DIVISION X.-DRUGS ACTING ON THE NERVOUS SYSTEM

A, Drugs acting on the Peripheral Endings of Motor Nerves.Conium, which is no longer official, is the important drug of this class. While others have a special action on the motor nerve terminations, in most of them this is more or less over-shadowed by other effects. This action, however, is not made use of in medicine.

Drugs paralyzing the termination of the motor nerves in muscle:

Drugs stimulating the termination of motor nerves in muscle:

(1) Aconite.

(2) Pilocarpine.

(3) Strychnine (slightly).

It is possible that some of the beneficial action of strychnine in certain patients may be due to its slight action on motor nerves, but otherwise these drugs are not employed for this purpose.

B. Drugs acting on the Peripheral Endings of Sensory Nerves.The knowledge of the action of this group is necessarily derived almost entirely from observations on man.

Drugs which Stimulate the Termination of Sensory Nerves.-These are the same as those already enumerated (see p. 467) as acting locally on vessels. When topically applied they give rise to pain, and, with most of them, the cause of the pain is the local inflammation they set up.

Therapeutics. It is for their action on the blood-vessels that local irritants are principally used, but they are not infrequently employed also for their counter-irritant effects. By their application to the skin it is probable that changes are induced in the caliber of the vessels and in the sensory nerves of internal organs, so that deepseated pain is thereby relieved. The heart and respiration are also reflexly stimulated by peripheral excitation of nerves, and hence counter-irritation is made use of to rouse persons who have fainted or become unconscious from opium poisoning, etc. It is essential that the action should be a prompt one, and the application of the faradic current is quite commonly employed as an external stimulus in such conditions.

Drugs which Depress the Terminations of Sensory Nerves. Of these there are two kinds: those which simply relieve pain, or local anodynes; and those which diminish sensibility, or local anesthetics.

Local Anodynes.-These have no action unless pain be present.

The first are the most powerful. The local anodyne action of opium has been disputed, and it is probable that many substances not included in this list which have been regarded as local anodynes have little if any claim to this designation. Cold is an effective local anodyne because of its depressant effect on sensibility, and so likewise is warmth, which relieves pain by diminishing tension, in consequence of the vaso-dilation which it primarily induces.

Therapeutics. Local anodynes, it may readily be supposed, are called for in a great variety of conditions, and while often of service as adjuvants to internal treatment, they are especially useful in those affections in which it is not possible to remove the cause of the pain or irritation present.

Local Anæsthetics.—These are cocaine, betaeucaine, phenol, and extreme cold, whether produced by ice or by the evaporation of ethyl chloride, or of ether. In the performance of operations, the ether spray has the disadvantage of stiffening the parts so that it is only useful for a single incision as for opening furuncles. Upon a damp day it is ineffectual. Ethyl chloride sprayed from tubes by the heat of the hand is the best method and the one most frequently employed at present. Cocaine and betaeucaine, which produce a high degree of local insensibility, are largely employed, generally hypodermatically.

C. Drugs Acting on the Trunks of Nerves.-These, if taken for a considerable period, give rise to chronic neuritis, with much proliferation of the areolar tissue and also fatty degeneration of the nervefibers. During the earlier stage of the inflammation marked pain and tingling are experienced, but later these are replaced by numbness and diminished sensation as the function of the nerves becomes more and more depressed, and finally paralysis, often accompanied by trophic lesions, results.

D. Drugs Acting on the Spinal Cord.-After the administration of certain drugs it is found that a slight peripheral stimulus will produce such marked reflex action that convulsions will result. That this is due to stimulation of the spinal cord, it is determined in the following way: If the cord is cut across and convulsions are still caused by such slight stimulus, it is evident that these cannot be of cerebral origin, since in that instance they would not take place below the point of section. On the other hand, if the drug does not cause convulsions when, previously to its injection into the circulation, the vessels of the cord have been ligated, it is inferred that its action is not on the muscles or nerves. Other experiments going to show that the action is on the cord are the following: If when the drug is injected into vessels by which it reaches the cord quickly, convulsions appear sooner than when it is injected into other vessels; if convulsions do not occur when the cord is destroyed; if when the destruction of the cord is gradually produced by pushing a wire down the vertebral canal, the convulsions cease from above downward as the destruction proceeds.

