Drugs Which Stimulate The Termination Of Sensory Nerves

These, when applied locally, cause pain. They are the same as the local vascular irritants which have already been enumerated (p. 60); in fact, most of them give rise to pain by causing local inflammation. There is no need to repeat the list.

Therapeutics

Local irritants are chiefly employed for their action on the vessels, but as they are also counter-irritants, their application to the skin, while causing some pain there, will often relieve a deep-seated pain. Although pain is always referred to the periphery, it is appreciated centrally, and therefore peripheral stimulation of nerves, which also reflexly excites the heart and respiration, is used to rouse people from unconsciousness, such as that of fainting, opium poisoning, etc. For these purposes the stimulus must be prompt, hence the application of the faradic current to the skin is a good means to employ.

Drugs Which Depress The Terminations Of Sensory Nerves

Of these there are two kinds: those which only relieve pain, or local anodynes; and those which diminish sensibility, or local anaesthetics.

Local Anodynes

These have no action unless pain be present. They are -

(1) Aconite.

(2) Carbolic acid.

(3) Menthol.

4 Diluted hydrocyanic acid. (5) Veratrine.

(6) Ether.

(7) Alcohol.

(8) Chloroform.

These must be allowed to evaporate.

(9) Chloral hydrate. (10) Belladonna.

(11) Stramonium.

(12) Hyoscyamus.

(13) Opium.

(14) Sodium bicarbonate.

(15) Zinc oxide.

In the above list the most powerful are placed first Many other sub - stances are said to be local anodynes, but their claim to the title is doubtful. Cold is a powerful depressant of sensibility, and therefore it is an excellent local anodyne; so also is warmth, for heat primarily dilates the vessels, and thus relieves tension, which is a very powerful factor in causing pain.

Therapeutics

It is clear that the scope for the employment of local anodynes is very wide. If possible, the first thing is to remove the cause of the pain, but often, as in neuralgia and many forms of pruritus, we cannot do this.

Local Anaesthetics

These are ethyl chloride, rhigolene, cocaine, eucaine, carbolic acid, and extreme cold, whether produced by ice or the ether spray. This spray was formerly employed to produce local anaesthesia before doing small operations, but it has the disadvantage of stiffening the parts so that it is only useful for 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. Eucaine and cocaine, which produce a high degree of local insensibility, are largely employed.

D. Drugs acting on the trunks of Nerves. - These are of greater pathological than pharmacological interest. If taken for a long time they produce chronic inflammation of the nerves, which is shown by the great increase of the fibrous tissue between the nerve-fibres and the fatty degeneration of the fibres themselves. During the earlier stages the irritation of the nerves causes much pain and tingling; later, as they lose their function, numbness, with loss of sensation, and paralysis set in, often accompanied by trophic lesions. For further details books on medicine must be consulted.

The drugs producing peripheral neuritis are -

(1) Lead. (2) Mercury.

(3) Arsenic.

(4) Alcohol.

E. Drugs acting on the Spinal Cord. - The difficulties of experiment are so great that we know nothing of the action of drugs on the sensory portions of the cord. We are also ignorant of the action of drugs on the motor fibres. The following method is adopted to discover whether a drug acts on the cells of the anterior cornua. Suppose we are studying a drug which stimulates them. After the drug has been given, a slight peripheral stimulus will produce such marked reflex action that convulsions will ensue upon the stimulation. If the cord is cut across and the convulsions follow the stimulus as before, it is clear that these cannot be of cerebral origin, for in that case they would not take place below the point of section. Again, if before injection of the drug into the circulation, the vessels of the cord are ligatured, and then the drug causes no convulsion, it is clear that it acts on the cord, and not on the muscles Of nerves. These results are confirmed, if, when the drug is injected into vessels by which it reaches the cord quickly, convulsions occur sooner than when it is thrown into other vessels; also if convulsions do not take place when the cord is destroyed; and lastly if, when the destruction is gradually caused by pushing a wire down the vertebral canal, the convulsions cease from above downwards as the cord is destroyed.

The drugs increasing the irritability of the anterior cornua are -

(1) Strychnine.

(2) Brucine.

(3) Ammonia.

(4) Thebaine.

(5) Chloroform.

(6) Ether.

(7) Ergot.

(8) Opium.

(The last four only slightly, and early in their action.)

Therapeutics

It is very rarely that we can do any good in spinal diseases by attempting to stimulate the anterior cornua; but strychnine is occasionally given for cases of paralysis due to diseases of the spinal cord.

Drugs which depress the activity of the anterior cornua:

(1) Physostigmine.

(2) Bromides.

(3) Alcohol.

(4) Chloroform.

(5) Ether.

(6) Ergot. (7) Opium.

(8) Mercury.

(9) Zinc salts.

(10) Silver salts.

(11) Sodium salts.

(12) Potassium salts.

(13) Lithium salts.

(14) Antimony salts.

(15) Arsenical salts.

(16) Camphor.

(17) Amyl nitrite.

(18) Sodium nitrite.

(19) Chloral hydrate.

(20) Carbolic acid.

(21) Apomorphine.

(22) Veratrine.

(23) Turpentine.

(24) Saponin.

(25) Emetine.

(26) Gelsemium.

(27) Colchicum.

Of these, apomorphine, alcohol, chloroform, ether, arsenic, camphor, morphine, carbolic acid, chloral hydrate, nicotine, and veratrine first excite slightly before depressing.

Therapeutics

These drugs are of very little use in medicine for their action on the spinal cord. Physostigmine is by far the most powerful, and has been occasionally used in obscure nervous diseases accompanied by convulsions, as tetanus.

Ergot has a very peculiar action in producing sclerosis of the posterior columns of the cord. Lead sometimes causes atrophy of the anterior cornual cells, and long-continued abuse of alcohol probably causes slight degeneration of the cord as a whole.

F. Drugs acting on the Brain. - The action of these cannot be localized nearly so accurately as can that of drugs acting on the spinal cord and nerves. Drugs acting on the brain illustrate two very important general laws.

First, the law of dissolution, which, when stated as it applies 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 highest, and so on; till 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 very well exemplified in the case of alcohol, for 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.

Another law very well exemplified by drugs which act on the brain is that when a drug in moderate doses excites a function, in large doses it often paralyzes it. For example, a person under the influence of chloroform, soon after its administration, tosses his arms about in a disorderly way, but they subsequently become motionless, and cerebral stimulants may also be hypnotics.