General principles of zoology . eripherylies a layer of nerve-fibres (the whitematter of human anatomy); next isa central portion formed of ganglion-cells and nerve-fibres (the gray matter),which is marked off from the central FIG. 72.—Ladder nervous sys- . . , .... tem of Porcciiio scaber (AS- canal by a special epithelium. sel). A, brain; B, ventral cord, connected with the brain RelatlOHS bctWCCn the NCfVOUS by the oesophageal commis- sures; 6, a cord formerly re- System and the Skin.—r or almost garded as sympathicus. (Af-ter Leydig.) all animals, it has been ascertained that the nervous s
General principles of zoology . eripherylies a layer of nerve-fibres (the whitematter of human anatomy); next isa central portion formed of ganglion-cells and nerve-fibres (the gray matter),which is marked off from the central FIG. 72.—Ladder nervous sys- . . , .... tem of Porcciiio scaber (AS- canal by a special epithelium. sel). A, brain; B, ventral cord, connected with the brain RelatlOHS bctWCCn the NCfVOUS by the oesophageal commis- sures; 6, a cord formerly re- System and the Skin.—r or almost garded as sympathicus. (Af-ter Leydig.) all animals, it has been ascertained that the nervous system arises from the ectoderm. Therefore,in many animals, the nerve-cords and the ganglionic masses,lie permanently in the skin; in others, only during thedevelopment, later becoming separated by splitting off orby infolding, and thus coming to lie in the deeper layersof the body (Fig. 9, p. 46). III. Sensory Organs. Sensations of the Lower Animals.—What we knowof the character of the external world is founded upon ex-. GENERAL ORGANOLOGY. 149 periences gained through our sensory organs. We thusknow the external world only in so far as it is accessible tothe senses, directed and carefully controlled by the powerof judgment. If things exist outside of ourselves whichhave no influence upon our senses, we can form no con-ception of them. It follows from this proposition that wecan gain knowledge of the natural capacity of the sensoryorgans of animals only by analogy with our own experi-ences. We must extend to the whole animal kingdom
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
