96 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



separate, having each a characteristic fundamental func- 

 tion, viz., the epithelial or nutritive system, the blood 

 system, and the nervous system (Fig. 69). The func- 

 tion of the first is the exchange of matter with the ex- 

 ternal world — foreign commerce; of the second, ex- 

 change of matter between different parts of the body 

 — internal carrying trade ; of the third, the exchange of 

 intelligence, both foreign and interior. Putting aside 

 the other two (which, however, are also differentiated), 

 we take only the nervous system. In vertebrates this 

 again is differentiated into three subsystems, viz., the 

 conscio-voluntary, the reflex, and the ganglionic. Put- 

 ting aside again all but the conscio-voluntary, and tak- 

 ing here only the sensory fibers, these are again differ- 

 entiated into five kinds, viz., the five special senses. 

 Even these are probably further differentiated — i. e., dif- 

 ferent kinds of colors, tones, feelings, etc. — and per- 

 ceived by different fibers differently specialized. 



Now as different as these kinds of sensation are from 

 one another, so different that they can not be conceived 

 the one in terms of another, yet they are all probably 

 but modifications of one another and all refinements of 

 the lowest — viz., feeling. It will be interesting, then, to 

 trace the gradations between them in several respects. 



i. Vibrations. — Coarse vibrations are perceived by 

 the nerves of common sensation. We call them jars or 

 tremors. If these increase in number and decrease in 

 size until they lose their separateness — i. e., until there 

 are sixteen in a second, then they appear in conscious- 

 ness in another and entirely different' form — as sound — 

 and we have a special nerve adapted to perceive these 

 more rapid vibrations. As the vibrations become more 

 and more rapid they are perceived as higher and higher 

 musical pitch, until they reach some thirty thousand to 

 forty thousand in a second, which is the acutest sound 



