ACTIVITY.
THE nerve-cells, considered as to their intrinsic proper ties, individually participate in all the general phe nomena of the life of cells. Like all their fellows they have their history, their genealogy, their periods of growth and decay. They are subject to alternate phases of repose and labour, and, like them, are all gifted with a specific histological sensibility which gives them special dynamic characters.
It is the blood alone that makes them live and feel ; it is it alone which, as sole agent of their incessant activity, percolates everywhere through the nervous tissue, and carries with it the elements of all life and all movement.
This is so true, that if we succeed in momentarily suspending the circulation in the encephalon, the whole vital machinery stops at once, and every phenomenon of nervous activity is at the same instant interrupted.
Decapitated animals are by this very fact deprived of all cerebral functionment, and, it is a very remarkable fact, that if we succeed in artificially restoring to the cerebral tissue the nutritive materials of which it was deprived ; if, by means of injections of defibrinated blood, such as Brown-Sequard has experimented with, we succeed in giving their habitual stimulation to the nerve-cells, the signs of life come back as if by en chantment, and the head of a dog, thus momentarily revived, will still afford ephemeral manifestations of a conscious perception of external things.* In man the more or less complete arrest of the blood in the brain, produces accidents which are sometimes overwhelming, faintings, and loss of consciousness with stupor ; and it is now recognized, thanks to the labours of modern physiology, that the greater number of those apoplectiform seizures, which were formerly attributed to a sanguineous plethora in the plexuses of the brain, should on the contrary be ascribed to a more or less complete arrest of the course of the blood in the capillary plexus. The attacks observed in these circumstances may be legitimately attributed to a sort of asphyxia of certain regions of nerve-cells (princi pally those of the sensorium, when we have to do with losses of consciousness, vertigos, and fainting-fits) ; the nervous elements being stupefied for an instant, in con sequence of the more or less complete suspension of the arrival of their nutritive materials.t
The continuity of the sanguine irrigation is, then, the sine qua won of the regular working of the cerebral cells. It is at the expense of the juices exhaled from the walls of the capillaries, that they feed themselves and con tinually repair the losses sustained by their integral constitution.* Plunged into the midst of this humid atmosphere surcharged with phosphates, of which the materials are incessantly renewed, they extract from this vivifying medium the elements of their reconstitution ; like living beings plunged into the terrestrial atmosphere, borrow ing from the surrounding air the pabulum vita. which enables them to live, and sustains them. Thus it is that they successfully endure their expenditure of the phos phorated element during the period of their diurnal activity, and that they can maintain themselves in equilibrium as regards their receipts and expenditure.
This truth was very clearly brought to light by the ingenious researches of Byasson, who has pertinently shown that every cerebral cell in functioning expends its phosphorized materials, and that this waste resulting from cerebral activity, like the natural physiological excretions, is drained away from the organism by passing out in the urine, in the form of sulphates and phosphates, which serve as a chemical measure of the intensity of cerebral work done in a given time.t These facts show, then, the enormous influence which the blood exercises upon the vegetative phenomena of the life of the nerve-cells, and to what an extent their individual dynamic activity, and consequently the life of the whole system, depends upon it.