from which are given off the various branches that distribute arterial blood to every part of the brain-substance. After traversing this, the blood returns by the Veins, greatly altered in its chemical composition; especially as regards the loss of free oxygen, and its replacement by various oxy-compounds of carbon, hydrogen, phosphorus, etc., that have been formed by a process analogous to combustion. Now, if one, two, or three of the Arterial trunks be tied, the total supply of blood to the Brain is diminished; but, in virtue of the "Circle of Willis," no part is entirely deprived of blood; and the functional activity of the Brain is still maintained. If, however, the fourth artery is compressed so as to prevent the passage of blood, there is an immediate and complete suspension of activity; the animal becoming as unconscious as if it had been stunned by a severe blow, but recovering as soon as the blood is again allowed to flow through the artery. In fact, the "stunned" state produced by a blow on the head is not directly dependent upon the effect of that blow on the Brain, which may have sustained no perceptible injury whatever; the state of insensibility being due to the paralysis of the Heart and suspension of the Circulation, induced by the "shock:" and the like paralysis with the same result may be produced by a blow on the Epigastrium (acting on the great "solar plexus"of nerves), or some overpowering Mental emotion.—Again, there is a curious affection termed Hysteric Coma, which consists in the sudden supervention of complete insensibility, and the equally sudden and complete return of conscious intelligence, without any other indication of Brain-disorder. The insensibility may come on while the patient is talking, so as to interrupt the utterance of a sentence; and, the moment that it passes off, the series of words is taken up and completed, without the patient being aware that it has been interrupted. With our present improved knowledge of the action of the "vaso-motor" system of Nerves in producing local contractions of the Arteries, and of its liability to be influenced by those Emotional irregularities in which Hysteria essentially consists, we can scarcely doubt that this affection is due to a temporary disturbance of the Circulation through that agency.—Further, if the Blood transmitted to the Brain, though not deficient in quantity, be depraved in quality by the want of Oxygen and the accumulation of Carbonic acid (as happens in Asphyxia), there is a gradually increasing torpor of the Mental Faculties, ending in complete insensibility.
Thus the dependence of Mental activity of even the most elementary kind, upon the Physical changes kept up by the circulation of oxygenated Blood through the Brain, can be shown experientially to be just as direct and immediate as is the dependence of the Electric activity of a Galvanic battery upon the analogous changes taking place between its Metals and its exciting Liquid.—If we say that Electricity is the product of Chemical change in the one case, I see
