one, from the place of the common derivation of their nerves, wherever there are antagonist muscles that derive nerves from the same trunk, as in the limbs, and muscles of respiration.
Cor. It appears from this method of considering the contractions and relaxations of muscles, that there is a certain degree of hardness or contraction in muscular fibres, which may be supposed just to balance each degree of force with which vibrations descend into the muscular fibres; and that while this equilibrium subsists, the contraction can neither be increased nor abated.
In order to make this appear, it will be proper to distinguish the motory vibrations, or those which descend along the nerves of the muscles into their fibres, into the five following classes:
First, then, We are to conceive, that those sensory vibrations which are excited in the external organs, and ascend towards the brain, when they arrive, in their ascent, at the origins of motory nerves, as they arise from the same common trunk, plexus, or ganglion, with the sensory ones affected, detach a part of themselves at each of these origins down the motory nerves; which part, by agitating the small particles of the muscular fibres, in the manner explained in the sixteenth proposition, excites them to contraction.
Secondly, The remainder of the sensory vibrations, which arrives at the brain, not being detached down the motory nerves in its ascent thither, must be diffused over the whole medullary substance. It will therefore descend from the brain into the whole system of motory nerves, and excite some feeble vibrations, at least, in them. The same may be observed of ideal vibrations, generated in the brain by association; these must pervade the whole medullary substance, and, consequently, affect all the motory nerves in some degree.
Thirdly, The heat of the blood and pulsation of the arteries, which pass through the medullary substance, must always excite, or keep up, some vibrations in it; and these must always descend into the whole system of muscles. And I apprehend, that, from these two last sources taken together, we may account for that moderate degree of contraction, or tendency thereto, which is observable in all the muscles, at least in all those of healthy adults, during vigilance.
Fourthly, When vivid vibrations are excited in membranes of an uniform texture, by a stimulus of any kind, they seem to run over the whole extent of such membranes, and by this means to have a great influence in contracting all the muscles that lie near any part of this membrane, though they be remote from the
