CHAPTER XLVII.

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.

[Verse: "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_.

[Verse: "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.

[Verse: "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_.

[Verse: "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_.