Another very important antagonism is that between drugs acting on the respiratory centre and spinal cord. The mode of action of these drugs is difficult to explain on account of our imperfect knowledge of the physiology of the structures on which they act. Chloral lessens the excitability of these structures, strychnine increases it. These drugs have to a certain extent an antagonistic action, so that a fatal dose of strychnine may be so antagonised by chloral as to prevent death; and a small quantity of strychnine may prevent death from chloral. Atropine has an exciting action on the respiratory centre, somewhat like strychnine though very much less marked; and atropine also will antagonise chloral. It has also an antagonistic action to aconite, which has a peculiar depressing influence on the respiratory centre.

The sedative action of chloral enables it to antagonise picro-toxine which has a stimulant action on the brain causing convulsions. Opium and belladonna have to a certain extent an antagonistic action to one another. The first point which appeared to indicate an antagonistic action was their different effect on the pupil; but probably the point on which they chiefly antagonise one another is their action on respiration, atropine acting as a stimulant and morphine as a depressant to the respiratory centre.

The alkaloids of tea, coffee, and allied substances, viz. theine or caffeine, cocaine and guaranine, are antagonistic to morphine. These alkaloids in small and moderate doses increase the irritability of the brain, spinal cord, heart, and vaso-motor system, and in large doses paralyse them. Morphine and these alkaloids to a certain extent counteract one another, so that a lethal dose of one may be prevented from causing death by administering the other.

The antagonism of drugs is also marked in regard to their action on the glandular system; thus the excessive salivation produced by physostigma, pilocarpine, and bromal may be arrested by atropine, which also arrests the excessive secretion from the skin caused by pilocarpine, and the secretion from the mucous membrane of the lungs produced by bromal.

The following table shows the most important examples of antagonism. The lethal and antagonistic doses have only been ascertained for a few. When the remark ' not antagonistic ' occurs in the table, it means that the second drug mentioned will not prevent death from a lethal dose of the first, although the first will prevent death from a lethal dose of the second.

Table Showing The Antagonism Of Drugs

Lethal dose. - I.

Lethal dose. - II.

Antidotal doses

Aconitine. . .

Atropine....

1/900 th

7

1/750 th

' ' . . .

Digitalin. . . .

1/900 th

1

1/600 th

" . .

Strychnine . . .

1/900 th

1/288 th

1/750 th

Alcohol . .

Strychnine . . .

----

1/288 th

Ammonium chloride

Chloral ....

----

Atropine . . .

Aconitine . . .

7

1/900 th

not antagonistic

" . . .

Bromal-hydrate . .

7

1 1/4

not antagonistic

" . . .

Chloral-hydrate . . .

7

7

" . . .

Hydrocyanic acid . .

7

" . . .

Jaborandi . . .

7

" . . .

Muscarine . . .

7

" . . .

Morphine . . .

7

3

not antagonistic

" . . .

Physostigmine . .

7

1/25 th

" . . .

Phytolacca . . .

7

" . . .

Pilocarpine . . .

7

" . . .

Quinine ....

7

Barium . .

Sodium sulphate . . .

" . . .

Potassium salts . .

Bromal-hydrate

Atropine....

1 1/4

7

Brucine . .

Chloral ....

Calabarine . .

.. .

Carbolic acid .

" . . . .

Chloral . .

Ammonium chloride .

" . .

Atropine....

" . .

Brucine ....

" . .

Calabarine . . .

" . .

Carbolic acid . . .

" . .

Codeine ....

" . .

Physostigma . . .

" . .

Picrotoxine . . .

" . .

Strychnine . . .

" . .

Thebaine . . .

Chloroform .

Amyl nitrite . . .

Cocaine . .

Morphine . . .

Codeine . .

Chloral ....

Digitalin . .

Aconitine . . .

" . .

Muscarine . . .

" . .

Saponin ....

Gelsemium

Opium ....

" . .

Atropine. . . .

Lethal dose. - I.

Lethal dose. - II.

Antidotal doses

Morphine. .

Atropine ....

" . .

Caffeine ....

" . .

Chloroform . .

" . .

Cocaine ....

" . .

Daturine . . .

" . .

Hyoscyamine . . .

" . .

Nicotine . . .

" . .

Physostigma . . .

Muscarine .

Atropine. . .

Opium . .

" . .

" . .

Gelsemium

" . .

Veratrum viricle . .

Physostigma .

Atropine. . . .

" . .

Chloral ....

" . .

Morphine . .

Saponin . .

Digitalin. . .

Strychnine .

Alcohol ....

" . .

Chloral ....

" . .

Hydrocyanic acid .

" . .

Nicotine ....

" . .

Nitrite of amyl

Thebaine . .

Chloral ....