Page:EB1911 - Volume 19.djvu/419

 common to the whole system, or for which a place does not conveniently occur elsewhere.

Three kinds of tissue are found in the nervous system, nerve fibres, nerve cells, and a supporting tissue called neuroglia. may be medullated or non-medullated, but, whichever they are, they consist of the long process or axon of a nerve cell; in a non-medullated nerve this process is either naked or enclosed in a delicate membrane called the primitive sheath or neurilemma but in a medullated nerve the process or axis cylinder is encased by a white. fatty substance called myelin, and so the term “myelinated” is often used instead of “medullated” for these nerves (see fig. 1). Outside this white sheath the neurilemma is present in most nerves, but is lost when they are massed to form the white matter of the central nervous system and in the optic nerve. At regular intervals the myelin is interrupted by some substance which stains deeply with silver nitrate, and these breaks are known as nodes of Ranvier. They do not, however, affect the axis cylinder. In a large nerve, such as the median, the nerve fibres are collected into small bundles called funiculi, enclosed in a connective tissue sheath, the perineurium, and separated from it by a lymph space. From this sheath delicate processes penetrate among the fibres, and these are known as the endoneurium. The funiculi are collected into bundles called fasciculi, and the whole nerve consists of a variable number of fasciculi surrounded by a dense fibrous sheath, the epineurium. The various bundles do not remain distinct, but break up and rearrange themselves, so that following them up with the scalpel is a difficult and tedious work. The nerve fibres, however, never join one another and are often several feet in length.

are unipolar, bipolar or multipolar. Unipolar cells are found in the ganglia on the posterior roots of the spinal nerves, and only give off an axon or axis cylinder process; this, however, soon divides in a T-shaped manner, and all these cells were originally bipolar, though the cell has grown away from its two axons (or, as they are often regarded, axon and dendrite), leaving a stalk joining it to them at right angles. Bipolar cells are found as an embryonic stage of unipolar, though in fish they persist in the spinal ganglia throughout life. They are also sometimes found in the sympathetic ganglia. Multipolar cells are found in the brain and cord, and are best studied in the anterior horns of the grey matter of the latter, where they are nearly visible to the naked eye (see fig. 2). Of their many processes only one is an axon, and it becomes the axial cylinder of a motor spinal nerve. The other fibres are called dendrites, and break up into delicate branches some of which surround, but, it is generally believed, are not actually continuous with, neighbouring cells or their processes. It is known that the axons are made up of delicate fibrils, and it is thought by some observers that there is actual continuity between some of these and those of an adjacent neuron, as the combination of a nerve cell, its axon and dendrites, is called. The cells of Purkinje in the cerebellum show a particularly rich arborization of dendrites (see, fig. 7). Nerve cells have generally a large clear nucleus.

is the delicate connective tissue which supports and binds together the nervous elements of the central nervous system. One part of it, which lines the central canal of the cord and ventricles of the brain, is formed of columnar cells, and is called ependyma, while the rest consists of small cells with numerous processes which sometimes branch and sometimes do not. These fibres interlace with one another to form a delicate felt-work which is unmixed with nervous elements on the surface of the grey matter of the brain (see, figs. 7 and 15), though elsewhere it is interwoven with them.

.—Sensory nerves end by breaking up into fibrillae or by various tactile organs. In the former case the minute fibrils, of which it has been shown that the axons or nerve fibres consist, separate and end among epithelial cells of the mucous membrane or skin. In the latter case the nerve fibres lose their coating of myelin and end in one of the seven following organs:—

1. End bulbs of Krause (fig. 3, A), oval bulbs composed of elongated cells among which the nerve fibrils end in knobs or coils; each is surrounded by a sheath of neurilemma, and the organs are found in the lips, tongue, conjunctiva, epineurium of nerves, synovial membranes of joints, and in the glans penis et clitoridis, where they have a mulberry-like appearance.

2. Pacinian corpuscles (fig. 3, B) are large enough to be seen by the naked eye, and are oval bodies made up of a series of concentric capsules of connective tissue rather resembling the structure of an onion; in the centre of this is a structureless core, at the distal extremity of which the nerve fibre ends in one or more knobs. These bodies are found in the palm and sole, in the mesentery, the genital organs and in joints. 3. Tactile corpuscles of Meissner and Wagner (fig. 3, C) are oval bodies found in certain of the skin papillae and mucous membrane, especially of very sensitive parts like the hand and foot, lips, tongue and nipple. They are oval and made of a connective tissue capsule from which septa enter the interior. The nerve fibre generally takes a spiral course through them, loses its myelin sheath, and ends by breaking up into its fibrils, which eventually become bulbous.

4. Tactile corpuscles of Grandry are found in the skin of those parts devoid of hair, and consist of a capsule containing two or more largish cells, between which the nerve fibre ends in the so-called tactile discs.

5. Ruffini’s endings are flattened oval bodies with a thick connective tissue capsule, in which the nerve fibre divides into many