The startings and convulsions which happen to children from gripes, to women from disorders of the uterus, and to all persons in general from certain poisons, seem to be of the third kind, or to arise from vigorous vibrations in the abdomen, suddenly checked, and running into the whole system by means of the intercostal nerve.
It has been observed already, that convulsive motions are apt to return from less and less causes perpetually, on account of the vestiges which they leave of themselves, and the power of associated circumstances. I will add here, that seeing a person in convulsions is apt to occasion them in such as are of nervous and irritable frames; and that there is reason to believe, that some persons, who have been enthusiasts or impostors, have been able to throw themselves into convulsions by a semi-voluntary power, and particularly, as it seems, by introducing strong ideas, and internal feelings.
It is commonly observed, that yawning is apt to infect a whole company, after one person in it has set the example; which is a manifest instance of the influence of association over motions originally automatic.
It was necessary to deliver many things which properly relate to this proposition under the twenty-first, in order to make the derivation of voluntary motion from automatic, by means of association, in some measure intelligible to the reader. I will now resume the subject, and add what I am able for the full explication and establishment of the theory proposed.
Walking is the most simple of the three kinds of voluntary motion here mentioned, being common to the brute creation with man, whereas handling and speaking are, in a manner, peculiar to him. His superiority in this respect, when compared with the superiority of his mental faculties, agrees well with the hypothesis here advanced concerning handling and speaking, viz. their dependence on ideas, and the power of association.
The new-born child is unable to walk on account of the want of strength to support his body, as well as of complex and decomplex motory vibratiuncles, generated by association, and depending upon sensations and ideas by association also. As he gets strength, he advances likewise in the number and variety of compound motions of the limbs, their species being determined by the nature of the articulations, the position of the muscles, the automatic motions excited by friction, accidental flexures and extensions made by the nurse, &c. When he is tolerably
