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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.

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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.

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I.—­1.  The nervous system has its origin from the brain, and is distributed to every part of the body.  Those nerves, which serve the senses, principally arise from that part of the brain, which is lodged in the head; and those, which serve the purposes of muscular motion, principally arise from that part of the brain, which is lodged in the neck and back, and which is erroneously called the spinal marrow.  The ultimate fibrils of these nerves terminate in the immediate organs of sense and muscular fibres, and if a ligature be put on any part of their passage from the head or spine, all motion and perception cease in the parts beneath the ligature.

2.  The longitudinal muscular fibres compose the locomotive muscles, whose contractions move the bones of the limbs and trunk, to which their extremities are attached.  The annular or spiral muscular fibres compose the vascular muscles, which constitute the intestinal canal, the arteries, veins, glands, and absorbent vessels.

3.  The immediate organs of sense, as the retina of the eye, probably consist of moving fibrils, with a power of contraction similar to that of the larger muscles above described.

4.  The cellular membrane consists of cells, which resemble those of a sponge, communicating with each other, and connecting together all the other parts of the body.

5.  The arterial system consists of the aortal and the pulmonary artery, which are attended through their whole course with their correspondent veins.  The pulmonary artery receives the blood from the right chamber of the heart, and carries it to the minute extensive ramifications of the lungs, where it is exposed to the action of the air on a surface equal to that of the whole external skin, through the thin moist coats of those vessels, which are spread on the air-cells, which constitute the minute terminal ramifications of the wind-pipe.  Here the blood changes its colour from a dark red to a bright scarlet.  It is then collected by the branches of the pulmonary vein, and conveyed to the left chamber of the heart.