NERVOUS SYSTEM 237 traction of the muscles to which the nerve is distributed. 2. These contractions occur at the moment of making and breaking the cur- rent, and not while it is passing. 3. "When the current' is made to pass from the centre to- ward the periphery, it is called the direct cur- rent; when from the periphery toward the centre, the inverse current ; and it is observed that contractions occur at the moment of ma- king the direct current, and at that of break- ing the reverse. 4. Continuance of the direct current exhausts the power of the nerve, but the reversal of the current will restore it; hence the value of the interrupted current, ob- tained from the electro-magnetic machines, as a remedial agent. Nature of Nervous Force. We can judge of the nature of the nervous force only by its effects. The muscular contraction caused by the irritation of a nerve is due to the development of a peculiar vital force in the nerve structure, which is unlike any of the known physical forces. It bears certain anal- ogies to electricity, and by some authors has been supposed to be identical with it. The rea- sons for this supposition are: 1, the identity of their effects on muscular fibre ; 2, the rapid- ity of their action without producing any ap- preciable effect on the parts between the point of irritation and the point affected ; 3, the ex- treme sensibility of nerves to the electric cur- rent ; and 4, the phenomena of electrical fishes. None of these reasons are sufficient to establish the identity of the nervous and electrical forces, while the experiments of Longet, Matteucci, Prevost, Dumas, and others, showing that the more delicate tests can detect no galvanic cur- rent in the nerves, and that they are inferior to some other substances as conductors, prove conclusively that they are essentially distinct and different forces. With regard to the in- disputable electrical phenomena that are ob- served in the torpedo and gymnotus, it is well ascertained that they depend on the existence in these animals of a special organ which gen- erates electricity. The function of nerves, as has been remarked, is internuncial. They possess only limited capacity for the develop- ment of nervous force. Their office seems to be to conduct sensations from the periphery to the centre, and impulses from the centre to the periphery. Their connection with a centre is essential for the appreciation of external im- pressions or the exercise of mental stimulus. The determination of the sensitive or motor character of a nerve is effected by direct experi- ment on living animals, and by the effects of disease or injury of the nerves. The different properties belonging to the two roots of the spinal nerves were discovered by Sir Charles Bell about 1810, and established by Magen- die in 1822. Magendie operated by dividing, in the living animal, first, the posterior roots of the lumbar and sacral nerves, after which sensibility was lost in the corresponding limbs, while the power of motion remained ; second- ly, the anterior roots only, after which the power of motion was abolished, but sensjU bility remained ; and finally, at the same time both anterior and posterior roots, after which motion and sensibility were alike destroyed. These experiments have been repeated by nu- merous observers, and the conclusion is es- tablished that the anterior root of each spinal nerve is motor and the posterior sensitive. (For a description of the functions of the encephalic nervous centres and the cranial nerves, see BRAIN.) Functions of the Spinal Cord. The spinal cord consists of two anterior and two posterior columns made up of longi- tudinal nerve fibres. Each lateral half encloses a crescentic mass of gray vesicular nerve matter. The fibres of the spinal nerves originate from the gray matter within the cord, while others extend longitudinally through the cord, and are connected, either directly or indirectly, with the brain. The discovery of the separate func- tion of the roots of the spinal nerves was the first step in unravelling the important offices of the spinal cord. Experiments on living ani- mals have established the following points in its physiology : 1. That the anterior and pos- terior columns of the cord correspond in their properties to the anterior and posterior roots of the spinal nerves; for when the anterior column is irritated, convulsive motion is pro- duced in the parts below the point of division ; and when the posterior column is irritated, a painful sensation is produced. 2. When the anterior and posterior columns are both divided, it is found that irritation of the portion of the anterior column attached to the brain produces no effect, while irritation of the lower segment causes convulsive action, showing that in the anterior column the nervous force travels from within outward and not from without in- ward. When, on the other hand, the portion of the posterior column attached to the brain is irritated, pain is produced, while irritation of the portion below the division causes no pain, showing that in the posterior column the nervous force travels from without inward and not from within outward. 3. The motor nerves derived from the right side of the body enter the right lateral half of the cord, and make their connections from below upward on the same side until they reach the medulla oblongata, where they cross over to the left side ; the same course is followed by the mo- tor nerves of the opposite side, which pass up the left anterior column till they reach the medulla, when they cross to the right side. This crossing of the motor nerves in the an- terior columns of the medulla is termed decus- sation. The consequence of this arrangement is, that when an injury is inflicted on the right half of the anterior column below the medulla oblongata, the paralysis is on the same side ; but when the injury is above the point at which decussation commences, the paralysis will be on the opposite side. 4. The experiments of Dr. Brown-Sequard show that there is a cross- ed action in the sensitive as well as in the
