curls which frequently appear to the eye in muscles, after boiling or roasting, and the rhomboidal pinnulae taken notice of by Dr. Hales in the abdominal muscles of a living frog, when under contraction, all seem to favour this fifth supposition.
Dr. Pemberton conjectures, that the cause of the contraction of muscular fibres is no other than the common cause of the cohesion of the small particles of the muscular fibres increased. And this seems very probable; for the muscles are hard during contraction, soft during relaxation; and hardness and softness are evidently nothing but variations in the cohesion of the small particles of bodies. Neither is this conjecture at all repugnant to the supposition of an electrical attraction above made, or to the doctrine of vibrations; for electricity may reach to small distances, without being excited by friction, and flow from the same principle as the cohesion of bodies, as Sir Isaac Newton has observed. It may therefore be the general cause of cohesion, and may be excited in the muscular fibres in an extraordinary degree, whenever extraordinary vibrations are communicated to them. Or, if we suppose the cause of cohesion to be something distinct from electricity, it may, however, be increased by vibrations of the small cohering particles.
For, when the fibres are in a state of contraction, they are hard; and this hardness, if it be supposed to extend to the small particles (which is no unreasonable supposition), must render the particles of these particles, i.e. the particles supposed in these propositions to be agitated with vibrations, indisposed to receive these vibrations; but the free admission of these vibrations is by supposition the cause which excites the attractions of the particles, and the consequent contraction of the muscle. It follows therefore, that the hardness which impedes these vibrations, must also lessen the attraction and contraction; or, in other words, that the contraction of a muscle, when carried to a certain degree, must check itself, and bring on a relaxation, after a time sufficient for the proper causes to take effect.
In like manner, when a muscle is relaxed, the vibrations which descend along the motory nerves pass freely into the muscular fibres, increase the attractions of the particles, and bring on the opposite state, that of contraction; and so on alternately.
The fibres of the relaxed muscle may also be considered as under a state of distention to a certain degree, and, consequently, as liable to an increase of vibrations upon this account. To which we may farther add, that since vibrations are hindered from passing into the contracted muscle, in the manner just now explained, they will pass with greater force into the relaxed
