Page:Observations on Man 1834.djvu/98

 

proposition is the immediate consequence of admitting the doctrines of vibrations and association, in the manner in which they have been asserted in the foregoing propositions. It contains the theory of the voluntary and semi-voluntary motions; to facilitate the application of which theory in the next proposition, I shall deliver the principal cases of this, in the following corollaries.

. I. The motory vibrations of the five classes mentioned Prop. XVIII. will generate a propensity to corresponding motory vibratiuncles.

. II. These motory vibratiuncles will affect the brain, as well as the motory nerves along which they descend; and, indeed, their descent along the motory nerves will be principally owing to their being first excited in the brain. This is sufficiently evident in the motory vibratiuncles which are derived from the motory vibrations of the second and third classes. As to the motory vibrations of the other classes, it is evident, that the brain is strongly affected by the sensory vibrations which give birth to them, and consequently, that a proportional affection of the brain must take place in the motory vibratiuncles derived from them.

. III. The motory vibratiuncles will cohere to one another, by associations both synchronous and successive. Hence the simple parts, of which complex and decomplex motions are compounded, may cohere closely, and succeed readily to each other.

. IV. The motory vibratiuncles will also cohere to ideal ones by association. Common ideas may therefore excite motory vibratiuncles, and consequently be able to contract the muscles, provided the active powers lodged in their fibres and blood globules be sufficiently exalted for this purpose.

. V. If we suppose the ideal vibratiuncles to be so much increased from the causes mentioned, Prop. XIV. as to be equal in strength to the usual sensory vibrations, the motory vibratiuncles connected with them by aossciation must be supposed to be increased proportionably. Hence ideas may occasion muscular motions of the same strength with the automatic motions.

. VI. The third and last connexion of the motory vibratiuncles is that with sensory vibrations, foreign to them, i.e. such as had no share in generating the motory vibratiuncles under consideration. Particular motions of the body may therefore by association be made to depend upon sensations, with which they have no natural and original connexion.

. VII. As muscular motion has three connexions deducible