Even if the instrument be free from fallacy, we still have difficulty in ascertaining the real action of the drug on the circulation, inasmuch as the blood-pressure is much affected by movements, and by anaesthetics. If the animal is not anaesthetised we may get untrustworthy results from the straining or movements it may make, and if it is anaesthetised, the anaesthetic may greatly alter the power of the heart, or the sensibility of the nerve-centres either to the direct action of the drug upon them, or to its reflex action through the afferent nerves. In order to get rid of movement, and at the same time to prevent the vascular centres from being much depressed, curare is sometimes used instead of an anaesthetic. Perhaps, almost equally good results may be obtained by using ether as the anaesthetic, carefully regulating the supply so as to abolish sensation without greatly affecting the medulla. The reasons why this is possible are discussed at p. 204. In order to regulate the supply of ether, we use a stopcock, by which pure ether, or pure air, or an admixture of both in any desired proportion, can be passed into the lungs (Fig. 73, p. 211).

Other fallacies arise from the mode of injecting the drug, and this has sometimes led to false results : thus drugs are not unfrequently injected into the jugular vein, as it is very conveniently situated for the purpose. In this way, however, they are carried directly to the heart, and act much more strongly upon it, than they would do if absorbed from other parts of the body. In the case of irritant salts, for example, time is not afforded for their irritant properties becoming lessened by chemical combination with the constituents of the blood. If the solution injected contain particles which will not pass through the pulmonary capillaries, or if it is likely to cause coagulation of the blood, it may plug up the pulmonary vessels and give rise to dyspnoea and convulsions.

Both these objections are avoided when the drug is injected under the skin, or into the peritoneal cavity. Absorption from the skin is slower than from the peritoneum. In some experiments this is a disadvantage : in others, however, it is an advantage.

Another fallacy sometimes arises from the solution of carbonate of sodium used to prevent coagulation. In order to prevent the blood from passing too far into the tube connecting the artery with the kymograph, it is usual to introduce the solution of carbonate of sodium into the tube by a syringe (vide Fig. 86) or otherwise, under a pressure very little less than the usual blood-pressure of the animal experimented on. If the blood-pressure be lowered much by stoppage of the heart or dilatation of the vessels, the solution of carbonate, or bicarbonate of sodium, runs into the arteries and may cause convulsions and death. Thus stoppage of the heart by irritation of the vagus, or by the action of a drug, may sometimes appear to be followed by results which are not really due to it, but only to the conditions under which the experiment has been made.