(1) Drugs which excite the activity of the anterior cornua are—

(1) Strychnine.

(2) Ammonia.

(3) Chloroform.
(4) Ether.

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(5) Ergot.
(6) Opium.

The last four produce this result only slightly, and early in their action.

Therapeutics. Strychnine is at times useful for paralysis resulting from diseases of the spinal cord, but with this exception it is rare that affections of the cord are benefited by stimulation of the anterior

cornua.

(2) Drugs which depress the activity of the anterior cornua:

Of these, apomorphine, alcohol, chloroform, ether, opium, ergot, arsenic, camphor, phenol, hydrated chloral, and veratrine first excite slightly before they depress.

Therapeutics. So far as their action on the spinal cord is concerned, these drugs are of very little practical utility. Physostigmine, which is by far the most powerful, has been used to some extent in tetanus and other diseases accompanied by convulsions.

E. Drugs Acting on the Brain.-The action of these can by no means be so distinctly localized as that of drugs acting on the spinal cord and nerves. Drugs acting on the brain illustrate: (1) The law of dissolution, which, in pharmacology, is as follows: When a drug affects functions progressively, those first affected are the highest in development; that is to say, they are the last acquired by the individual and the last to appear in the species. The next affected are those next to the highest, and so on; until finally the lowest of all from an evolutionary point of view, that is to say, the functions of respiration and circulation, are affected. This law is well exemplified in the instance of alcohol, under the influence of which the first functions to be disordered are those of the intellect, especially the highest, such as judgment and reason; then follow disorders of movement, and finally death from failure of respiration and circulation. (2) When a drug in moderate doses excites a function, in large doses it often paralyzes it. Cerebral stimulants may thus also be hypnotics.

Drugs Acting on the Motor Centers of the Brain.-To investigate these, the motor area of the cortex is exposed: (1) Note, before and after the administration of the drug, the strength of the electric current which it is necessary to apply to this area to produce corresponding movements, or, (2) Observe the strength of current necessary to elicit a movement and then allow the wound made by the trephine to close; after which the drug is regularly administered to the animal for several weeks; the opposite motor area is then exposed, and the strength of the current required for the same purpose is noted. Drugs diminishing the activity of the motor area of the cortex are

(1) Alcohol.

(2) Anæsthetics.

(3) Hydrated chloral.

(4) Potassium bromide.
(5) Sodium bromide.
(6) Ammonium bromide.

It is on account of this action that bromides are largely employed in epilepsy and other convulsive affections.

Drugs increasing the activity of the motor area of the cortex are(3) Physostigmine.

(1) Atropine.

(2) Strychnine.

They are not used in medicine for this purpose.

(1) General Cerebral Stimulants.-These produce general excitation of the mental faculties, and this is not infrequently followed by confusion, incoherence and delirium, the character of the latter varying to some extent with the particular drug employed. In many instances the stimulation is soon replaced by a paralyzing influence.

Therapeutics. These are of the greatest importance in their therapeutic applications, and some of them are taken habitually as cerebral stimulants in various parts of the world.

(2) General Cerebral Depressants.-These are commonly divided into three classes: Hypnotics, Narcotics and General Anæsthetics. HYPNOTICS OR SOPORIFICS are drugs which produce sleep, closely resembling, if not identical with, natural sleep. It is known that during sleep the brain is anæmic, and it is probable that the anæmia is the result of depression of the activity of a neurone from retraction of the terminal filaments of the dendrons.

The hypnotics are

(1) Opium.

(2) Hydrated chloral.

(3) Bromides.

(4) Sulphonethylmethane.
(5) Sulphonmethane.

(6) Paraldehyde.

(7) Alcohol.

(8) Scopolamine.
(9) Cannabis.
(10) Ethyl carbamate.
(11) Lactucarium.

Therapeutics. In insomnia the underlying condition should be removed if possible. These drugs should be resorted to with the greatest reluctance on account of the danger of habituation. Hydrated chloral, if used with great caution, paraldehyde, sulphonethylmethane, and sulphonmethane are perhaps the most satisfactory, but the use of hypnotics is apt to be greatly abused. It is well to remem