"Next the long nerves unite their silver train, And young SENSATION permeates the brain; 270 Through each new sense the keen emotions dart, Flush the young cheek, and swell the throbbing heart. From pain and pleasure quick VOLITIONS rise, Lift the strong arm, or point the inquiring eyes; With Reason's light bewilder'd Man direct, And right and wrong with balance nice detect. Last in thick swarms ASSOCIATIONS spring, Thoughts join to thoughts, to motions motions cling; Whence in long trains of catenation flow Imagined joy, and voluntary woe. 280 [Footnote: _And young Sensation_, l. 270. Both sensation and volition consist in an affection of the central part of the sensorium, or of the whole of it; and hence cannot exist till the nerves are united in the brain. The motions of a limb of any animal cut from the body, are therefore owing to irritation, not to sensation or to volition. For the definitions of irritation, sensation, volition, and association, see additional Note II.] "So, view'd through crystal spheres in drops saline, Quick-shooting salts in chemic forms combine; Or Mucor-stems, a vegetative tribe, Spread their fine roots, the tremulous wave imbibe. Next to our wondering eyes the focus brings Self-moving lines, and animated rings; First Monas moves, an unconnected point, Plays round the drop without a limb or joint; Then Vibrio waves, with capillary eels, And Vorticella whirls her living wheels; 290 While insect Proteus sports with changeful form Through the bright tide, a globe, a cube, a worm. Last o'er the field the Mite enormous swims, Swells his red heart, and writhes his giant limbs. [Footnote: _Or Mucor-stems_, l. 283. Mucor or mould in its early state is properly a microscopic vegetable, and is spontaneously produced on the scum of all decomposing organic matter. The Monas is a moving speck, the Vibrio an undulating wire, the Proteus perpetually changes its shape, and the Vorticella has wheels about its mouth, with which it makes an eddy, and is supposed thus to draw into its throat invisible animalcules. These names are from Linneus and Muller; see Appendix to Additional Note I.] V. "ORGANIC LIFE beneath the shoreless waves Was born and nurs'd in Ocean's pearly caves; First forms minute, unseen by spheric glass, Move on the mud, or pierce the watery mass; These, as successive generations bloom, New powers acquire, and larger limbs assume; 300 Whence countless groups of vegetation spring, And breathing realms of fin, and feet, and wing.