In producing local anaesthesia we obtain a similar condition of the sensitive plexuses, and prevent the evolution of pain. When the anaesthetic agent is applied it acts locally upon the individual sensibility of the nerves of the region. It chills them in a man ner, hinders them from becoming heated, as regards the excessive production of painful vibrations, and main tains them at the low pitch of general sensibility. The anstheticized regions, in fact, cease to disengage pain, while they are still conductors of sensitive impressions.
Pain being only the expression of the histological of the nervous elements risen to an extra physiological pitch, we can understand how, being always identical with itself as regards its genesis, it may reveal itself in a different manner according to the different nature of the nervous plexus thrown into agitation.
Thus pain presents itself in various modes according as it affects such and such a sensorial plexus. If it be the retina which is affected, we know that when light is too intense its sensibility is developed to excess, and leads to a reverberation excessively painful for the sen sorium. It is the same with the acoustic nerves, when violent and s' rident sounds produce contusions of their natural sensibility. The olfactory and gustatory plex uses have also their own forms of suffering, and every one knows how painfully the contact of bitter and acrid substances, or that of foetid emanations, affects the sensorial plexuses thus brought into play. Finally, when our viscera are attacked in their sensitive ele ments, we all know that they complain in their own fashion to the sensoriztm, that they reveal their suffer ing in a peculiar manner, and that the manifestations of pain vary with the tissues engaged, the regions invaded that, in a word, the semeiology of pain, as regards its different characters and modes, has a special physiognomy which all physicians can appreciate.
If we now pass to the examination of the processes of pain in the central i eg:ons of the nervous system, we shall see that they are developed in a manner similar to that we have just explained, and that the morbid re actions of the sensorium have a method similar to that of the morbid processes of the peripheral regions.
The plexuses of the sensorium, in the substance of which sensitive impressions are diffused, are normally insensible, like our nerves, which, when in activity, silently transmit and elaborate sensorial impressions, without our having a notion of all their minute opera tions.
It is not always so. Just as the peripheral plexuses are susceptible of exaltation in presence of too-energetic vibratory excitations, or by the occurrence of a local dis turbance of their habitual state of existence—lb the plexuses of the sensorium are susceptible of excessive heating,* and of exaltation when a too-vivid peripheral impression, or a too-prolonged excitation comes to rever berate through their meshes, and thus cause them to rise to the vibratory pitch of pain.
We know that the absence of repose for the brain, prolonged vigils, uninterrupted intellectual work, moral emotions, engender a local heating of the cerebral sub stance, cephalalgia, and aching of the brain. The calling into activity of the cerebral cell, in an extra-physiological manner, at the same time abnormally develops its histo logical sensibility, and induces, as a necessary conse quence, prolonged erethism and pain, in the manner we have just pointed out.
We all know by experience, how painfully a piece of taskwork which does not provoke an intellectual appetite, is done—it is an effort which the brain makes against the grain ; and how, on the contrary, when the task is a pleasant one, there is a fascination in setting to work, and a rapidity in the execution. The natural spontaneity of the brain thus supplies the place of effort.
All those who have suffered from headache know how exquisite is the sensibility of all regions of the sensorium ; how painful a thrill is produced by the least noise from without, the slightest shock of the thoughts which tra verse the brain. They know also that silence, and sleep —that is to say the cessation of every source of cerebral excitement—are the only efficacious means for charming away these painful crises through which the sensibility of the sensorium has to pass.* More than this, a comparative examination of the manner in which the central and peripheral regions of the nervous system behave in presence of agents, shows us a new connection between the modes in which sensibility is developed in these two opposite regions.