EVOLUTION OF NERVOUS SYSTEM 85 



Thus a whole group of peripheral parts, often the peripheral 

 ends of afferent neurones, are concerned with receiving such 

 environmental changes as are likely to excite activity in the 

 organism. Thus the eye is receptive for light, the ear for 

 sound, the organs of touch for changes of pressure and the 

 like. These organs, commonly designated as sense organs, 

 but more appropriately called receptors, constitute the first 

 large class of parts in the neuromuscular mechanism. The 

 second class includes the central apparatus proper, composed 

 of the central ends of the afferent neurones, the internuncial 

 neurones, and the central ends of the efferent elements. This is 

 the portion of the system, corresponding to a central telephone 

 station, in which the nervous impulses arriving from the re- 

 ceptors are directed toward the appropriate channels of re- 

 sponse and in which are stored those records of past experi- 

 ence that modify or otherwise qualify the subsequent responses. 

 This second portion is represented in man by the brain, and the 

 spinal cord, and by the sympathetic system and may be desig- 

 nated from one of its chief functions as the adjuster mecha- 

 nism. Finally there is to be mentioned the third group of parts, 

 not truly nervous themselves, but under the control of the 

 nervous system, the muscles, the electric organs, the glands, 

 the luminous organs and the like, all of which enable the 

 animal to act in some particular way on the environment. 

 These have been appropriately spoken of as effectors and com- 

 plete the list of necessary parts in the neuromuscular mecha- 

 nism. 



A nervous system composed of the three types of neurones 

 already mentioned and organized into receptors, adjusters, 

 and effectors is characteristic of most of the higher animals. 

 It is found not only in the vertebrates from man to the fishes, 

 but also in the mollusks, in the arthropods, and in the higher 

 worms such as the annelids. Even such a lowly organized 

 creature as an earthworm has a nervous system developed 



