the case of the winged insect, with head, antennæ or feelers projecting from the head, wings, and legs, leading to a more marked appearance of separate combinations, giving greater prominence to the head. When from this we rise to the fish, thence to the bird, thence to the quadruped, we find the head made conspicuously the central organ of the entire nerve system of the animal, while it occupies the front position in the body. It is no longer one of a set or series of knots; nor even the largest or more conspicuous in a graduated order of centres; but in the head of the animal is found that which is the true nerve centre for the whole nerve system, designated the brain. In the case of the vertebrates, not only does the skeleton afford the solid frame-work on which the muscular system is built, but the back-bone contains within it the main column of nerve fibres, which are given out at the several joints according to the requirements of the body.

If meanwhile we concentrate attention on our own bodies, we may by the aid of personal experience find easy illustration of the prominent features of the nerve system. We shall take first the two distinct lines of nerves

already mentioned, the one set concerned with sensibility, the other with movement of the muscles. From the tips of the fingers there run lines of nerve fibre, which are brought into combination at the wrist, and are carried up the arm, and onward by the shoulder and upper portion of the back-bone to the head. These are the nerves of sensibility, by means of which, as by telegraph wires, the slightest impression made on the tips of the fingers is instantly conveyed to the great nerve centre in the brain. Distinct from these is another set of nerves issuing from the brain, and descending the arm, giving off its fibres as it passes to the several muscles above the elbow, next to those above the wrist, and next to the muscles of the hand and fingers. These are the nerves of movement, by means of which the whole arm may be brought into action at pleasure, or the hand may be set to work, while the arm is at rest.

These two sets of nerves-the sensory and motor-are exactly similar in structure, consisting of an outer covering, within which floating in a white fluid is a thread which constitutes the nerve proper. The outer covering provides for isolation of the fibre, from

other fibres laid alongside of it, just as copper wire is isolated by a gutta-percha covering when the two connecting lines from an electric battery are laid down in close proximity as in the arrangement for electric bells. By this provision the nerve fibres are completely isolated making it possible to distinguish sensory impressions so as to tell which finger has been touched. The similarity of structure in the two lines of nerves is a striking fact in view of the completely distinct functions fulfilled. This leads to a special explanation of the provision for different modes of action. This is secured by diversity in the terminal arrangements for the two classes of nerves. The nerves of sensibility have a peculiarly sensitive arrangement spread under the skin, constituting an end-bulb or touch organ. In certain parts of the body more sensitive than others, such as the tips of the fingers, there are additional minute corpuscles, grouped alongside of the nerve, liable to contract under the slightest pressure, and which add greatly to the sensitiveness of the particular parts about which they cluster. The terminal arrangements of the motor nerves are quite different. The nerve fibres pass into

the substance of the muscle to be moved by them, and the nerve fibre is subdivided and distributed, so as to bring the several parts of the muscle under control. These fibres are so laid and connected, that a whole set of muscles can be moved simultaneously, being made to work in perfect harmony.

The vital activity of this whole arrangement of nerve fibres, including sensory and motor in one system, depends upon living connection of all with the great nerve centre in the brain, where the nerve energy is provided which keeps all in functional activity. Only, there is this striking difference with the two sets of fibres, that in the case of the sensory nerve the pulsation of energy is upwards to the brain, in the case of the motor nerve it is downwards towards the muscle. There is no scientific explanation yet reached of this contrast of molecular action. But by means of it the one order of nerves plays the part of a vehicle of impression providing for knowledge of what is without, the other order fulfils the part of an instrument for moving the muscular system which is part of the organism itself.

These two orders are not, however, to be