the cell processes become continuous with the fibres of the white matter, while others anastomose with those of adjoining cells and form a plexus.

Structure of the White Matter. The white matter, found for the most part in the interior of the brain, on the surface of the spinal cord, and in almost all the nerves of the cerebro-spinal and sympathetic systems, consists of minute tubules or fibres, the ultimate nerve filaments, which in the perfectly fresh condition, are apparently structureless and homogeneous; but when carefully examined after death are seen to consist of three distinct portions, (1) a tubular membrane; (2) the white substance of Schwann; (3) the axis cylinder.

The Tubular membrane, investing the nerve filament, is thin, homogeneous, and lined by large, oval nuclei, and presents, in its course, annular constrictions; it serves to keep the internal parts of the fibre in position, and protects them from injury.

The White substance of Schwann, or the medullary layer, is situated immediately within the tubular membrane, and gives to the nerves their peculiar white and glistening appearance. It is composed of oleaginous matter in a more or less fluid condition; after death it undergoes coagulation, giving to the fibre a knotted or varicose appearance. It serves to insulate the axis cylinder, and prevents the diffusion of the nerve force.

The Axis cylinder occupies the centre of the medullary substance. In the natural condition it is transparent and invisible, but when treated with proper reagents, it presents itself as a pale, granular, flattened band, albuminous in character, more or less solid, and somewhat elastic. It is composed of a number of minute fibrillæ united together to form a single bundle. (Schultze.)

Nerve fibres in which these three structural elements coexist are known as the medullated nerve fibres. In the sympathetic system, and in the gray substance of the cerebro-spinal system, many nerves are destitute of a medullary layer, and are known as the non-medullated nerve fibres.

Gray or Gelatinous nerve fibres, found principally in the sympathetic system, are gray in color, semi-transparent, flattened, with distinct borders, finely granular, and present oval nuclei.

The diameter of the gelatinous fibres is about the go of an inch; of the medullated fibres, from 0 to 100 of an inch.

Ganglia are small bodies, varying considerably in size, situated on the posterior roots of spinal nerves, on the sensory cranial nerves, alongside of the vertebral column, forming a connected chain, and in the different viscera. They consist of a dense, investing, fibrous membrane, containing

in its interior gray or vesicular cells, among which are found white and gela. tinous nerve fibres. They may be regarded as independent nerve centres. Structures of Nerves. Nerves are rounded or flattened cords extend

ing from the centres to the periphery; they are surrounded externally by a sheath, the neurilemma, composed of fibrous and elastic tissue forming a stroma, in which blood vessels ramify, from which the nerves derive their nourishment.

A Nerve consists of a greater or less number of ultimate nerve filaments, separated into bundles by fibrous septa given off from the neurilemma. The nerve filaments pursue an uninterrupted course, from their origin to their termination; branches pass from one nerve trunk into the sheath of another, but there is no anastomosis or coalescence with adjoining nerve fibres.

A Plexus is formed by a number of branches of different nerves interlacing in every direction, in the most intricate manner, but from which fibres are again given off to pursue their independent course, e. g., brachial, cervical, lumbar, sacral, cardiac plexuses, etc.

SPINAL NERVES.

Origin. The spinal nerves are thirty-one in number on each side of the spinal cord, and arise by two roots, an anterior and posterior, from the anterior and posterior aspects of the cord respectively; the posterior roots present near their emergence from the cord a small ganglionic enlargement; outside of the spinal canal the two roots unite to form a main trunk, which is ultimately distributed to the skin, muscles and viscera.

The Function of the Anterior Roots is to transmit motor impulses from the centres outward to the periphery. Irritation of these roots, from whatever cause, excites convulsive movements in the muscles to which they are distributed; disease or division of these roots induces a condition of paresis or paralysis.

The Function of the Posterior Roots is to transmit the impressions made upon the periphery to the centres in the spinal cord, where they excite motor impulses, or to the brain, in which they are translated into conscious sensations. Irritation of these roots gives rise to painful sensations; division of the roots abolishes all sensation in the parts to which they are distributed.

The ganglion on the posterior root influences the nutrition of the sen. sory nerve; for if the nerve be separated from the ganglion, it undergoes degeneration in the course of a few days, in the direction in which it carries impressions, i. e., from the periphery to the centres; if the nerve be

divided between the ganglion and the cord, the central end only undergoes degeneration. The nutrition of the anterior root is governed by nerve cells in the gray matter of the cord; for if these cells undergo atrophy, or if the nerve be divided, it undergoes degeneration outward.

Nerve Terminations. (1) Central. Both motor and sensory nerve fibres, as they enter the spinal cord and brain, lose their external invest ments, and retaining only the axis cylinder, ultimately become connected with the processes of the gray cells.

(2) Peripheral. As the nerves approach the tissues to which they are to be distributed, they inosculate freely, forming a plexus from which the ultimate fibres proceed to individual tissues.

Motor Nerves. In the voluntary or striped muscles the motor nerves are connected with the contractile substance by means of the "motorial end plates," when the nerve enters the muscular fibre the tubular membrane blends with the sarcolemma, the medullary layer disappears, and the axis cylinder spreads out into the form of a little plate, granular in character, and containing oval nuclei.

In the unstriped or involuntary muscles, the terminal nerve fibres form a plexus on the muscular fibre cells, and become connected with the granular contents of the nuclei.

