substance, but an uniform pressure, susceptible of few or no modifications, and consequently highly unsuitable to the great variety of the phænomena that are to be solved by it. This argument therefore tends to shew, that sensation, thought, and motion, must all be performed by vibrations.
In order to shew this, let us make the following suppositions:
First, That vibrations descend along the motory nerves, i.e. the nerves which go to the muscles, in some such manner as sound runs along the surfaces of rivers, or an electrical virtue along hempen strings.
Secondly, that these vibrations, when they arrive at the muscular fibres, are communicated to them, so that the small particles of these fibres shall be agitated with like vibrations.
Thirdly, That the vibrations thus excited in the fibres, put into action an attractive virtue, perhaps of the electrical kind, which lies concealed in the particles of the fibres, or in the blood-globules, or both. That the blood-globules of animals are electrical, may be conjectured from the electricity of those of the muscle-shell fish, observed by Dr. Hales; and that the red blood has a principal share in muscular contraction is highly probable, from the red colour of all the great muscles of the body, and from the weakness of all young animals, and of such as want a due share of red blood. At the same time, it appears from exanguious and transparent animals, that pale fibres, and colourless fluids, have all the necessary requisites for muscular contraction, in certain degrees.
Fourthly, We must now suppose, in consequence of the three foregoing suppositions, that each muscular fibre, and consequently the whole muscle, is made shorter by this increase of attraction in its particles; whilst yet their approach to each other is so small, as that the whole bulk of the muscle is but little diminished; for though the length of the muscle is lessened, its other dimensions are increased.
Fifthly, If we suppose the small ultimate fibres of the muscles to bend alternately to the right and left as an eel does, at exceedingly short intervals, agreeably to Dr. Lower, this may somewhat assist us to conceive in what manner a muscle may be shortened, and yet so increased in breadth and thickness, as to remain of nearly the same dimensions. For if these flexures be increased by the increase of the attraction of the parts, the whole muscle will become shorter and thicker, as it is found to be in contraction; and conversely, when the flexures are drawn out, the muscle will be longer and thinner, i.e. in a state of relaxation. The small wrinkles which have been observed in the muscular fibres, by Leeuwenhoek, and others, the wavings and
