Small doses of any of them dilate the blood vessels from an action on the vaso-motor centre in the medulla oblongata, as a result of which the heart beats more rapidly and the blood circulates more freely; but larger doses have a general depressing effect upon the circulatory system.
Morphine exercises its effects chiefly upon the cerebrum and the medulla oblongata in man.
Similarly the depressant action on the respiratory centre in the medulla oblongata occurs only after the administration of enormous doses.
Whether effected entirely by action on the nerve terminals, or by an additional influence upon the vaso-motor centre in the medulla oblongata, atropine certainly causes extreme dilatation of the blood-vessels, so much so that the skin becomes flushed and there may appear, after large doses, an erythematous rash, which must be carefully distinguished, in cases of supposed belladonna poisoning, from that of scarlet fever: more especially as the temperature may be elevated and the pulse is very rapid in both conditions.
As the vaso-motor centre in the medulla oblongata is also stimulated, as well as the contractions of the heart, there is thus trebly caused a very great rise in the blood-pressure.