or by the electrometer (pp. 307, 313). Movement as evidence of the passage of motor impulses, the negative variation as evidence of the passage of centripetal or centrifugal impulses, are objective signs. Sensation, as evidence of the passage of sensory impulses, is a subjective' sign. Movement, as evidence of the passage of sensory impulses, is a mixed sign-i.e. a movement is seen, and inferred to be an expression of sensation. Objective signs are less liable to fallacy than subjective or mixed signs. Hence, as a rule, it is easier to ascertain loss of motion than loss of sensation.

Nerve, in common with all protoplasm, possesses excitability, and the salient characteristic of this excitability in the case of nerve is its transmission along the fibre; hence it is termed conductivity. The transmission is not, as in the case of circulation, any actual transmission of matter, but only the transmission of a state of matter from particle to particle. The direct local excitability of nerve to artificial stimuli, applied at any point of its course, is sometimes distinguished from its indirect excitability to the impulse starting from that point and transmitted onwards. The distinction has its justification in experimental and in clinical facts, for it sometimes happens that the local excitability to a direct stimulus is lost at a part of the nerve that can still be traversed by impulses transmitted from above. This is apt to occur clinically during the progress of recovery from paralysis, owing to previous lesion or disease of motor nerve; it has been observed that muscles may be voluntarily set in action through motor nerves which have not yet recovered their electrical excitability. The converse may be experimentally demonstrated, viz. a nerve may be rendered impermeable while still remaining directly excitable.

Stimuli may be natural or artificial. Natural stimuli may arise in the brain or at the periphery, and their consequent impulses may pass from the brain to the periphery, or from the periphery to the brain. All appreciable qualities of objects in the surrounding world are natural stimuli at the sensory periphery, all consequent volitional impulses and ideas of intended movements are natural stimuli at the supreme motor centre. The natural stimuli at the sensory periphery are the physical qualities of objects that excite smell, sight, hearing, taste, touch, and possibly muscular sense,' thermic sense,' and 'pathic sense.' To these qualities we give the names smell, light, sound, taste, smoothness or roughness, resistance and extension, weight, heat, cold, pain. All these stimuli affect us best when we meet them

half-way by some action of our own, by a state of voluntary attention, or by actual movements. An object is best seen when it is looked for, a sound best heard when it is listened for. We generally sniff to smell, and move in our mouths substances that are to be tasted, and movement on our part is necessary for anything like delicate appreciation of the nature of a surface, or the weight and size of an unseen object.

Artificial stimuli are such as are applied experimentally. They may be mechanical, chemical, thermic, or electrical, and the latter may be in the form of induced, constant, or static electricity. Of these various kinds of stimuli, the electrical, in the form of induced currents, is that of most frequent and convenient experimental application. The effects of the constant current have also been exhaustively studied. Those of static electricity are only incidentally observed, this form being rarely employed in the laboratory; 'unipolar stimulation' (i.e. only one pole being applied to the nerve) is due to a discharge of static electricity, and is demonstrated as a fallacy to be guarded against in experiments.

Mechanical stimulation.-A sudden smart blow upon a motor nerve causes muscular contraction; a blow upon a sensory nerve causes pain. This is easily experienced on the ulnar nerve, which is a mixed nerve composed of sensory and of motor fibres. When it is struck just where it lies upon the bone, there result the sensation of tingling or pain, and a twitch of the muscles that are served by the nerve. In the laboratory, mechanical stimulation is applied by means of Heidenhain's tetanomotor, or of an instrument devised for the same purpose by Tigerstedt. This form of stimulation supplies the best available means for the demonstration of electrotonic alterations of excitability in the intrapolar region of a nerve during the passage of the constant current.

Of thermic stimuli there is little to be said; if heat be applied suddenly it may act as a stimulus; also, upon a change from a medium of low to one of high temperature, nerves are prone to fall into a tetanic state, the muscles that they supply becoming contracted, and so remaining for long periods. The action of chemical stimuli has been carefully studied, especially as regards their comparative action upon muscle and upon nerve, and, apart from experiments in which the fallacy due to self-excitation through closure of a nerve or muscle-current could occur, we may name ammonia as acting specially upon muscle, while

glycerin or common salt would be selected for the excitation of nerve; removal of water is in this case the excitant-a nerve is stimulated as it dries.

Effects of the constant current. These are studied upon nervemuscle preparations. The nerve is laid across two unpolarisable electrodes, through which it receives the current from a battery. The effects to be studied are-(1) those that take place when the current commences and ceases to flow through the nerve, i.e. at make and at break; (2) electrotonic currents, which spread along the nerve on either side of the two electrodes; (3) alterations of excitability, which accompany these electrotonic currents.

Make and break effects. Pflüger's law. When the current is made or when it is broken, or at both these events, there is contraction of the muscle, which is evidence that the nerve has been stimulated. In other words, the constant current stimulates the nerve when it commences and when it ceases to pass, but does not stimulate the nerve while it is passing. If now attention be given to the strength of current and to its direction, it is found that the contractions at make and at break of the current appear or fail to appear in a regular order. This is called Pflüger's law of contractions.

The terms ascending and descending are those in common use, and signify direction of current in the nerve in relation to nerve-centre. Current is ascending' when its direction in the nerve is from muscle towards centre, 'descending' when its direction in the nerve is from centre towards muscle. But it simplifies matters to attend particularly to the points where the current enters and leaves the nerve; the point where the current enters is the anode (+), the point where the current leaves is the kathode (-).

The explanation of the above 'law' or formula is as follows:As stated below, when a current commences to flow, i.e. at make of a current, excitability of the nerve is diminished at and near the anode, increased at and near the kathode. A sudden increase of excitability is equivalent to a stimulus; therefore at make of a current the stimulus is at the kathode. As stated in the next paragraph, when a current ceases to flow, i.e. at break of the current, excitability at and near the anode suddenly recovers up to and beyond its normal level, while at and near the kathode it suddenly falls down to and beyond its normal. A sudden change of excitability from below normal to such normal, or above it, is equivalent to a sudden increase of excitability, and furnishes a

stimulus; therefore at break of a current the stimulus is at the anode.

With a current of 'medium' strength there is contraction at make and break of ascending and of descending currents, con

tractions at make being excited from the kathode (-), contractions at break being excited from the anode (+), and the formula reads

With a 'weak' current only the more efficient stimulus is effective. The sudden increase at the kathode when the current is made, is more effectual than the sudden release at the anode when the current is broken. Hence, with a weak current contractions appear at make only with either direction of current, and the formula reads

With a 'strong' ascending current the point of stimulation at make is the kathode, but between it and the muscle lies the anode at which excitability is diminished. With such a strong current the diminution is sufficient to block the passage of the stimulus from the kathode. Hence the make contraction fails to appear. At break, the stimulus at the anode, having no obstacle between it and the muscle, produces a contraction. With a strong

descending current the anode does not separate the kathode from the muscle, and the make stimulus at the kathode produces a contraction. At break, however, the stimulus is produced at the anode, between which and the muscle there intervenes the kathode where excitability suddenly diminishes; the diminution is sufficient to block the stimulus from the anode, and the formula reads

Ritter's tetanus.-It frequently happens that at break of the galvanic current the muscle enters into tetanus; this is due to an after-anodic excitation, and the effect is also demonstrable by the galvanometer (positive polarisation current,' p. 370).

The formula of contraction on man.-The above statements are based upon experiments with the nerves of frogs; experi