By paralysis is understood a loss of power in the muscles to contract, and consequently greater or less impairment of voluntary motion. There is also a second form of paralysis, by which a part may be deprived of the sense of feeling. The former constitutes paralysis of motion, the latter paralysis of sensation.

They frequently occur together, but when this is so the loss of power usually exceeds that of sensation. Each may exist alone. The more common of the two as separate ailments is paralysis of motion.

Paralysis is not a disease, but a symptom of some disorder going on either in the brain or spinal cord, or the nerves connecting them with the paralysed part. If for any reason the brain fails to develop and to give out voluntary impulses, the influence of the will ceases to act upon those parts over which it ordinarily exercises control.

The nerves, although healthy in themselves, receiving no nervous force from the affected centre, become incapable of exciting muscles to contract.

On the other hand, the brain or nerve centre may be perfectly free from disease, but owing to some pressure or disease in the course of the nerves the impulses developed by the healthy brain fail to travel along the diseased nerves, with the result that paralysis ensues in the part to which they are distributed.

The nerve centres and the nerves are liable to become disabled from a number of diverse causes. Lack of nourishment, the consequence of insufficient blood supply; certain poisons introduced from without or formed within the body, such as lead or the accumulation of urinary or biliary products in the blood; mechanical injury or pressure; rupture of blood-vessels in or upon the brain; the formation of tumours; inflammation and its consequences, are all found to produce paralysis at one time or another. From what has been stated it will be seen that paralysis may originate - (1) in the brain (cerebral), (2) in the spinal cord (spinal), (3) in the nerves connected with the one or the other (peripheral paralysis). Hence it follows that the disease presents a considerable variety of forms, of which only those of the more common kind will be considered here.