152 



NERVOUS CORRELATION 



of effectors. One who studies carefully the activities of 

 an organism of this kind will perceive that the adjustor 

 mechanism must be more comj)licated than anything that 

 has been pictured thus far. For it can be shown that a 

 single effector may be played upon now by one receptor 

 and now by another. Also, that any given receptor may 

 influence sometimes one and sometimes another of the 

 effectors. Evidently the adjustor system in such an ani- 

 mal consists of something more than nerve cells extending 

 directly from individual receptors to individual effectors, 

 since that arrangement offers no means by which any re- 

 ceptor could establish connection with any effector other 

 than the particular one with which its adjustor nerve-cell 

 connects. 



Fio. 42. — Diawam to sugj?est the organization of a simple nervous 

 system. From the receptors, R and R', sensory nerve cells, S, lead 

 to the ganglion, G. Thence motor nerve cells, M, lead to the effectors, 

 E and A". It is seen that within the ganglion each of the sensory 

 nerve cells has contact with both motor nerve cells. 



The Structure of the Nervous System. — The 

 method by which flexibility of connection between re- 

 ceptors and effectors is established in animals is by means 

 of an arrangement comparable to a telephone installa- 

 tion. Every receptor has nerve-cells leading from it to 

 a region in the body which we may call a nerve center, 

 or. to use the technical name, a ganglioji (see Fig. 42). 

 The flisturbances aroused in the receptors will be con- 

 ducted from them to this center instead of directly to 

 an effector. Each effector in turn has nerve-cells leading 



