The nervous system is the seat of intelligence, of sensation, and of motion; it is the centre of action for the organism, and it presides over all the phenomena which together constitute life. Its spinal and superficial portions—the cord and the nerves—take part only in the functions of sensation, of motion, and of organic life: but the encephalon contributes at once to the material and mental functions.
Observers have succeeded in distinguishing in the nerves and spinal cord the apparatus which is specially devoted to sensation, from that which presides over motion. But the knowledge which we have attained concerning the special functions of the different parts of the encephalon is as yet very limited, and for the most part hypothetical. Comparative physiology teaches us that some portions are extremely sensitive and irritable, while others are not affected by external agents. The medulla oblongata, the pons Varolii, and quadrigeminal bodies, are most allied to the spinal cord, and anatomy can follow thither the medullary fascicles endowed with sensibility, but beyond them these same fascicles become insensible in the greater and lesser brain, the optic beds, etc. It appears that after having transmitted the external impression, they change their nature, becoming an integral portion of the organ in which the sensation is produced, and submitted to the apprehension of the intelligence. We are no less embarrassed if we attempt to specify the portions of the encephalon which preside over motion. As for the seat of the intellectual faculties, we cannot doubt that it is situated in the encephalon, but science possesses no exact data regarding the part played by the different organs contained in the cranial cavity in the elaboration of thought, and the soul cannot perceive the mysterious tie which unites it with these organs.
The nervous system, which gives motion and sensibility to every part of the body, is itself absolutely dependent on the circulation. It determines and regulates its progress by exciting the action of the heart, but it must itself be excited by the afflux of blood which the arteries bring to it; and just as the heart slackens or even stops its action under the influence of certain impressions, the functions of the brain, spinal cord, and nerves are inevitably suspended when the blood does not come to awaken the nervous energy. Any impediment to circulation of the blood induces paralysis, more or less complete, in the parts beyond the obstacle, and no sooner does the nourishing fluid stop its motion, or even slacken, in its course toward the brain, than syncope or fainting supervenes; that is to say, the functions of the encephalon are weakened or cease altogether.
In giving a summary description of the nervous system, we have proceeded from centre to circumference, but in describing the nervous functions the reverse course seems preferable.