Hence it appears that the black blood either is not an excitant capable of keeping up the cerebral action, or that it acts in a deleterious manner, upon the brain. The injection by the carotid of various other substances will produce analogous effects.
I have killed animals in this way with ink, oil, wine, and water coloured with indigo. The greater number of the excrementitious fluids, such as urine, bile, the mucus of catarrhs, occasion death also by their simple presence on the brain. The serosity of the blood is fatal, but not as quickly so. Now it is certainly upon the substance of the brain, and not upon the internal surface of the arteries, that these different substances exert their influence. I have injected them all into the crural artery. In this way they are none of them mortal, but occasion always a torpor, amounting even to paralysis at times.[92]
The black blood is doubtless fatal to the brain, the brain becoming at once a tonic from its presence. In what way does it act? I do not pretend to determine the manner; for this were only to begin a series of conjectures.
By this time we are authorised to conclude, that in asphyxiæ, the circulation which continues for some time after the interruption of the chemical functions of the lungs, interrupts the cerebral functions, from its being composed of black blood only. The fact is proved in another manner, for the movements of the brain continue to be made as usual.
If the cerebral mass be exposed, and the creature asphyxiated, the animal life will be extinguished, but the motion of the brain will be apparent still. Since then the latter cause of life subsists, the cause of death must be in the nature of the fluid, by which the organ is penetrated.
Nevertheless, if any affection of the brain coincide with asphyxia, the death which is occasioned by the latter, will be quicker than is usually the case. Strike a dog a violent blow upon the head, and then if he be deprived of air, he will die on the instant. In asphyxiating another animal already in a state of stupor, from compression of the brain, I observed that the vital functions were interrupted somewhat sooner, than when the brain is untouched during that operation; but the consequences hitherto deduced, may be supported by other experiments.
If in asphyxia the black blood suspend the action of the cerebral mass, it is evident that the black blood taken from the arteries of an animal dying of asphyxia, and injected into the brain of another, will be the cause of death.
The experiment will be found to succeed—cut the trachea, of a dog, and tie it up hermetically; then in the course of two or three minutes, open the carotid and receive into a syringe the blood, which flows from the vessels; inject it into the brain of another animal, and it will die.