i8 



THE NERVOUS SYSTEM 



complicated reactions required, long lines of communication have been estab- 

 lished between peripheral sense organs and muscle-fibers in widely separated 

 parts of the body. 



The sensory elements and the lines of communication constitute the nervous 

 system and, together with the musculature, the neuromuscular mechanism. 

 It is well to keep in mind the fact that the nervous system was developed for the 

 purpose of enabling the musculature to react to changes in the environment of 

 the organism. But in all higher animals the nervous system responds not only 

 to stimuli from without but also to stimuli from within the body, and helps to 



Itl 



Fig. 1. Stages in the differentiation of the neuromuscular mechanism: A to C, Hypothetic 

 early stages: A, epithelial stage; B, muscle cell at the stage of the sponge; C, partially differen- 

 tiated nerve-cell in proximity to fully differentiated muscle-cell; D, nerve- and muscle-cell of 

 coelenterate stage; E, a type of receptor-effector system found in many parts of sea-anemones, in- 

 cluding not only receptors, r, with their nerve-nets, and of muscle cells, w, but also of ganglion 

 cells, g, in the nerve-net; F, section at right angles to the sphincter of the bell of a jellyfish (Rhizos- 

 toma): e, epithelium of the subumbrellar surface; n, nervous layer; w, muscle layer. (Parker.) 



bring about an internal adjustment of part with part. Here again it acts as a 

 sensitive mechanism for receiving stimuli and conducting them to the appro- 

 priate organs of response. These organs through which the nervous system 

 produces its effects are known as effectors. While muscles and glands are by 

 far the most important effectors, we must also include certain pigmented cells 

 (or chromatophores) and electric and phosporescent organs under this heading. 

 Except for the reactions produced through such effectors the nervous system 

 would be meaningless. 



We can best understand the significance of the nervous system if we trace 

 its early history. This, as it has been interpreted by Parker (1919), makes an 



