(SENSATION).
THE nervous system being constituted, as we have just explained, by a central axis, plunging by its lateral roots into the surrounding tissues, and crowned at its superior extremity by a central ganglion, the brain, gifted with its special activity, we shall now see how the phenomena of sensibility, existing per se as funda mental histological properties, behave in presence of the machinery which the nervous system places at their disposal ; how they become incorporated with it ; how, arriving in the form of centripetal excitation, they be come refracted in the plexuses, reappearing as a centrifugal reaction, through the peculiar influence of the new media they have put in requisition ; and how at last, in the most elevated regions of their journey, they come to play a primary part in the evolution of the essential phenomena of psycho-intellectual activity.
In taking their departure from the peripheral regions of the nervous system, which physiologically represent the frontiers of the organism, sensitive impressions, wherever they may have originated, once implanted in their tissues in the form of vibratory agitations, follow their natural channels towards the central regions.
Some are extinguished in certain interposed gang lionic masses ; others advance further, become dis persed in the grey regions of the cord and transformed, either instantaneously or in a more or less gradual manner, into excito-motor reactions—these being the phenomena of unconscious sensibility.
Others, finally, endowed with an altogether special vitality, pursue their course, converge, mount up to the sensorium and come into contact with the psycho-intel lectual operations for which they provide the indispens able food—these being the phenomena of conscious sensibility, or sensation, to which they give birth.
We shall successively pass in review the mode of genesis and distribution of these two special groups of sensitive contingents.
Unconscious Sensibility.—Unconscious sensitive exci tations are derived from two orders of peripheral plexuses : r. From the plexus of vegetative life of the pathetic.
2. From the plexus of general and special sensibility.
These latter originate in common with the excita tions destined to ascend to the sensorium ; but they are extinguished on the way, and are destined to produce reflex actions (automatic actions) in the interior of the plexuses of the spinal cord.
1. Sensitive excitations radiating from the plexus of vegetative life, if we take them from their origin, only expand within a limited radius. They follow the threads of the sympathetic, which are distributed ad infinitum throughout the organism, and only manifest their presence by vaso-motor phenomena, capable of modifying, in a more or less direct manner, certain branches of local circulation.
This special order of sensitive impressions is con densed in special ganglionic masses, which represent small local centres, and are the primitive types of the first traces of a nervous system in the lower species.
Sometimes they are capable of radiating to a dis tance, and thus traversing several ganglionic masses and vibrating even as far as the grey plexus of the spinal cord, of which they thus provoke the secondary activity. Thus it is that the sensibility of the intestinal mucous membrane excites the secretion of the juices' destined to co-operate in digestion ; that the sensi bility of the uterus laden with the product of concep tion leads to development of the breasts ; that in abnormal conditions certain abnormal sympathies are developed, so that we see, for instance, the irritation of the urethral mucous membrane exercise an influence upon certain articular surfaces ; and that the irritation of certain peripheral nerves leads to the sudden occur rence of tetanic phenomena and of certain, so called, refri: convulsions.
Sometimes also, when certain peripheral regions in which they originate are intensely affected, and have risen to the pitch of pain, the excitations of sensi bility become capable of an action more penetrating still, and even of reaching the sensorium, where they are perceived, and whither they carry, as it were, the cry of some organ of vegetative life shaken in its essential constitution.