INTRODUCTION 19 



of the sensory neurones in the vertebrates or the sensory 

 cells in the integument of the earthworm. These organs 

 have for their function the reception of the external stim- 

 uli and the production of the sensory impulses. The re- 

 ceptors are connected by nerve-fibers with the central 

 nervous organ or adjuster composed of the central ends 

 of the sensory and the motor neurones and of the inter- 

 nuncial neurones. Here the impulses arriving from the 

 receptors are directed toward the appropriate groups of 

 muscles by which the animal may respond to the stimulus 

 and, if the animal is highly organized, impressions are 

 made upon the adjuster which, as memories, may become 

 more or less permanent parts of the animal's nervous 

 equipment. Finally the adjusters are connected by nerve- 

 fibers with the third set of elements, the effectors, which 

 as muscles, electric organs, glands, etc., enable the animal 

 to react on the environment. Thus three physiological 

 categories are to be distinguished which in the order of 

 their sequence in action are sense organs or receptors, 

 central nervous organs or adjusters, and muscles or other 

 effectors. 



It is to be noted in passing, that the physiological 

 scheme just outlined includes a wider range of parts than 

 is generally admitted under the head of the nervous sys- 

 tem. The additional parts are the effectors, which, as 

 will be shown later, form as truly a part of the whole sys- 

 tem as do the sense organs or the central nervous organs. 

 Since the term nervous system does not ordinarily include 

 the effectors, it is perhaps best to designate the whole 

 chain of related parts, receptors, adjusters, and effectors, 

 as the neuromuscular mechanism, and in dealing with the 

 elementary nervous system it will be found important 

 to keep this relation in mind, for in such an inquiry, the 



