DISLOCATION DISMAL SWAMP 135 sulphurous and carbolic acids. The others, however, may be used to prevent or arrest the development of injurious gases from decom- posing solids and fluids, or to neutralize them when formed. DISLOCATION (Lat. dis, apart, and locus, place), in surgery, that displacement in the osseous system which results from the direct application of force or other long continued cause. All the joints are liable to dislocation, but it most commonly occurs to those which possess the greatest mobility ; hence the shoul- der joint, is the most frequent seat of this acci- dent. The head of the humerus or bone of the upper arm, forming a ball-and-socket joint in connection with the scapula or shoulder blade, is regulated in its motions by very strong muscles, and is but slightly impeded in its free motions by the very shallow socket in which it rests. While this arrangement bestows great freedom of action upon this joint, it renders it liable to dislocation in almost every direction. The most common is that which occurs when the arm is elevated above the head, by means of which the head of the humerus is thrown into the armpit. Next in frequency is the dis- location of the hip joint, which is generally produced by a sudden blow upon the knee when the thigh is flexed toward the abdomen, whereby the head of the thigh bone is drawn backward by the action of the gluteal muscles upon the dorsuin of the ileum or pelvis. The jaw bone is often thrown out of place in laugh- ing, and much more frequently in yawning. This accident sometimes occurs while speak- ing under undue excitement. It may be easily remedied by placing the thumbs on the back teeth so as to press them downward while the chin is raised by the fingers slowly up- ward. Care should be taken, however, to re- move the thumbs quickly on the restoration of the joint, or they may be painfully compressed between the teeth. The chief difficulty in re- storing a dislocation consists in the opposition offered by the muscles, rendered acutely irri- table by the unnatural position of the head of the luxated bone. In some instances this is overcome by reducing the heart's action by general bleeding. The warm bath and emetics are likewise used to relax the muscles, and with the same view tobacco moistened with water is sometimes laid upon the abdomen until it induces sickness and a disposition to syn- cope. But the safest and most efficient means of securing complete relaxation of the muscles is probably the etherization of the patient; and in this way a dislocation may sometimes be reduced with the exertion of a comparatively slight degree of force. The surgeon in re- ducing a luxated joint endeavors, by a steady application of force exerted in the direction of the joint, either to fatigue the muscles, or seize some moment when they are relaxed to slip the bone into its socket. Various degrees of force and different appliances are used to effect this object. In the case of the shoulder joint the surgeon frequently forms a lever of the arm, with the heel of his boot placed in the armpit for a fulcrum, and by pressing the arm inward over this toward the body, overcomes the resistance of the muscles, and restores the joint. In the case of the hip joint, the force required is necessarily more considerable, and pulleys are often resorted to, by which means not only a greater but a steadier traction is ex- erted. A recent dislocation is much more easily reduced than one of long standing ; in- deed, no time should be permitted to elapse between the accident and an attempt at its re- duction, for every hour adds to the uncertainty of success. In geology, the term dislocation is applied to the change in the position of rocks caused by their being torn from their original place, either by upheaval or subsidence. DISMAL SWAMP, Great, a large morass in Vir- ginia and North Carolina, extending 40 m. S. from near Norfolk in the former state, and 25 m. E. and W. The soil consists of black vege- table matter to the depth of 15 ft., saturated with water, yielding to the tread of man, and during a large part of the year covered in many places with stagnant pools. Several small streams flow through it, and in the centre is Lake Drummond, 6 m. long and 3 m. wide, the surface of which is 21 ft. above tide water. The swamp is for the most part covered with a dense growth of cypress, juniper, gum, and cedar, and upon the drier ridges that intersect it are found the beech and oak. Much of the most valuable timber, however, has been cut down, and large quantities have been obtained from beneath the surface, where the fallen trunks have been preserved by the wetness of the soil. The Seaboard and Roanoke and the Norfolk and Petersburg railroads pass through the N. border. The great channel of trans- portation is the Dismal Swamp canal, made by the assistance of the national government and the state of Virginia, which connects the W. branch of Elizabeth river with the Pasquotank. It is 6 ft. deep, supplied chiefly by Lake Drum- mond, with which it is connected by a feeder, and passes for 20 m. through the swamp, af- fording an outlet not only for timber but for much of the agricultural produce of the E. part of North Carolina. Steam power is used upon it, and the tolls amount to about $20,000 a year. The Chesapeake and Albemarle canal also passes through the swamp, connecting the E. branch of Elizabeth river with Currituck sound, and admits vessels of considerable size. In 1870, 4,382 vessels of all classes passed through it, and the revenue from tolls and towage was $58,734. This canal contains a single lock, 40 by 220 ft., and is fed by tidal action. Several minor canals connect the main- land with Lake Drummond. A stage road runs parallel to the Dismal Swamp canal from the N. border to Elizabeth City, N. C. Roads are made in the swamp by laying logs 8 or 10 ft. long side by side on the surface of the soil or " sponge." They are passable by mules and
