_The circulation of the blood is sufficiently quick to account even for the operation of those poisons which act most rapidly by influencing the nerve-centres._
There is no poison whatever which acts so quickly on distant parts that the circulation cannot previously have had time to conduct it to them. By means of an instrument invented by M. Poisseuille, Dr. Blake found that a chemical substance traversed the whole circulation of a dog in nine seconds, and of a horse in twenty seconds.[20] The results of Hering were similar. M. Volkmann, in the tenth chapter of his work on Hæmadynamics, states, as the result of several experiments, that the whole circulation in an adult man occupies exactly 65.76 seconds.
Now a poison that operated by nervous connexion would probably operate directly when it touched the stomach. This is not the case even with Hydrocyanic acid. This, the most sudden of all poisons, before it takes effect, allows sufficient time to elapse for the blood to conduct it to the brain. Blake made an interesting experiment upon it. He placed some on the tongue of a dog, having first fitted a tube into the larynx, so as to prevent the vapour from passing into the lungs. The effect did not commence until sixteen seconds had elapsed, and forty-five were required for its completion. This allowed of time for absorption.
Thus it is proved that poisons act when introduced into the system at any point; that vascular connexion is required for this action; and that the rapidity of the circulation is in all cases quick enough to account for it.
But this last is only a proof of _possibility_, and does not by itself show that a substance may not nevertheless act through the nerves. And to the experiments on nervous connexion some may object that no conclusions on this point can be drawn from trials made on isolated and exposed nerves. So we may imagine a person to be still incredulous as to the truth of the Proposition, that medicines must pass into the blood before they can act. But a fourth consideration will suffice to bring this probability as close as possible to a certainty.
_The great majority of medicines have been detected in the blood, and found in the secretions formed out of it._