2d. The chemical class of motions includes all the various appearances of chemistry. Many of the facts, which belong to these branches of science, are nicely ascertained, and elegantly classed; but their laws have not yet been developed from such simple principles as those above-mentioned; though it is probable, that they depend on the specific attractions belonging to the particles of bodies, or to the difference of the quantity of attraction belonging to the sides and angles of those particles. The chemical motions are distinguished by their being generally attended with an evident decomposition or new combination of the active materials.
3d. The third class includes all the motions of the animal and vegetable world; as well those of the vessels, which circulate their juices, and of the muscles, which perform their locomotion, as those of the organs of sense, which constitute their ideas.
This last class of motion is the subject of the following pages; which, though conscious of their many imperfections, I hope may give some pleasure to the patient reader, and contribute something to the knowledge and to the cure of diseases.
SECT. [II].
EXPLANATIONS AND DEFINITIONS.
[I]. Outline of the animal economy.—[II]. [1]. Of the sensorium. [2]. Of the brain and nervous medulla. [3]. A nerve. [4]. A muscular fibre. [5]. The immediate organs of sense. [6]. The external organs of sense. [7]. An idea or sensual motion. [8]. Perception. [9]. Sensation. [10]. Recollection and suggestion. [11]. Habit, causation, association, catenation. [12]. Reflex ideas. [13]. Stimulus defined.
As some explanations and definitions will be necessary in the prosecution of the work, the reader is troubled with them in this place, and is intreated to keep them in his mind as he proceeds, and to take them for granted, till an apt opportunity occurs to evince their truth; to which I shall premise a very short outline of the animal economy.