The nervous system may be considered

in two divisions: (1) The central system, which consists of the brain

and spinal cord, and (2) the peripheral system, which comprises the

sensory and motor neurones connecting the periphery and the internal

organs with the central system and the specialized end-organs of the

senses. The sympathetic system, which is found as a double chain of

nerve connections joining the roots of sensory and motor nerves just

outside the spinal column, does not seem to be directly related to

consciousness and so will not be discussed here. A brief description of

the nervous system will help us better to understand how its parts all

work together in so wonderful a way to accomplish their great result.



THE CENTRAL SYSTEM.--In the brain we easily distinguish three major

divisions--the cerebrum, the cerebellum and the medulla oblongata.

The medulla is but the enlarged upper part of the cord where it connects

with the brain. It is about an inch and a quarter long, and is composed

of both medullated and unmedullated fibers--that is of both white and

gray matter. In the medulla, the unmedullated neurones which comprise

the center of the cord are passing to the outside, and the medullated to

the inside, thus taking the positions they occupy in the cerebrum. Here

also the neurones are crossing, or changing sides, so that those which

pass up the right side of the cord finally connect with the left side of

the brain, and vice versa.



THE CEREBELLUM.--Lying just back of the medulla and at the rear part of

the base of the cerebrum is the cerebellum, or little brain,

approximately as large as the fist, and composed of a complex

arrangement of white and gray matter. Fibers from the spinal cord enter

this mass, and others emerge and pass on into the cerebrum, while its

two halves also are connected with each other by means of cross fibers.




the crura; P, pons; Mo, medulla oblongata; Ce, cerebellum; Sc, spinal

cord.]



THE CEREBRUM.--The cerebrum occupies all the upper part of the skull

from the front to the rear. It is divided symmetrically into two

hemispheres, the right and the left. These hemispheres are connected

with each other by a small bridge of fibers called the corpus

callosum. Each hemisphere is furrowed and ridged with convolutions, an

arrangement which allows greater surface for the distribution of the

gray cellular matter over it. Besides these irregularities of surface,

each hemisphere is marked also by two deep clefts or fissures--the

fissure of Rolando, extending from the middle upper part of the

hemisphere downward and forward, passing a little in front of the ear

and stopping on a level with the upper part of it; and the fissure of

Sylvius, beginning at the base of the brain somewhat in front of the

ear and extending upward and backward at an acute angle with the base

of the hemisphere.





The surface of each hemisphere may be thought of as mapped out into four

lobes: The frontal lobe, which includes the front part of the hemisphere

and extends back to the fissure of Rolando and down to the fissure of

Sylvius; the parietal lobe, which lies back of the fissure of Rolando

and above that of Sylvius and extends back to the occipital lobe; the

occipital lobe, which includes the extreme rear portion of the

hemisphere; and the temporal lobe, which lies below the fissure of

Sylvius and extends back to the occipital lobe.



THE CORTEX.--The gray matter of the hemispheres, unlike that of the

cord, lies on the surface. This gray exterior portion of the cerebrum is

called the cortex, and varies from one-twelfth to one-eighth of an

inch in thickness. The cortex is the seat of all consciousness and of

the control of voluntary movement.





THE SPINAL CORD.--The spinal cord proceeds from the base of the brain

downward about eighteen inches through a canal provided for it in the

vertebrae of the spinal column. It is composed of white matter on the

outside, and gray matter within. A deep fissure on the anterior side and

another on the posterior cleave the cord nearly in twain, resembling the

brain in this particular. The gray matter on the interior is in the form

of two crescents connected by a narrow bar.



The peripheral nervous system consists of thirty-one pairs of

nerves, with their end-organs, branching off from the cord, and twelve

pairs that have their roots in the brain. Branches of these forty-three

pairs of nerves reach to every part of the periphery of the body and to

all the internal organs.



It will help in understanding the peripheral system to remember that a

nerve consists of a bundle of neurone fibers each wrapped in its

medullary sheath and sheath of Schwann. Around this bundle of neurones,

that is around the nerve, is still another wrapping, silvery-white,

called the neurilemma. The number of fibers going to make up a nerve

varies from about 5,000 to 100,000. Nerves can easily be identified in a

piece of lean beef, or even at the edge of a serious gash in one's own

flesh!



Bundles of sensory fibers constituting a sensory nerve root enter the

spinal cord on the posterior side through holes in the vertebrae. Similar

bundles of motor fibers in the form of a motor nerve root emerge from

the cord at the same level. Soon after their emergence from the cord,

these two nerves are wrapped together in the same sheath and proceed in

this way to the periphery of the body, where the sensory nerve usually

ends in a specialized end-organ fitted to respond to some certain

stimulus from the outside world. The motor nerve ends in minute

filaments in the muscular organ which it governs. Both sensory and motor

nerves connect with fibers of like kind in the cord and these in turn

with the cortex, thus giving every part of the periphery direct

connection with the cortex.



The end-organs of the sensory nerves are nerve masses, some of them,

as the taste buds of the tongue, relatively simple; and others, as the

eye or ear, very complex. They are all alike in one particular; namely,

that each is fitted for its own particular work and can do no other.

Thus the eye is the end-organ of sight, and is a wonderfully complex

arrangement of nerve structure combined with refracting media, and

arranged to respond to the rapid ether waves of light. The ear has for

its essential part the specialized endings of the auditory nerve, and is

fitted to respond to the waves carried to it in the air, giving the

sensation of sound. The end-organs of touch, found in greatest

perfection in the finger tips, are of several kinds, all very

complicated in structure. And so on with each of the senses. Each

particular sense has some form of end-organ specially adapted to respond

to the kind of stimulus upon which its sensation depends, and each is

insensible to the stimuli of the others, much as the receiver of a

telephone will respond to the tones of our voice, but not to the touch

of our fingers as will the telegraph instrument, and vice versa. Thus

the eye is not affected by sounds, nor touch by light. Yet by means of

all the senses together we are able to come in contact with the material

world in a variety of ways.





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