PART I. 



ORGANS AND FUNCTIONS OF ANIMAL LIFE. 



These must be treated under four groups or systems 

 of organs, viz. : i. Nervous system. 2. Sense organs. 

 3. Muscular system. 4- Skeletal system. These are 

 closely connected in the performance of the functions of 

 animal life. The general way in which they co-operate 

 is shown in the diagram (Fig. 13). 



Fig. 13. — Diagram showing essential parts of an apparatus of exchange be- 

 tween the external world and consciousness : JVC, nerve center ; sc, sen- 

 sory cell ; sf, sensory fiber ; SS, sensory surface ; mc, motor cell ; mf, 

 motor fiber ; M, muscle. Arrowheads show the direction of transmis- 

 sion. 



We have (1) an impression on a sense organ, SS; (2) 

 a transmission inward along a sensory fiber to a nerve 

 center N.C., say the brain; (3) a change of some kind 

 in a sensory cell, s.c, which awakens conscious sensation ; 

 (4) an influence of some kind transferred by a connect- 

 ing fiber to a motor cell, m.c. ; (5) an impulse transmitted 

 outward along a motor fiber, m.f., to a muscle, M, and de- 

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