HO IV ANIMALS KNOIV THINGS 



the shape of very numerous white threads, too small to 

 be detected by the naked eye. These very small nerve- 

 threads or fibers end at last in connection with certain of 

 the tissue-cells. All the sense-cells of the retina, ear, 

 nose, tongue, and skin are con- 

 nected with minute nerve-fibers 

 as are also all the muscle-fibers. 

 Now all the nerve-fibers from both 

 sense-cells and muscle-cells run 

 to the central portions of the 

 nervous system, the brain and 

 spinal cord, and are there in some 

 way definitely connected with one 

 another, thus making pathways 

 over which everything that affects 

 the eye, ear, and other sense- 

 organs may affect the muscles. 



The nervous svstem of all ver- F''=-,6'5.-Bram of a cat, dor- 

 sal surface ; /, olfactory 

 tebrates is on the same general bulbs; //. cerebral hemi- 



plan, being, however, less com- ^^^ ^^^^"TAfte; 



plex in the lower forms. All Reighard and Jennings.) 



animals with a definite nervous system have nerve-fibers 

 connecting both sense-cells and muscle-cells with 

 certain central parts. They differ, however, in the 

 arrangement of these parts. And since they differ also 

 in muscular arrangement, and in the kind and position of 

 the sense-organs, the arrangement of the nerve-fibers 

 connecting muscles and sense-organs with these central 

 parts differs accordingly. 



In the worms, Crustacea, and insects, which have much 

 the same body-plan, the central nervous system (figs. 67 

 and 68), consists of a chain of ganglia (small nerve-centers) 

 along the ventral portion of the body, this chain being con- 

 nected at the anterior end by a cord on each side of the 

 gullet, with a large liead ganglion which stands in the 