In the glands nerve fibres have been traced to the glandular cells, where they form a branching plexus from which fibres pass into their interior and become connected with their substance, and thus influence secretion.

Sensitive Nerves terminate in the skin and mucous membranes, in three distinct modes, e. g., as tactile corpuscles, Pacinian corpuscles, and as end bulbs.

The tactile corpuscles are found in the papillæ of the true skin, especially on the palmar surface of the hands and fingers, feet and toes; they are oblong bodies, measuring about of an inch in length, consisting of a central bulb of homogeneous connective tissue surrounded by elastic fibres and elongated nuclei. The nerve fibre approaches the base of the corpuscle, makes two or three spiral turns around it, and terminates in loops. They are connected with the sense of touch.

The Pacinian corpuscles are found chiefly in the subcutaneous cellular tissue, on the nerves of the hands and feet, the intercostal nerves, the cutaneous nerves, and in many other situations. They are oval in shape, measure about the of an inch in length on the average, and consist of concentric layers of connective tissue; the nerve fibre penetrates the corpuscle and terminates in a rounded knob in the central bulb. Their function is unknown.

The end bulbs of Krause are formed of a capsule of connective tissue in which the nerve fibre terminates in a coiled mass or bulbous extremity; they exist in the conjunctiva, tongue, glans penis, clitoris, etc.

Many sensitive nerves terminate in the papillæ at the base of the hair follicle; but in the skin, mucous membranes, and organs of special sense their mode of termination is not well understood.

PROPERTIES AND FUNCTIONS OF NERVES. Classification. Nerves may be divided into two groups, viz. :— (1) Afferent or centripetal, as when they convey to the nerve centres the impressions which are made upon their peripheral extremities or parts of their course. They may be sensitive, when they transmit impressions which give rise to sensations; reflective or excitant, when the impression carried to the nerve centre is reflected outward by an efferent nerve and produces motion or some other effect in the part to which the nerve is distributed.

(2) Efferent or centrifugal, as when the impulses generated in the centres are transmitted outward to the muscles and various organs. They may be motor, as when they convey impulses to the voluntary and invol. untary muscles; vaso-motor, when they regulate the calibre of the small blood vessels, increasing or diminishing the amount of blood to a part; secretory, when they influence secretion; trophic, when they influence nutrition; inhibitory, when they conduct impulses which produce a restraining or inhibiting action.

The Axis Cylinder is the essential conducting agent, the white substance of Schwann and tubular membrane being probably accessory structures, protecting the axis from injury, and preventing the diffusion of nerve force to adjoining nerves.

The properties of sensation and motion reside in different nerve fibres. Motor nerves can be destroyed or paralyzed by the introduction of woorara under the skin, without affecting sensation; the sensibility of nerves can be abolished by the employment of anesthetics without destroying motion.

Irritability. Nerves conduct peripheral impressions to the centres, and ́ motor impulses to the periphery, in virtue of their possessing an ultimate and inherent property, denominated neurility, nervous irritability, or excitability, which is manifested as long as the physical and chemical integrity of the nerve is maintained.

Nerve degeneration. When nerves are separated from their trophic or nutritive centres, they degenerate progressively in the direction in which they conduct impressions. In motor nerves, from the centre to the periphery; in sensory nerves, from the periphery to the centres.

Nerve force is not identical with electricity. Nerves do not possess the power of generating force, or of originating impulses within themselves, but propagate only the nervous impulses which are called forth by chemical, physical and mechanical stimuli from without, and by volitional acts, normal and pathological conditions from within.

Phenomena of Muscles and Nerves. The muscles are the motor organs of the body and constitute a large per cent. of the body weight. Muscles are of two kinds, striated and non-striated or involuntary. The striated muscles consist of bundles of fibres, the fasciculi, held together by connective tissue. Each muscle fibre is about 1⁄2 to 11⁄2 inches long, and possesses a delicate homogeneous membrane, the sarcolemma, in the interior of which is contained the contractile substance, which presents a striated appearance. During life this substance is in a fluid condition, but

after death undergoes stiffening.

The non-striated muscles form membranes which surround cavities, e. g., stomach, arteries, bladder, etc. They are composed of elongated cells without striations and contain in their interior one or more nuclei.

Muscular tissue is composed of water, an organic contractile substance, myosin, non-nitrogenized substances, such as glycogen, inosite, fat, and inorganic salts. When at rest the muscle is alkaline in reaction, but during and after contraction it becomes acid.

Muscles possess the properties of (1) Contractility, which is the capability of shortening themselves in the direction of their long axis, and at the same time becoming thicker and more rigid. (2) Extensibility, by means of which they are lengthened in proportion to weights attached. (3) Elasticity, in virtue of which they return to their original shape when the force applied is removed.

The contractility of muscles is called forth mainly by nervous impulses, descending motor nerves, which originate in the central nervous system; but it can also be excited by the electric current, the application of strong acids, heat, or by mechanical means.

Phenomena of a Muscular Contraction. When a single induction shock is propagated through a nerve, the muscle to which it is distributed undergoes a quick pulsation, and speedily returns to its former condition. As is shown by the muscle curve, the contraction, which is at first slow, increases in rapidity to its maximum, gradually relaxes and is again at rest, the entire pulsation not occupying more than the of a second.

The muscular contraction does not instantly follow the induction shock, even when the electrodes are placed directly upon the muscular fibres themselves; an appreciable period intervenes before the contraction,