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Thou art' where friend meets friend,
Beneath the shadows of the elm to rèst;
Thou art! where fóe meets fòe, and trumpets rend
Leaves' have their time to fáll,
And flowers to wither at the north-wind's breath, And stárs to sèt—but àll,
Thou hast all seasons for thine own, O Death!
THE NERVOUS SYSTEM.
Anterior, (L., from ante) fore, before
Choroid, (chorion, eidos, G.)
Cornea, (cornu, L.) Lit., horny.
Crystalline, (crystallos, G.) clear, resembling crystal.
Ganglion, (G. Pl. ganglia,) a small mass of nervous matter.
Iris, (L. and G.) Lit., the rainbow.
Posterior, (L. from post) hinder, after
THE NERVOUS CENTRES AND THE NERVES.
THE human body, as we have learned in former lessons, is moulded upon a framework of bones, which are capable of being moved by the excitement and contraction of the muscles attached to them. It is by means of the nervous system that this excitement is produced, and the consequent motions regulated. The same system is also the seat of sensation, that is, of the faculties of seeing, hearing, tasting, smelling, and touching, which give us our knowledge of surrounding objects, and of those feelings, such as hunger, thirst, weariness, and the like, which make us aware of the condition of our own organs. And, still further, the same nervous system contains the organ of intelligence, and thus is connected, in some mysterious way, with the spiritual and immortal part of our nature.
There are two kinds of matter of which this nervous system is composed. First, a grey cellular substance, generally collected in masses called centres; and, secondly, a white fibrous substance, of which are formed, besides portions of the centres themselves, the whole of the nerves connecting these centres with all parts of the body. The great nervous centre of the human body is made up of the brain and spinal cord, with which the nerves of motion and sensation are connected. There are, however, smaller masses of nervous matter, known as ganglia, in different parts of the body. These ganglia send out nerves to the heart, liver, intestines, and other internal organs.
The brain is generally regarded as consisting of two parts, the brain proper (a a) (fig. 20), and the little brain (b).
The brain proper is divided longitudinally into two hemispheres, on the outer surface of which the grey nervous matter is arranged in folds or convolutions, as seen in the figure. These two hemispheres are united by an internal mass of the white matter of which nerves are composed. Below this are several ganglia, in immediate connection with the upper and most important part (cc) of the spinal cord, which, being thicker than the rest of the cord, is often called
the bulb. With the latter is also connected the little brain, which is situated behind, and presents in its anterior an appearance somewhat similar to the branches of a tree. It is composed of the same substances as the brain proper; and so also is the spinal cord, only in the latter the grey matter is inside, and the white outside.
Forty-three pairs of nerves proceed from the great centre which has just been described. Twelve of these pairs have their origin within the skull, and the remaining thirty-one in the spine. Each of the spinal nerves has two roots, one in the posterior, the other in the anterior part of the spinal cord. Experiment has shown that the posterior root is a nerve of sensation, the anterior a nerve of motion. The two combine together to form a compound nerve, but, at the other end, the former spread themselves over the skin, the latter terminate in the muscles.
The nerves keep up a kind of telegraphic communication between the muscles, the organs of sense, and the general surface of the body on the one hand, and the brain and spinal cord on the other. The brain seems to be the immediate residence of the mind, but the nature of their connection has ever been, and most probably will ever be, unknown to the wisest of men. We know, however, some of the results of their union. No sooner has the mind willed or resolved that any one of the fingers shall be moved, than there issues a message to that effect along the nerve that leads to the muscle whose services are wanted. Immediately the muscle performs, by contraction, the duty required of it, and the finger moves. Try, now, how fast such a movement can be performed, and remember that, every time a single finger wags backwards and forwards, two such messages must be forwarded and obeyed. Yet we feel no difficulty in wagging all the fingers at once, and that with great rapidity, moving at the same time the head, the arms, the legs, and the toes!
There are other motions not dependent on the will, and some have been wisely placed altogether beyond its control. For example, the heart never ceases to beat, even when we
are asleep. It would not be well for us if its motion were under our management or dependent on our care, for we should be apt to let it stop, and when it stops we die. To understand the origin of such involuntary motions, notice what happens when anything approaches too near the eye. That organ, so easily injured, has been made cautious, if the expression may be allowed, in guarding itself against danger. As soon as any object threatens to touch it, an alarm is despatched to the nervous centre, and forthwith an answer returns, in virtue of which the proper muscles contract, and the eyelid closes. An impression is telegraphed inwards, and, whether it produces a sensation or not, is followed by a message outwards, which regulates the motion to be performed.
It is scarcely necessary to add that this whole subject borders on a region where much is dark and unknown. Verily "we are fearfully and wonderfully made." The mechanism itself we may examine and describe, but its union with a living and life-giving spirit, and the mode of that spirit's action upon it, are entirely beyond our comprehension.
THE brain appears to be the seat of sensation, as well as of intelligence and will. To it a vast multitude of impressions, made upon the internal or external organs, are conveyed by the connecting nerves. In the case of the external senses, of which five are usually enumerated, the organs may be said to collect, the nerves to carry, intelligence. By virtue of this varied apparatus, the mind sees, hears, tastes, smells, and feels: the particular nature of each sensation depending on the organ from which it comes, and on the object by which it is excited. Thus, snow gives us, through the eye, the sensation of whiteness; through the tongue or fingers, the sensation of cold. Salt appears similar to the eye, but has a very different effect upon the tongue.
Of the organs of sense, the eye
is that whose construction
is best understood. In a full-grown man, it is a ball or
globe of about nine-tenths of an inch in diameter, formed by a very dense, tough, and opaque white fibrous membrane, called the sclerotic coat, which surrounds and protects the internal parts. Into the front of this spherical chamber is inserted a circular transparent portion, like a very small watch-glass, which serves the purposes of a window. This part, which is nearly half an inch in diameter, is called the cornea. A little behind the cornea is the iris, a sort of circular curtain for the purpose of regulating the quantity of light to be admitted. The iris has a round hole or opening in its centre, called the pupil, which contracts in strong light, and expands again when the light is diminished. It is the iris that determines the colour of different eyes. It is seen through the cornea as blue, black, or brown, while the pupil appears as a black spot in the centre. Continuous with the iris, and extending backwards as an internal lining beneath the sclerotic coat, is a dark membrane, called the choroid coat, and within it again, like a second lining, is another membrane, called the retina. The retina is connected with the optic nerve, which, proceeding from the back part of the eyeball, connects the eye with the brain. But the most exquisite piece of mechanism in the eye has yet to be noticed. This is the crystalline lens, which is situated