The therapeutical relations of the nervous system are as extensive as those of the whole body itself. Pain, for example, is constantly associated with local disease, and many of the most distressing diseases of the viscera are disturbances of nervous mechanisms. Here we must confine ourselves chiefly to the therapeutical relations of the higher nervous centres, representing sensation, consciousness, and voluntary motion, especially to the means by which we may relieve pain in general, produce unconsciousness, and induce sleep. The student must also clearly understand that we are approaching the therapeutics of the nervous system from the physiological side, i.e. the treatment of symptoms only. The treatment of the pathological processes, such as haemorrhage, degeneration, syphilis, which constitute these diseases and cause these symptoms, is another "and even more important part of the management of this class of cases, and one which falls under other heads.

I. Physiological Relations

Nervous tissue is a kind of protoplasm with highly specialised properties, which may be resolved into the one great property of displaying or discharging force when brought into contact with certain influences. We name this property irritability; the influence which calls it forth, an irritant; the act of calling it forth, irritation. If the effect be the display of more force than ordinary, we speak of the influence as a stimulant, and of the act or result as stimulation. If the effect of irritation be the display of less force than ordinary, we say there has been depression-that the influence is a depressant. Much discussion is still going on as to the nature of irritation, stimulation, and depression, but the points just indicated are clear enough for our present purpose.

Plan of the Nervous System.-The nervous system, though forming one continuous mass of nervous tissue, is built up of a number of centres, which are connected with an irritable surface, and with the organs of force. An impression made on the surface by an irritant is conveyed by an afferent nerve, or tract, to the centre; effects there some change upon the protoplasm; and either remains as potential energy, or flows out again through efferent tracts and nerves, as an impulse, to the organs of force-the muscles, glands, vessels, etc. This process is spoken of as reflex action. Nervous substance is, however, not simply irritable, or capable of being brought into action by an impression from without. It can also originate action. It is automatic as well as reflective. The automatic action of the higher centres is the basis of the emotions, of the intellect, and of the will, and is continually modifying the impulses flowing out of the reflex centres, and vice versa, by means of connecting fibres or tracts. In the same way the viscera, such as the heart, are innervated by automatic centres in the medulla or cord, and these are constantly influenced by impressions reaching them from all sides. The highest centres are in the convolutions; the simple automatic reflex centres in the basal ganglia, cerebellum, medulla, and cord-the whole constituting a series of successive centres, the central nervous system, joined to each other by tracts which associate or co-ordinate the impulses, whilst the outlying systems of ganglia, chiefly automatic in their action, are called the sympathetic.

Now we find, when we come to consider the action of drugs and other remedies on the nervous system, that certain of them affect one centre, some another; some afferent parts, others efferent or motor parts; that some drugs affect the lower centres only some the centres of emotion and intelligence only; and that others again interfere chiefly with the co-ordinating mechanism. We must therefore attempt to arrange the parts of the nervous system on something like a definite plan, before we can comprehend the action of drugs upon it.

Plan of the Nervous System.

I. The terminal irritable apparatus, on the surfaces of the body, and in the organs. II. The afferent nerves.

III. The posterior of cornuaof the cord.

IV. The convolutions.

V. The basal ganglia and cerebellum. VI. The medulla oblongata.

VII. The antero-lateral tracts and anterior cornua of the cord. VIII. The co-ordinating fibres between the different centres, especially in the cord, where they form definite columns.

If we were to add to this plan, we might put in the viscera with their nerves. These we have relegated to other chapters; and all that need be indicated at present is that most of the viscera are governed by centres in the medulla, cord, or cerebrum, an arrangement which is partly reflex: that the efferent nerves between the centres and the viscera are intimately connected with the sympathetic chain; and that the viscera have also intrinsic ganglia, by which their automatic action is chiefly carried on.

Sensation.-Sensation is a cerebral state, referable to an impression received through an afferent nerve. This generally originates at the periphery, more rarely in the afferent nerve or tract, but is in every case referred to the periphery. In this way an impression (peripheral) becomes a sensation (cerebral), and a sensation in turn may or may not travel onwards into a still higher part of the cerebrum, where it becomes a perception, a part of consciousness, a mental act. Of the various perceptions, common sensibility alone demands special notice here. The tissues and organs in health are sensitive, but not the seat of actual sensations. Very slight disturbance, however, is sufficient to arouse perception or consciousness of the condition of the organs, of which pain is an example, and we therefore assume the constant existence of a quiescent sense, called common sensibility.

Motion.-All movement may be said to originate as an impulse in a nervous centre, whence it is conveyed to muscles or muscular organs by efferent or motor nerves. Thus an impulse arising in the automatic action of the cerebral cells travels from the higher to the lower centres; here it joins the reflex impulse, proceeding by reflexion from these centres; and the mixed impulse courses through the motor nerves to a special terminal apparatus, say in a muscle, by which the motor nerve is brought into relation with the organ. Just as a perception in the cerebrum may be referable to a condition of any part of the afferent or sensory side of the nervous system, so muscular contraction may be produced by stimulation of any part of the efferent or motor side, from the convolutions to the muscle itself; and what is of special interest to the therapeutist, it frequently originates, wholly or in part, in stimulation of some part of the sensory side, reflected through the centres.