are associated together to form large plexuses containing collections of nerve-cells. Three such plexuses exist: namely, the pre-vertebral cardiac plexus, placed at the base of the heart. The pre-vertebral solar, or epigastric plexus, placed around the coeliac axis. The pre-vertebral hypogastric plexus placed in front of the last lumbar vertebra. From these large plexuses branches are distributed to the thoracic, abdominal, and pelvic viscera. The nerves of the sympathetic system consist almost exclusively of the grey or non-medullated variety.

Properties and Composition of Nerves.

Nerve tissue contains a considerable quantity of water, varying from 70 per cent in the white to as much as 85 per cent in the grey substance.

The chemical composition of the brain may be stated as follows:

The albuminous substances are chiefly derived from the axis cylinder and the protoplasm of the cells, and they appear to be formed of a substance which is supposed to resemble fibrin and myosin; it is, however, not soluble in dilute acids, or in a solution of potassium nitrate. The albumin of the medullary substance is soluble in dilute acids, and resembles casein. The salts, which form about 2 per cent, are composed chiefly of phosphates. The chief extractives are elastin, kreatin, leucin, xanthin, hypo-xanthin, lactic, and uric acids. Nerves, when in a state of rest, present a neutral or faintly alkaline reaction, but they become acid after death or on long-continued irritation.

Uses of Nerve Tissue.

Nerve-fibres are capable of conducting impulses from the point to which a stimulus has been applied either towards the centre or to the periphery. Those which conduct towards the periphery are called motor or efferent, and those which conduct towards the centre are called sensory or afferent nerves. Most of the spinal nerves are mixed nerves: that is, they consist of both motor and sensory fibres, and

these, though associated together in the same nerve, are separate at their origin. Each spinal nerve arises by two roots—an anterior, which is motor; and a posterior, which is sensory, and has a ganglion on it. In man, a sensory nerve impulse has been calculated to travel at the rate of about 140 feet a second.

Nerve-cells are either automatic, originating the stimuli which are conveyed to the efferent nerves; or they are reflex centres, receiving a stimulus from a sensory nerve and transferring it to a motor; or to another sensory nerve. When an impulse passes through a nerve cell a considerable augmentation of energy takes place.

CHAPTER VII.

Classification of Muscle-Parts of a Muscle-Microscopical appearances and structure of Muscle, Fibres, and Fibrilla-Connexion of Fibres with their Tendons -Vascular Supply -- Nerves and Lymphatics of Muscles-Structure of unstriated Muscle and of the muscular Fibre of the HeartPhysical properties and chemical Composition of Muscle-The uses of muscular Tissue.

MUSCLE.

CLASSIFICATION OF MUSCLE.-In speaking of muscle tissue we may classify it either histologically, and divide it into the striated, and unstriated, or physiologically, when it may be divided into the quick-contracting, and slow-contracting—the quick-contracting being represented by the striated, and the slow-contracting by the unstriated variety. Muscle is frequently classed under the heads voluntary and involuntary, the striated variety representing the voluntary and the unstriated the involuntary; but this classification is misleading, as we have the notable instance of the muscle of the heart, which is striated but involuntary, and the equally important ex

ception of the ciliary muscle of the eye, which is unstriated but voluntary in its action. In birds the ciliary muscle is of the striated variety.

STRUCTURE OF MUSCLE.-The fibres of striated muscles are aggregated together to form distinct masses or muscles, which vary much in size and shape. They terminate at each end in a tendon,-either abruptly or by a process of gradual continuity,—by which they are attached to the bones. The muscular fibres, which are made up of minute fibrils, are collected into bundles called fasciculi. The fibrils run parallel with one another in the fibres, and the fibres are parallel in the fasciculi. The fasciculi also are, as a rule, parallel with one another, although some converge towards the terminal tendon; but in the ordinary skeletal muscles they do not anastomose. Every muscle is invested externally by a continuous sheath of more or less dense areolar tissue, which sends in processes between the fasciculi or bundles of fibres, by means of which they are associated together to form the muscle (P. V. Fig. 18). Finer prolongations of the sheath, entering