951. What are nerves?

The nerves are branches of the brain and the spinal cord; they are distributed in great numbers to all the active and sensitive parts of the body.952. What is the spinal cord?

The spinal cord is a long and large cord of nervous matter, which extends from the brain through a continuous tube formed by corresponding hollows in the bones of the back. It serves as a nervous trunk for the distribution of nerves, just as the aorta distributes branches of blood-vessels.953. Why is the spinal cord placed in the grooves formed by the back-bone?

Being a very vital part of the system, and from the delicacy of its structure liable to injuries, it is set in the back-bone for protection; and so great is its security that it is only by force of an unusual kind that it can be injured.

"A sound heart is the life of the flesh: but envy is the rottenness of the bones."—Proverbs xiv.

954. How can branches proceed from it, if it is so securely encased in bone?

Because in the bones, on each side of the spinal cord, there are smaller grooves for the transmission of the nervous branches.955. Of what does the nervous system consist?

Of the brain, the spinal cord, and the branches which are called nerves.

A. B. Veins of the fore-arm.

B. Canal formed in the muscle, through which a trunk-vein emerges.

C. Canal formed in the muscle, through which a large nerve emerges.

D. Canal through which a vein enters to communicate with the deep muscles of the arm.956. What is the constitution of a nerve?

It consists of a thin membrane, or sheath, surrounding a greyish oily matter, which forms the nervous marrow. In the centre of this marrow is usually found a small fibre, which is supposed to be the essential part of the nerve; and most nerves consist of a number of these sheaths enclosing fibres running in parallel directions.957. What is the nervous fluid?

The term nervous fluid is used to express our ideas of the mode by which the brain and spinal cord influence the remote parts: just as we say the electric fluid, without knowing that such a fluid exists. It is the most convenient form of expression.958. How many classes of nerves are there?

There are:—

1. The nerves of motion.
2. The nerves of sensation.
3. The nerves of special sense.
4. The nerves of sympathy.

"Having many things to write unto you, I would not write with paper and ink; but I trust to come unto you, and speak face to face, that our joy may be full."—II John.

959. What are the nerves of motion?

The nerves of motion are those which, in obedience to the will, stimulate the muscles to act, and apportion the amount of stimulation they convey to the degree of exertion required.

A A A. The facial nerve emerging from underneath the ear, and distributing branches to the cheeks, temple, forehead, &c. This nerve excites the muscles of the face, and is chiefly instrumental in producing the expressions of the countenance under the changing emotions of the mind.

B B B. Muscles by which various motions are imparted to the head, face, mouth, &c., under the stimulus of the nerves.960. What are the nerves of sensation?

The nerves of sensation are those which impart a consciousness to the brain that its commands to the nerves of motion have been obeyed, and how far they have been fulfilled.

"Oh that men would praise the Lord for his goodness, and for his wonderful works to the children of men."—Psalm cvii.

Let us perform a simple experiment, which will more clearly illustrate the phenomena of motion and of sensation, which we are now describing, than a great deal of writing upon the subject. You hold in your hand this book: close it, and set it upon the table; lay your hands passively upon your lap, and then will your hand, to take up the book, which is the same as to say, command your hand to take up the book. What occurs? The hand, immediately obeying your desire, stretches forward to the book, and takes hold of it. How do you know that you have hold of it? You see that you have: but were your eyes closed, you would be equally aware that the hand had reached the book, and fulfilled your wishes. It is by the nerves of sensation that you are made aware that the hand has fulfilled your instructions.

Consider what took place in the simple action. In the first instance, a desire arose in your mind to take up the book. The brain is the organ of the mind; and having branches either proceeding from itself, or from the spinal cord, to every part of the body—branches that traverse like telegraphic wires throughout every part of the system,—it transmitted instructions along the nerves that proceed to the muscles of the arm and hand, directing them to take up the book. This was done instantly; and as soon as it was done you became conscious that your will had been obeyed—because the nerves sent back a sensation to the brain acquainting it that the book had been taken up, and that at the moment of the dispatch it was in the firm hold of the hand.

In all the varied motions of the body this double action of the nerves takes place. It is obvious that without an outward impulse from the brain, upon which the desire of the mind first made an impression, no motion of the muscles of the arm and the hand could have taken place; and it is also obvious that without an inward impulse from the nerves to the brain you would not have known that the muscles had fulfilled your instructions. The hand might have dropped by the side of the book, or have gone too far, or not far enough, and you would not have been aware of the result, but for an inward communication through the nerves.

We are not now speaking of the nerves which endow us with the sense of feeling, because they are regarded as separate and distinct from those nerves that produce in us consciousness of muscular response. When we walk, rise, or sit, we are made conscious, without any special feeling being exerted, that the muscles have placed the limb, or the body, in the desired position, that it is set down safely and firmly, and that we may repose upon it securely without further attention. We refer the impressions made by the book upon the nerves of the hand, and which enable us to tell whether it feels hot or cold, whether its surface is rough or smooth, and so on, to the special sense of feeling. The consciousness of muscular action is a separate and distinct function; and it is generally believed that the same nerves that convey the command of the will outward, bringing back the intimation that the will has been obeyed, but that different fibres of the nerves convey the outward and the inward impulses. A single nerve may therefore be likened to a double wire connected with the electric telegraph: one transmitting despatches in one direction, and the other in the opposite direction.961. What are the nerves of special sense?

The nerves of special sense are those through which we hear, see, feel, smell, and taste.

"For the Lord seeth not as man seeth; for man looketh on the outward appearance, but the Lord looketh on the heart."—Samuel xvi.

962. What are the nerves of sympathy?

The nerves of sympathy, or the system of sympathetic nerves, are those which are distributed to the internal organs, and which are independent of the will. They regulate the motions of the heart, the lungs, the stomach, &c., and stimulate the organs of secretion, so that those organs work in harmony with each other.

As the internal organs are all more or less dependent upon each other, and unite their functions for similar ends, it is obvious that there should prevail among them a mutual consciousness of their state. Otherwise, when the stomach had formed chyme, the liver might have no bile ready to fulfil its office; the absorbents might be in a state of rest at the moment when nutrition was set before them; and the heart might beat slowly, while the lungs were in active exertion to obtain additional blood to support an active exercise. The sympathetic system of nerves therefore regulates and harmonises these internal functions.