Besides this power which the nervous system possesses of receiving impressions originating outside of itself and actively replying to them, it appears also to possess the power of originating within itself changes the result of which are sensations, movements, and trophic alterations. In other words, it can act automatically.
The apparatus for the performance of these various functions consists of the end-organs, the nervous centres, and the nerves.
The end-organs are peripheral mechanisms for the reception of impressions. The structure and mode of action of some of them, as the eye and the ear, are pretty well understood, while others, as those connected with the sense of touch, temperature, etc., are but imperfectly known. It is probable that there are also peripheral mechanisms which facilitate the delivery of the impulses coming from the nerve-centres to the organs, tissues, muscles, glands, etc.
The nervous centres are made up of nerve-cells variously connected with each other. They are immediately concerned in receiving impressions conveyed to them by the nerves and transforming them into sensations, or transmitting them to other organs, causing reflex actions, or in originating sensations and impulses.
The nerves are organs which, connected at one extremity with the end-organs and at the other with the nervous centres, convey peripheral impressions to the centres, and impulses and influences from the centres to the various organs of the body.
As it is with diseases of the peripheral nerves that we are now concerned, let us begin by looking more closely into their structure and functions.
The nerves appear to the naked eye as white strands of variable size, which a close inspection shows to be made up of threads or fibrils (best seen when the cut end of a nerve is examined) bound together by fine connective tissue and scantily supplied with blood-vessels. A microscopic examination shows that each of the fibrils visible to the naked eye is made up of a great number of fibres. These are the medullated nerve-fibres, and they extend unbroken between the nerve-centres, with the cells of which they are connected, to the various organs and tissues, with which they also enter into organic union.
If we examine the structure of a medullated nerve-fibre, we find it to consist of a central thread called the central axis or axis-cylinder, in which close microscopic investigation shows a longitudinal striation, indicating that it is made up of fibrillæ. Surrounding the central axis like a sheath is the white substance of Schwann, composed of an oleo-albuminous substance, myeline, to which the nerves owe their white appearance. According to some observers, the white substance of Schwann is pervaded by a meshwork of fibres. Surrounding the white substance of Schwann is the sheath of Schwann, a structureless membrane having at intervals upon its inner surface nuclei, around which is a small amount of protoplasm.
At intervals along the course of the nerve-fibres are seen constrictions which involve the sheath and white substance of Schwann, but which do not affect the central axis, which passes unbroken the points of constriction. These are the nodes of Ranvier. Each space on the fibre beneath the nodes of Ranvier contains one of the nuclei of the sheath of Schwann, and probably, together with the white substance of Schwann, represents a cellular element. Diseased conditions sometimes respect the limits of these cellular elements.
The central axis is the true conducting part of the nerve-fibre, and it is probable that each of the fibrillæ of which it is composed has a separate peripheral termination and possesses the power of isolated conduction. The white substance of Schwann and the sheath of Schwann protect the central axis and seem to be connected with its nutrition.