INTBODUCTION 17 



vertebrates (Fig. 1). In this complex the sensory neu- 

 rones, whose cell-bodies lie in the dorsal ganglia, extend 

 from the integument through the dorsal roots to the gray 

 matter of the cord. Motor neurones, whose cell-bodies 

 are situated within the gray matter of the cord, reach 

 from this region to the muscle-fibers which they control. 

 These two classes of neurones would seem to be sufficient 

 for all ordinary reflex operations, but the cord contains 

 within its limits other neurones which serve to connect 



Fio. 1. Diagram of the primary sensory s and motor m neurones of the spinal cord of a 

 vertebrate showing their connections with the integument i and with muscle fibers mf. 



one part of its structure with another. These neurones, 

 which have been called internuncial neurones, are inter- 

 polated between the sensory and motor elements just de- 

 scribed 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 af- 

 ford in the region of the cerebral cortex the material basis 

 for all intellectual operations. 



The plan of neuronic arrangement as exemplified in 

 the vertebrates also obtains in animals as lowly organ- 

 ized as the earthworm (Fig. 2). In this form the sensory 



2 



