The separate microscopic threads or fibers, bound together in cords of variable size, form the nerves. Each strand or cord is surrounded and protected by its own sheath of connective tissue, made up of nerves. According to its size a nerve may have one or many of these strands. The whole nerve, not unlike a minute tendon in appearance, is covered by a dense sheath of fibrous tissue, in which the blood-vessels and lymphatics are distributed to the nerve fibers.
[Illustration: Fig. 112.--Medullated Nerve Fibers.
A, a medullated nerve fiber, showing the subdivision of the medullary sheath into cylindrical sections imbricated with their ends, a nerve corpuscle with an oval nucleus is seen between the neurilemma and the medullary sheath; B, a medullated nerve fiber at a node or constriction of Ranvier, the axis cylinder passes uninterruptedly from one segment into the other, but the medullary sheath is interrupted. ]
263. The Functions of the Nerve Cells and Nerve Fibers. The nerve cells are a highly active mass of living material. They find their nourishment in the blood, which is supplied to them in abundance. The blood not only serves as nourishment, but also supplies new material, as it were, for the cells to work over for their own force or energy. Thus we may think of the nerve cells as a sort of a miniature manufactory, deriving their material from the blood, and developing from it nervous energy.
The nerve fibers, on the other hand, are conductors of nervous energy. They furnish a pathway along which the nerve energy generated by the cells may travel. Made up as they are of living nerve substance, the fibers can also generate energy, yet it is their special function to conduct influences to and from the cells.
[Illustration: Fig. 113.--Non-Medullated Fibers.
Two nerve fibers, showing the nodes or constrictions of Ranvier and the axis cylinder. The medullary sheath has been dissolved away. The deeply stained oblong nuclei indicate the nerve corpuscles within the neurilemma.]
264. The Nervous System Compared to a Telegraphic System. In men and other highly organized animals, nerves are found in nearly every tissue and organ of the body. They penetrate the most minute muscular fibers; they are closely connected with the cells of the glands, and are found in the coats of even the smallest blood-vessels. They are among the chief factors of the structure of the sense organs, and ramify through the skin. Thus the nervous system is the system of organs through the functions of which we are brought into relation with the world around us. When we hear, our ears are bringing us into relation with the outer world. So sight opens up to us another gateway of knowledge.