PARALYSIS, or AKINESIS, is a condition in which loss of voluntary or involuntary muscular movement occurs through defective innervation. Such a strict definition is desirable, as excluding cases in which motion is abolished by local or general morbid states not essentially nervous, as in acute articular rheumatism, ruptures of muscles or tendons, fractures, extreme asthenia, etc. Paresis is a term often employed to designate a paralysis partial in degree; it does not mean an essential muscular paralysis. Paralysis varies infinitely in extent and distribution, yet certain types are recognized as having diagnostic value, and their exact determination is of great importance in practice. (_a_) Hemiplegia, or paralysis of many muscular groups in one lateral half of the body. (_α_) Common Hemiplegia.—In this, the most frequent form, we find loss of voluntary motion in many muscles of one side of the face and body. This condition may or may not be preceded by apoplectic or epileptic symptoms: it may occur gradually or suddenly. Although it is customary to say that in hemiplegia a patient is paralyzed on one side, this is not strictly correct, as careful observation shows that (1) in the face only the lower facial muscles are distinctly affected; the tongue itself is rarely paralyzed, but its projecting muscular apparatus is, so that when protruded it deviates as a whole toward the paralyzed side; the eye-muscles and masticatory muscles are unaffected; (2) in the extremities the loss of power is greatest in the hand and foot, less so in the arm and thigh, very slight in the muscular groups of the shoulders and hips; (3) the muscles of the neck and trunk, the respiratory muscles, and in general the muscles of the vegetative life are practically unaffected. These facts may be summarized by the statement that in common hemiplegia the greatest paralysis is shown in those muscular groups whose action is most independent on either side of the body; or, in other words, in those whose functions are most highly differentiated and whose innervation is most cortical (from the motor centres in the cerebral cortex). Those muscular groups, on the other hand, whose action is usually or necessarily simultaneously bilateral or associated across the median line—or, in other words, whose innervation is largely spinal or subcortical—are least paralyzed; while the purely automatic or reflex muscular apparatuses, those having a strictly spinal or sympathetic innervation, are not at all affected. Common hemiplegia is rarely accompanied by hemianæsthesia. {43} It must not be forgotten that double hemiplegia may occur, in which case the symptoms are simply duplicated.