those of the systole of the heart. It was long supposed that the cessation of respiration occasioned that of the heart’s motion, in consequence of the black blood not having sufficient power to stimulate its fibres; but does not the right side of the heart, which, under all circumstances, contains de-oxygenated blood, contract with a vigour equal to that of the left? It was reserved for _Bichat_ to offer a true explanation of this phenomenon; he has very justly stated that, in consequence of the suspension of the respiratory function, the coronary vessels, by which the muscular structure of the heart is supplied, are compelled to carry black, instead of scarlet blood; a fact which in itself is quite adequate to explain the cause of the heart ceasing to contract; for the irritability of this, like that of every other muscle, can be alone maintained by duly oxygenized blood. But it remains to be shewn how the functions of the brain and nervous system stand related to those of the heart and lungs. Although the agency of nervous influence is necessarily involved in impenetrable obscurity, yet we shall not have much difficulty in proving, that _the brain_[18] _is immediately necessary to life only because the muscles of respiration owe their action to its influence_. _M. Lallemand_ has published the history of a fœtus, in which the brain and spinal marrow were equally deficient, notwithstanding which, it even exceeded the usual size, the heart was perfect, and it was evident that the circulation had been properly performed; no sooner, however, was the monster born than it perished, because the diaphragm and other muscles of respiration were unable to perform their functions without the aid of nervous excitement; no air was therefore inhaled into the lungs, and in a few minutes the heart ceased to contract from the deficient supply of oxygenized blood. If the phrenic nerves of a quadruped be divided,[19] the motion of the diaphragm ceases, and the animal breathes by the motion of the ribs alone, panting and respiring with difficulty and distress. If the spinal marrow be divided below the origin of the phrenic nerves in the lower part of the neck, no interruption is given to the transmission of the nervous influence to the diaphragm, but the ribs now become motionless, and respiration is performed by the diaphragm only; if the spinal marrow be divided in the upper part of the neck, above the origin of the phrenic nerves, the nervous influence is neither transmitted to the diaphragm, nor to the muscles which produce the motion of the ribs, and respiration is entirely suspended; under these circumstances the heart continues to contract for some minutes, after which it ceases, as there is no supply of blood which has received the influence of the air, and, consequently, the muscular fibres of the heart lose their excitability, and the blood is no longer circulated; if, however, the lungs be artificially inflated, before the action of