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_. [Illustration: Fig. 59.--SHOWING THE DISTRIBUTION OF NERVES AND VEINS, AND ILLUSTRATING THE MANNER IN WHICH THEY PASS THROUGH THE FLESH TO REACH THE PARTS TO WHICH THEIR FUNCTIONS BELONG.] 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. [Illustration: Fig. 60.--MUSCLES OF THE HEAD AND FACE, WITH NERVES DISTRIBUTED THERETO.]