Page:Popular Science Monthly Volume 35.djvu/397

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HE fundamental characteristic of the animal world, as distinguished from the vegetable world, lies in its different relations to the energies of matter. Every animal is a mechanism for the liberation of energy previously stored up, in great part, in the tissues of plants which serve as food for these higher forms of life; and the quantity and kinds of energy liberated in any animal are determined mainly by the degree of development of the muscular and nervous systems, the other tissues and organs of the body being subservient to these two, which have been well styled the master tissues.

But the animal differs from the plant, not only in the power of liberating energy, and thus acting on the outside world; it is also differently affected by the outside world, the energies of which play upon its living tissues as the wind upon the strings of an Æolian harp; and the sensitive organism thrills under these influences with responsive sensations of greater or less diversity and intensity according to the variety and grade of development of its sensitive organs.

The muscular and the nervous tissues, upon which depend the distinctively animal functions of sensation and spontaneous movement, develop together, and their relations, both anatomical and physiological, are of the most intimate character.

Rudimentary nerve-threads are found in the Hydra; first recognized by Klinenberg, they were regarded by him as partly nervous and partly muscular; and the most primitive fibers positively identified as true nerves serve as pathways of communication from the sensitive surface to the rudimentary nerve-centers, and from these centers to the equally rudimentary muscles of the simple animals to which they belong. In short, the primitive nervous system is merely an immature apparatus for the production of sensations and the excitation of movements of the kind called "reflex," since they are excited by a stimulus transmitted from the surface of the body to the nerve-centers and thence reflected, as it were, to the muscles; and a large proportion of the nerve-bundles which, with the centers, make up the nervous system of man, consists of fibers of communication between the muscles of the trunk and limbs and their stimulating centers in