We have seen that the cells of the organic tissues are like little packets of explosives, and that the changes which occur in the organism may be likened to their explosion and the setting free of the energy stored up in them. The end-organs of the special senses may be regarded as charged with explosives of extreme sensitiveness. Some are fired by a touch; the molecular vibrations of sapid or odorous particles explode others; yet others are fired by the coarser vibrations of sound; others, once more, by the energy of the ætherial waves. The visual purple is a highly unstable chemical compound of this kind; expose it for a moment to light, and it topples over to a new molecular arrangement, the colour being at the same time discharged. If the retina has been removed from the body, this is all that happens. But if (in the frog) it be replaced on the choroid layer from which it has been stripped, the visual purple is reformed. The explosive is thus reconstructed and the sensibility is restored. Thus, as fast as the explosives are fired off by sense-stimuli, so fast in normal life are they reconstituted and the sensibility restored. Meanwhile the explosion at the end-organs has fired the train of explosives in the nerve, and created molecular explosive disturbances in the brain. Thence the explosive waves pass down other nerves to muscles or glands, and, giving rise therein to further explosions, take effect in the activities of the organism. We shall have to consider these activities hereafter. We must now turn to the psychical or mental accompaniments of the explosive disturbances in the brain or other aggregated mass of nerve-cells. NOTES [EO] See abstract in _Nature_, vol. xxxiv. p. 515. [EP] See _Nature_, vol. xxxvii. p. 557. [EQ] "Sense-Organs and Perception of Fishes:" Journal of Marine Biological Association, New Series, vol. i. No. 3, p. 225. [ER] _Nature_, vol. xlii. p. 201. [ES] _Nature_, vol. xxxvi. p. 273. [ET] Journal of Marine Biological Association, New Series, vol. i. No. 3, p. 235. [EU] Mr. S. Klein mentions a similar fact in connection with _Bombyx quercus_ (_Nature_, vol. xxxv. p. 282). [EV] Journal of Marine Biological Association, New Series, vol. i. No. 2, p. 211. [EW] A friend of mine informs me that his limit is about 17,500 per second, 20,000 being quite inaudible. [EX] Journal of Marine Biological Association, New Series, vol. i. No. 3, p. 251. [EY] Of course, anglers will say that what may be true for pollack and other coarse and vulgar sea-fish does not apply to King Salmon or Prince Trout. [EZ] "Senses of Animals," p. 117. [FA] See a very interesting and lucid paper by Professor Crum Brown, whose name is intimately connected with this subject, in _Nature_, vol. xl. p. 449.