776. Some physiologists assert, that the medulla oblongata is the point at which excitement to motion commences, and sensation terminates; and also, that it possesses the power of originating motion in itself.
Observation. The medulla oblongata, unlike the brain, is highly sensitive; if slightly punctured, convulsions follow; if much injured, respiration, or breathing, immediately ceases.
777. It is remarkable that the nerves which arise from the right side of the spinal cord communicate with the left hemisphere of the cerebrum, and vice versa; this results from the crossing of the fibres in the medulla oblongata. It follows from this, that if the right side of the brain receives an injury, the parts of the opposite side of the body lose their sensibility and motion.
Observations. 1st. If the cranial nerves which are connected 349 by a single root are divided, only the sensation of the part to which they are distributed is lost. Thus, if the optic nerve is divided, the sense of vision disappears, but the motions of the eye are performed as readily as before. But, if the spinal nerves are divided, both sensation and motion of the part to which they lead are destroyed.
What portions have been removed without impairing the intellect? What is remarkable of the brain? 775. What is the influence of the brain upon the muscles? 776. What do some physiologists assert of the medulla oblongata? 777. What is remarkable of the nerves? Give the 1st observation relative to the cranial nerves.
2d. When the spinal cord is divided or compressed, as in fractures of the spinal column, all parts below the fracture are paralyzed, though the nerves leading to these parts may be uninjured.
3d. Again, one side of the body or one limb may become insensible, and the power to move it, be perfectly retained; or the reverse of this may happen—the power of motion will be lost while sensation remains. In the former instance, the function of the posterior, or sensitive column of the spinal cord on one side is destroyed; in the latter, the anterior, or motor column is affected.
4th. In some cases, both sensation and motion of one side of the body or one limb are destroyed. In such instances, both the anterior and the posterior columns of one side of the spinal cord are diseased.
778. Vigorous and controllable muscular contraction requires a sound and well-developed brain. If this organ is defective in these particulars, the movements will be inefficient, and may be irregular. The central organ of the nervous system must, likewise, be in an active condition, to induce regular, steady, and controllable muscular movements.
Observations. 1st. Persons who have suffered from apoplexy and other severe diseases of the brain, have an involuntary trembling of the limbs, which results from a weakened state of the nervous system.