This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Experiments on the outflow of blood from divided vessels, while the nervous system is intact, are sometimes made on frogs for the purpose of ascertaining the direct effect of drugs on the arterioles themselves; but this method is faulty, for the alterations consequent on the injection of the drug may be simply due to its local irritant action producing reflex contraction.
Such experiments are usually made by snipping off the toe of a frog, then injecting the drug into the lymph-sac and observing how many drops of blood exude in a given time from the toe before and after the injection.
It is obvious that if no change occur in the heart, and the openings of the divided vessels do not become obstructed by clots or otherwise, these experiments may give some indication regarding the contraction of the vessels; but the results are not trustworthy unless we can ascertain the condition of the heart. A modification of this experiment enables us to some extent to do this. The end of a toe on each foot having been snipped off, the nerve in one leg is divided and then the drug is injected into the lymph-sac. If it be then found that the flow of blood from the foot, whose vaso-motor supply has been destroyed by division of the nerve, continues unchanged or is even increased after the injection of the drug, while that from the other foot is diminished, we may conclude that the diminution is due to contraction of the vessels caused by the injection of the drug.
But it is incorrect to assume, as has sometimes been done,
Ccntralblatt f. d. med. Wiss., April 6, 1867, p. 234.
that this contraction is due to any specific action of the drug, either upon the muscular walls of the blood-vessels or upon the vaso-motor centre. There is here a fallacy similar to that already mentioned in respect to direct observation of the size of bloodvessels. Any irritation of a sensory nerve by pinching, scratching, heat, etc, may cause reflex stimulation of the vaso-motor centre and produce contraction of the vessels, and injection of strong saline solutions into the lymph-sac, having a local irritant action, will produce a similar effect.
As an example of this fallacy we may mention certain experiments with bromide of potassium. In such experiments it was found that injections into the lymph-sac were followed by contraction of the vessels of the toes, so that much less blood flowed after the injection. When the sciatic nerve was divided on one side the flow was not lessened but rather increased in the corresponding foot, at the same time that it was much diminished on the other side where the nerve was intact. This result clearly shows that after the injection the vessels in one foot contracted, and that this contraction was due to the effect of the injection on the vaso-motor centre, inasmuch as it did not occur in the foot whose vessels had been withdrawn from the influence of this centre by division of the nerves. From this fact the conclusion has been drawn that bromide of potassium has a special power of contracting blood-vessels generally, and on this conclusion theories of its action upon the nervous system have been based. Such theories, however, rest on a very untrustworthy foundation; for though contraction of the vessels no doubt followed the injection of a strong solution of bromide into the lymph-sac, this contraction was probably not at all due to any specific action of the bromide, but only to the reflex stimulation of the vaso-motor centre caused by its local irritant action at the place of application. If introduced in a dilute solution into the mouth instead of in a concentrated form into the lymph-sac, this local irritant action would be absent and probably no contraction of the bloodvessels would be produced.