Paralysis, Or Palsy (Gr. Paralysis Or Palsy Gr 130052 , relaxation), a loss of the power of motion in any part of the body. As the contractile power of the muscles depends upon their healthy organization and the integrity of their structure, anything which interferes with these qualities will diminish in a corresponding degree their power of action. Imperfect nutrition or atrophy of the muscles, their disuse, a fatty degeneration of their texture, and the action of certain poisons (see Lead), will all have this effect and destroy the power of motion by directly-affecting the muscular fibres themselves. A paralysis of this kind is called u muscular paralysis," since its cause resides in the substance of the muscular tissue, which has lost its natural properties. - Paralysis, however, is oftener due to injury or disease of the nerves or nervous centres." As muscular contraction is naturally excited during life by a stimulus communicated to the muscles through the nerves, when this communication is cut off by injury or disease of the nervous fibres, the natural movements in the corresponding region of the body are at once suspended. This is most distinctly marked in paralysis of those parts which are the seat of the voluntary motion, that is, the limbs and trunk.

If the nerves going to the right arm be divided or contused, or constricted by a ligature, voluntary motion is at once lost in the corresponding limb. The muscles themselves are uninjured, and are as capable of contraction as ever; but they cannot be called into action by any effort of the will, because the natural stimulus, which should be conveyed to them through the nerves from the brain, is cut off by the injury of the nervous trunks. A similar effect will be produced if the fibres of the brain itself be injured at the point where these nerves take their origin. - There are various forms of paralysis, corresponding to the different regions of the body affected and the extent of the affected portion. The following are the most important. 1. Hemiplegia, or paralysis of one lateral half of the body, that is, of the right arm and right leg, or the left arm and left leg, with the corresponding portions of the trunk. This is due to a circumscribed apoplexy or other injury which affects one side of the brain, and which, owing to the crossing of the fibres in the medulla oblongata, produces paralysis of the opposite side of the body. 2. Paraplegia, or paralysis of the two lower extremities with the lower part of the trunk.

This results from an injury to the spinal cord about its middle portion, which of course paralyzes all the parts below the seat of the injury, while those above, still preserving their connection with the brain, continue to have the power of voluntary motion. 3. Facial paralysis, or that affecting the superficial muscles of one lateral half of the face, so that the natural expression is lost in this region, and the features on the affected side are relaxed and vacant. This is owing to an injury of the seventh or facial nerve at some point in its passage from its origin in the brain to its termination in the muscles. 4. Local paralysis of any other part of the body, due to injury or disease of the special nerve distributed to that part. - Another important distinction in regard to paralysis is whether it is accompanied with loss of sensibility of the part, as well as loss of motion. As these two properties are conferred by two different sets of nervous fibres, and as these fibres may be injured separately or together, we may have paralysis of motion without loss of sensibility; loss of sensibility without loss of motion; or, finally, a paralysis of both at the same time.

The degree in which the power of motion and sensibility are affected in relation to each other, in any particular case of paralysis, will often throw much light on the precise seat of the injury or disease in the nervous system. (See Bkaest, Diseases of the, and Spinal Diseases.)