ANIMAL MOTION: various conjectures have been broached with a view to account for the origin of this important function in the animal economy: but, like most other springs of action, arising from a first cause, it is only in a slight degree cognizable to our senses, by its evident, mechanical effects.
Anatomists have, indeed, in their dissections demonstrated, that the contraction of the muscles causes motion, but by what peculiar process, or how produced, remains still doubtful, and involved in obscurity. Among other hypotheses advanced concerning animal motion, there prevails an opinion that it is occasioned by an impulse or irritation of the nerves; which, communicating with all parts of the body, produce muscular contraction, and consequent motion, either to a part or to the whole of the frame, in proportion to the force or frequency of the impression. The difficulty of comprehending, how mere impulse, or irritability of the nervous system, should alone be sufficient to produce such powerful effects, as often follow muscular contraction, has induced others, while they admitted this principle as a first cause of animal motion, to believe in the intervention of some other matter, which is the more immediate agent, in effecting a closer contact of the muscular fibres, and greater energy during the time of their contraction.
The existence of such a subtle matter, as may be capable of performing these wonderful phenomena, has been considered as highly probable; and is supposed to reside in the medullary substance of the nerves. This opinion has, lately, been in a great measure corroborated by the discovery of valves of various sizes attached to the nerves, which valves are found in greater or smaller numbers, accordingly as the animal is either of a quicker or slower motion.—See Muscles.