Part 14 (1/2)

IV. A certain quant.i.ty of stimulus produces irritation, which is an exertion of the spirit of animation exciting the fibres into contraction.

V. A certain quant.i.ty of contraction of animal fibres, if it be perceived at all, produces pleasure; a greater or less quant.i.ty of contraction, if it be perceived at all, produces pain; these const.i.tute sensation.

VI. A certain quant.i.ty of sensation produces desire or aversion; these const.i.tute volition.

VII. All animal motions which have occurred at the same time, or in immediate succession, become so connected, that when one of them is reproduced, the other has a tendency to accompany or succeed it. When fibrous contractions succeed or accompany other fibrous contractions, the connexion is termed a.s.sociation; when fibrous contractions succeed sensorial motions, the connexion is termed causation; when fibrous and sensorial motions reciprocally introduce each other, it is termed catenation of animal motions.

VIII. These four faculties of the sensorium during their inactive state are termed irritability, sensibility, voluntarily, and a.s.sociability; in their active state they are termed as above irritation, sensation, volition, a.s.sociation.

Irritation is an exertion or change of some extreme part of the sensorium residing in the muscles or organs of sense, in consequence of the appulses of external bodies.

Sensation is an exertion or change of the central parts of the sensorium, or of the whole of it, beginning at some of those extreme parts of it, which reside in the muscles or organs of sense.

Volition is an exertion or change of the central parts of the sensorium, or of the whole of it, terminating in some of those extreme parts of it, which reside in the muscles or organs of sense.

a.s.sociation is an exertion or change of some extreme part of the sensorium residing in the muscles or organs of sense, in consequence of some antecedent or attendant fibrous contractions; see Zoonomia, Vol. I.

The word sensorium is used to express not only the medullary part of the brain, spinal marrow, nerves, organs of sense and muscles, but also at the same time that living principle, or spirit of animation, which resides throughout the body, without being cognizable to our senses except by its effects.

ADDITIONAL NOTES. III.

Next when imprison'd fires in central caves Burst the firm earth, and drank the headlong waves.

CANTO I. l. 302.

The great and repeated explosions of volcanoes are shown by Mr.

Mitch.e.l.l in the Philosoph. Transact. to arise from their communication with the sea, or with rivers, or inundations; and that after a c.h.i.n.k or crack is made, the water rus.h.i.+ng into an immense burning cavern, and falling on boiling lava, is instantly expanded into steam, and produces irresistible explosions.

As the first volcanic fires had no previous vent, and were probably more central, and larger in quant.i.ty, before they burst the crust of the earth then intire, and as the sea covered the whole, it must rapidly sink down into every opening c.h.i.n.k; whence these primeval earthquakes were of much greater extent, and of much greater force, than those which occur in the present era.

It should be added, that there may be other elastic vapours produced by great heat from whatever will evaporate, as mercury, and even diamonds; which may be more elastic, and consequently exert greater force than the steam of water even though heated red hot. Which may thence exert a sufficient power to raise islands and continents, and even to throw the moon from the earth.

If the moon be supposed to have been thus thrown out of the great cavity which now contains the South Sea, the immense quant.i.ty of water flowing in from the primeval ocean, which then covered the earth, would much contribute to leave the continents and islands, which might be raised at the same time above the surface of the water. In later days there are accounts of large stones falling from the sky, which may have been thus thrown by explosion from some distant earthquake, without sufficient force to cause them to circulate round the earth, and thus produce numerous small moons or satellites.

Mr. Mitch.e.l.l observes, that the agitations of the earth from the great earthquake at Lisbon were felt in this country about the same time after the shock, as sound would have taken in pa.s.sing from Lisbon hither; and thence ascribes these agitations to the vibrations of the solid earth, and not to subterraneous caverns of communication; Philos. Transact. But from the existence of warm springs at Bath and Buxton, there must certainly be unceasing subterraneous fires at some great depth beneath those parts of this island; see on this subject Botanic Garden, Vol. II. Canto IV. l. 79, note. For an account of the noxious vapours emitted from volcanoes, see Botanic Garden, Vol. II.

Cant. IV. l. 328, note. For the milder effects of central fires, see Botanic Garden, Vol. I. Cant. I. l. 139, and Additional Note VI.

ADDITIONAL NOTES. IV.

So from deep lakes the dread musquito springs, Drinks the soft breeze, and dries his tender wings.

CANTO I. l. 327.