CHAPTER XV 

 NERVOUS CORRELATION 



Origin of the Nervous System. — In Chapter XIII 

 the fact was pointed out that in order for the motions of 

 animals to be correlated in accordance with the situations 

 existing in the environment the stimulation of the re- 

 ceptors must be conducted to the effectors, as we name 

 the muscular tissue, by means of an apparatus which is 

 called the adjustor mechanism. Early in the book 

 (Chap. II) it was stated that one of the fundamental 

 properties of protoplasm is sensitiveness or irritability, 

 a property by virtue of which the organism responds to 

 stimulation. A feature of this property is that the dis- 

 turbance set up by the stimulus spreads through the pro- 

 toplasm, so that parts at a distance share in the response. 

 The adjustor mechanism evidently depends on this ten- 

 dency of disturbances to spread through protoplasm to 

 points more or less distant from the stimulated point. 

 In introducing the general topic of nervous correlation, 

 which concerns itself with the action of the adjustor 

 mechanism, the various stages between the simplest pos- 

 sible arrangement and the highly complex structure of 

 the higher animals will be outlined as briefly as possible. 



Correlation in Protozoa. — That even so simple an 

 organism as a protozoan shows definite adjustment to its 

 environment is shown by the passage of disturbances 

 from the surface of the protoplasm throughout the mass 

 and so to those special parts of it which are capable of 

 motion. Thus, in Paramoecium, which swims by means 

 of cilia, the vigor of stroke, or often its direction, can be 

 sorn to vary according as the organism comes into contact 

 with one thing or another as it swims along. The cilia 



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