218 PARKER— ORIGIN AND SIGNIFICANCE OF [April 21, 



of neurones would seem to be sufficient for all ordinary reflex oper- 

 ations, but the cord contains within its limits many other neurones 

 which serve to connect one part of its structure with another. 

 These neurones, therefore, have been called association neurones, a 

 term which has unfortunately proved to be somewhat misleading 

 because of its use in psychology for quite a different range of 

 phenomena. The so-called association neurones are interpolated 

 between the sensory and motor elements just described and must 

 thereby lengthen and extend the courses of the reflex impulses. 

 Such neurones make up a large part of the substance of the cord and 

 doubtless increase enormously its internal connections. In the brain 

 they not only add to the nervous interrelations, but they afford in 

 the region of the cerebral cortex the material basis for all intellec- 

 tual operations. 



The plan of neuronic arrangement as exemplified in the verte- 

 brates also obtains in animals as lowly organized as the earthworm. 

 In this form the sensory neurones, whose cell-bodies are situated in 

 the integument instead of being gathered into special ganglia, ex- 

 tend, as in the vertebrates, from the skin to the central nervous 

 organs, the brain or the ventral ganglionic chain. The motor neu- 

 rones are essentially duplicates of those in the vertebrates in that 

 their cell-bodies lie within the central organs whence their fibers 

 extend to the appropriate musculature. Association neurones are 

 also abundantly present in the earthworm though their function here, 

 in contrast with that in the higher vertebrates, is pure nervous in- 

 tercommunication, for it is very unlikely that the earthworm pos- 

 sesses what in any strict sense of the word can be called intelli- 

 gence. Thus from a morphological standpoint, the nervous systems 

 of the higher animals, even including such forms as the earth- 

 worm, have much in common, their three sets of interrelated neu- 

 rones, sensory, motor, and association, being arranged upon an es- 

 sentially uniform plan. 



Considered from a physiological standpoint, the nervous system 

 with its appended parts as just sketched falls in the higher animals 

 into three well marked categories. On the exterior of these animals, 

 are to be foimd sense organs or receptors such as the free-nerve 

 terminations of the sensorv neurones in the vertebrates or the sen- 



