Enough has been said to shew that the brain is not _immediately_ necessary to the action of the heart; but _Mr. Brodie_ has very justly observed that the general proposition thus established, must not lead us to the conclusion that the heart is therefore incapable of being affected by violent impressions on the nervous system; the fact is quite otherwise, for although the brain may be removed, and the circulation be nevertheless maintained by artificial respiration, yet an injury inflicted on the brain, of another kind, may be followed by those immediately fatal consequences which decapitation itself would not produce. _Dr. Wilson Philip_ states that if the brain be violently crushed, the action of the heart is immediately stopped; and the fact is too notorious to be questioned, that a blow on the head is frequently succeeded by Syncope; there are but few circumstances, says _Mr. Brodie_, in the history of the animal œconomy which appears more remarkable than this fact, that _an injury of a part which is not immediately essential to the heart’s action, should nevertheless, under certain circumstances, have the effect of occasioning its immediate cessation_. The late researches of _Le Gallois_ may perhaps receive farther elucidation from the above proposition; this physiologist has stated that if a wire be introduced into the _Theca vertebralis_, and be moved upward and downward, so as to destroy the texture of the spinal marrow, the action of the heart presently ceases; and he from thence advances to the conclusion, not only that the spinal marrow is necessary to the heart’s action, but that every part of the animal body derives its vital properties from it; from what I have observed, says _Mr. Brodie_ (_Manuscript Notes_) in the repetition of the foregoing experiment, I should infer that the fact is correctly stated, as far as it relates to warm-blooded animals, but the conclusions are undoubtedly premature; and the history of the fœtus, as related by _Lallemand_, in which, notwithstanding the absence of the brain and spinal marrow, the child was even larger than usual, the heart perfect, and it was manifest that the circution had been duly performed, is in direct opposition to such a theory. We must here agree with _Mr. Brodie_, that such phenomena are quite incompatible with the doctrine in which the spinal marrow is supposed to be directly necessary to the existence of vitality in the system generally, and to the action of the heart in particular; and that we must therefore look for some other explanation of the effects which are produced by the destruction of the spinal marrow in warm-blooded quadrupeds.—May they not be explained by supposing them to be the effect of the shock which must necessarily attend the removal of the spinal marrow, which can never be effected with the facility that attends decapitation?